Good
Questions In C
!!Enjoy!!!
60.
How to obtain the current drive through C ?
Ans: We can use the function _getdrive( ) to obtain the
current drive. The _getdrive( ) function uses DOS function
0X19 to get the current drive number
#include <direct.h>
main( )
{
int disk ;
disk = _getdrive( ) + 'A' - 1 ;
printf ( "The current drive is: %c\n", disk ) ;
}
-------------------------------------------------------------------------------------------------
61.
How come the output for both the programs is different when
the logic is same?
main( )
{
int i, j ;
for ( i = 1, j = 1 ; i <= 5, j <= 100 ; i++, j++ )
{
gotoxy ( 1, 1, ) ;
printf ( "%d %d", i, j ) ;
}
}
main( )
{
int i, j ;
for ( i =1, j = 1; j <= 100, i <= 5; i++, j++ )
{
gotoxy ( 1, 1 ) ;
printf ( "%d %d", i, j ) ;
}
}
Output -> 5 5
Even if logic of both the programs is same the output of the
first program comes out to be 100, 100, but of the second
program it is 5, 5. The comma operator plays a vital role
inside the for loop. It always considers the value of the
latest variable. So, at the time of testing the condition in
for loop, the value of j will be considered in the first
program and value of i in the second.
-------------------------------------------------------------------------------------------------
62.
Can we get the x and y coordinate of the current cursor
position ?
Ans : The function wherex( ) and wherey( ) returns the
x-coordinate and y-coordinate of the current cursor position
respectively. Both the functions return an integer value. The
value returned by wherex( ) is the horizontal position of
cursor and the value returned by wherey( ) is the vertical
position of the cursor. Following program shows how to use the
wherex( ) and wherey( ) functions.
#include <conio.h>
main( )
{
printf ( "Just\n To\n Test\n Where\n the cursor\n goes" ) ;
printf ( "Current location is X: %d Y: %d\n", wherex( ),
wherey( ) ) ;
}
-------------------------------------------------------------------------------------------------
63.
How do I programmatically delete lines in the text window?
Ans: While writing programs that perform screen-based I/O, you
may want to-delete the current line's contents, moving one
line up, all of the output that follows. In such cases a
function called delline( ) can be used. Following code snippet
illustrates the use of function delline( ).
#include <conio.h>
main( )
{
int i ;
clrscr( ) ;
for ( i = 0; i <= 23; i++ )
printf ( "Line %d\r\n", i ) ;
printf ( "Press a key to continue : " ) ;
getch( ) ;
gotoxy ( 2, 6 ) ;
for ( i = 6; i <= 12; i++ )
delline( ) ;
getch( ) ;
}
-------------------------------------------------------------------------------------------------
64.
How do I get the time elapsed between two function calls ?
Ans: The function difftime( ) finds the difference between two
times. It calculates the elapsed time in seconds and returns
the difference between two times as a double value.
#include <time.h>
#include <stdio.h>
#include <dos.h>
main( )
{
int a[] = { 2, -34, 56, 78, 112, 33, -7, 11, 45, 29, 6 } ;
int s ;
time_t t1, t2 ; // time_t defines the value used for time
function
s = sizeof ( a ) / 2 ;
t1 = time ( NULL ) ;
sel_sort ( a, s ) ; // sort array by selection sort
bub_sort ( a, s ) ; // sort array by bubble sort method
t2 = time ( NULL ) ;
printf ( "\nThe difference between two function calls is %f",
difftime (
t2, t1 ) ) ;
}
In the above program we have called difftime( ) function that
returns the time elapsed from t1 to t2.
-------------------------------------------------------------------------------------------------
65.
How do I use swab( ) in my program ?
Ans: The function swab( ) swaps the adjacent bytes of memory.
It copies the bytes from source string to the target string,
provided that the number of characters in the source string is
even. While copying, it swaps the bytes which are then
assigned to the target string.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
main ( )
{
char *str1 = "hS eesll snsiasl not eh es as oher " ;
char *str2 ;
clrscr( ) ;
swab ( str1, str2, strlen ( str1 ) ) ;
printf ( "The target string is : %s\n", str2 ) ; // output --
She sells
snails on the sea shore
getch( ) ;
}
-------------------------------------------------------------------------------------------------
66.
Turbo C provides various command line compiler options which
we can use through TCC. The compiler options include :
displaying specific warning messages, generating 8087 hardware
instructions, using a filename for generating assembly code,
etc. Instead of compiler options being executed at command
line we can use these compiler options in our program. This
can be achieved using #pragma options. We can use various
flags with #pragma options to use the compiler options. All
these flags are available in turbo C's online help.
-------------------------------------------------------------------------------------------------
67.
I have an array declared in file 'F1.C' as,
int a[ ] = { 1, 2, 3, 4, 5, 6 } ;
and used in the file 'F2.C' as,
extern int a[ ] ;
In the file F2.C, why sizeof doesn't work on the array a[ ]?
Ans: An extern array of unspecified size is an incomplete
type. You cannot apply sizeof to it, because sizeof operates
during compile time and it is unable to learn the size of an
array that is defined in another file. You have three ways to
resolve this problem:
1. In file 'F1.C' define as,
int a[ ] = { 1, 2, 3, 4, 5, 6 } ;
int size_a = sizeof ( a ) ;
and in file F2.C declare as,
extern int a[ ] ;
extern int size_a ;
2. In file 'F1.H' define,
#define ARR_SIZ 6
In file F1.C declare as,
#include "F1.H"
int a[ ARR_SIZ ] ;
and in file F2.C declare as,
#include "F1.H"
extern int a[ ARR_SIZ ] ;
3. In file 'F1.C' define as,
int a[ ] = { 1, 2, 3, 4, 5, 6, -1 } ;
and in file 'F2.C' declare as,
extern int a[ ] ;
Here the element -1 is used as a sentinel value, so the code
can
understand the end without any explicit size.
-------------------------------------------------------------------------------------------------
68.
How to delete a line from text displayed on the screen?
Ans: Sometimes, specially when we are creating a text editor
like program we may wish to allow user to delete a line. We
can do so by using two functions namely clreol( ) and delline(
). The clreol( ) function deletes the line from the current
cursor position to the end of line. The delline() function
deletes the entire line at the current cursor position and
moves up the following line. Following program shows how to
use these functions.
#include <conio.h>
main( )
{
int i ;
for ( i = 1 ; i <= 20 ; i++ )
printf ( "This is Line %d\n", i ) ;
getch( ) ;
gotoxy ( 1, 7 ) ;
clreol( ) ;
getch( ) ;
gotoxy ( 1, 12 ) ;
delline( ) ;
getch( ) ;
}
-------------------------------------------------------------------------------------------------
69.
How do I programmatically insert lines in the text window?
Ans: We can insert a blank line in the text window using the
insline( ) function. This function inserts line at current
cursor position. While doing so, it shifts down the lines that
are below the newly inserted line.
#include <conio.h>
void main( )
{
printf ( "The little snail was slowly moving up. She
wanted\r\n" ) ;
printf ( "to reach the top of the tree. It was chilly\r\n" ) ;
printf ( "winter season. Most of the animals were resting
in\r\n" ) ;
printf ( "their nests as there was a heavy snow fall.\r\n" ) ;
printf ( "\r\nPress any key to continue:" ) ;
gotoxy ( 10, 2 ) ;
getch( ) ;
insline( ) ;
getch( ) ;
}
-------------------------------------------------------------------------------------------------
70.
What will be the output of the following program?
main( )
{
unsigned int num ;
int i ;
printf ( "\nEnter any number" ) ;
scanf ( "%u", &num ) ;
for ( i = 0 ; i < 16 ; i++ )
printf ( "%d", ( num << i & 1 << 15 ) ? 1 : 0 ) ;
}
Ans: The output of this program is the binary equivalent of
the given number. We have used bitwise operators to get the
binary number.
-------------------------------------------------------------------------------------------------
71.
Graphics
Building Mouse Cursors...
In text mode the mouse cursor appears as a block, whereas in
graphics mode it appears as an arrow. If we wish we can change
the graphics cursor to any other shape the way Windows does.
The mouse cursor in graphics mode occupies a 16 by 16 pixel
box. By highlighting or dehighlighting some of the pixels in
this box we can get the desired shape. For example, the
following bit-pattern can be used to generate the cursor which
looks like an hour-glass.
1111111111111111 0000000000000000
1000000000000001 0000000000000000
1111111111111111 0000000000000000
1000000000000001 0000000000000000
0100000000000010 1000000000000001
0010000000000100 1100000000000011
0000100000010000 1111000000001111
0000001001000000 1111110000111111
0000001001000000 1111110000111111
0000100000010000 1111000000001111
0010000000000100 1100000000000011
0100000000000010 1000000000000001
1000000000000001 0000000000000000
1111111111111111 0000000000000000
1000000000000001 0000000000000000
1111111111111111 0000000000000000
Mouse pointer bitmap Screen Mask the one's in the mouse
pointer bitmap indicate that the pixel would be drawn whereas
the zeros indicate that the pixel would stand erased. It is
important to note that the mouse pointer bit pattern is 32
bytes long. However, while actually writing a program to
change the pointer shape we need a 64 byte bit-map. This
provision is made to ensure that when the cursor reaches a
position on the screen where something is already written or
drawn only that portion should get overwritten which is to be
occupied by the mouse cursor. Of the 64 bytes the first 32
bytes contain a bit mask which is first ANDed with the screen
image, and then the second 32 bytes bit mask is XORed with the
screen image.
The following program changes the mouse cursor in graphics
mode to resemble an hour glass.
# include "graphics.h"
# include "dos.h"
union REGS i, o ;
struct SREGS s ;
int cursor[32] =
{
/* Hour-glass screen mask */
0x0000, 0x0000, 0x0000, 0x0000,
0x8001, 0xc003, 0xf00f, 0xfc3f,
0xfc3f, 0xf00f, 0xc003, 0x8001,
0x0000, 0x0000, 0x0000, 0x0000,
/* The mouse pointer bitmap */
0xffff, 0x8001, 0xffff, 0x8001,
0x4002, 0x2004, 0x1008, 0x0240,
0x0240, 0x0810, 0x2004, 0x4002,
0x8001, 0xffff, 0x8001, 0xffff,
} ;
main( )
{
int gd = DETECT, gm ;
initgraph ( &gd, &gm, "c:\\tc\\bgi" ) ;
if ( initmouse( ) == -1 )
{
closegraph( ) ;
printf ( "\n Mouse not installed!" ) ;
exit( ) ;
}
gotoxy ( 10, 1 ) ; printf ( "Press any key to exit..." ) ;
changecursor ( cursor ) ; showmouseptr( ) ;
getch( ) ;
}
initmouse( )
{
i.x.ax = 0 ; int86 ( 0x33, &i, &o ) ;
return ( o.x.ax == 0 ? -1 : 0 ) ;
}
showmouseptr( )
{
i.x.ax = 1 ; int86 ( 0x33, &i, &o ) ;
}
changecursor ( int *shape )
{
i.x.ax = 9 ; /* service number */
i.x.bx = 0 ; /* actual cursor position from left */
i.x.cx = 0 ; /* actual cursor position from top */
i.x.dx = ( unsigned ) shape ; /* offset address of pointer
image*/
segread ( &s ) ;
s.es = s.ds ; /* segment address of pointer */
int86x ( 0x33, &i, &i, &s ) ;
}
-------------------------------------------------------------------------------------------------
72.
Towers Of Hanoi
Suppose there are three pegs labeled A, B and C. Four disks
are placed on peg A. The bottom-most disk is largest, and
disks go on decreasing in size with the topmost disk being
smallest. The objective of the game is to move the disks from
peg A to peg C, using peg B as an auxiliary peg. The rules of
the game are as follows:
Only one disk may be moved at a time, and it must be the top
disk on one of the pegs. A larger disk should never be placed
on the top of a smaller disk. Suppose we are to write a
program to print out the sequence in which the disks should be
moved such that all disks on peg A are finally transferred to
peg C. Here it is...
main( )
{
int n = 4 ;
move ( n, 'A', 'B', 'C' ) ;
}
move ( n, sp, ap, ep )
int n ;
char sp, ap, ep ;
{
if ( n == 1 )
printf ( "\n Move from %c to %c ", sp, ep ) ;
else
{
move ( n - 1, sp, ep, ap ) ;
move ( 1, sp, ' ', ep ) ;
move ( n - 1, ap, sp, ep ) ;
}
}
And here is the output...
Move from A to B
Move from A to C
Move from B to C
Move from A to B
Move from C to A
Move from C to B
Move from A to B
Move from A to C
Move from B to C
Move from B to A
Move from C to A
Move from B to C
Move from A to B
Move from A to C
Move from B to C
This problem is the famous Towers of Hanoi problem, wherein
three pegs are to be employed for transferring the disks with
the given criteria. Here's how we go about it. We have three
pegs: the starting peg, sp, the auxiliary peg ap, and the
ending peg, ep, where the disks must finally be. First, using
the ending peg as an auxiliary or supporting peg, we transfer
all but the last disk to ap. Next the last disk is moved from
sp to ep. Now, using sp as the supporting peg, all the disks
are moved from ap to ep. ‘A’, B and C denote the three pegs.
The recursive function move( ) is called with different
combinations of these pegs as starting, auxiliary and ending
pegs.
-------------------------------------------------------------------------------------------------
73.
What would be the output of following program?
struct syntax
{
int i ;
float g ;
char c ;
}
main( )
{
printf ( "I won't give you any error" ) ;
}
Ans: The above program would get compiled successfully and on
execution it would print the message given in printf(). What
strikes in the above code snippet is the structure syntax
which is declared but not terminated with the statement
terminator, the semicolon. The compiler would not give any
error message for it, as it assumes that main( ) function have
a return type of struct syntax and hence would successfully
compile and execute the program.
-------------------------------------------------------------------------------------------------
74.
How to get the memory size ?
Ans: Consider the following program
#include <stdio.h>
#include <bios.h>
main( )
{
int memsize;
memsize = biosmemory( ) ;
printf ( "RAM size = %dK\n",memsize ) ;
return 0 ;
}
The function biosmemory uses BIOS interrupt 0x12 to return the
size of memory.
-------------------------------------------------------------------------------------------------
75.
Float Format
How does C compiler stores float values ?
Ans: In C, the float values are stored in a mantissa and
exponent form. While writing a number we specify the exponent
part in the form of base 10. But, in case of C compiler, the
exponent for floats is stored in the form of base 2.
Obviously, because, computer stores the numbers in binary
form. The C compiler follows an IEEE standard to store a
float. The IEEE format expresses a floating-point number in a
binary form known as `normalized' form. Normalization involves
adjusting the exponent so that the "binary point" (the binary
analog of the decimal point) in the mantissa always lies to
the right of most significant nonzero digit. In binary
representation, this means that the most significant digit of
the mantissa is always a 1. This property of the normalized
representation is exploited by the IEEE format when storing
the mantissa. Let us consider an example of generating the
normalized form of a floating point number. Suppose we want to
represent the decimal number 5.375. Its binary equivalent can
be obtained as shown below:
2 | 5
.375 x 2 = 0.750 0
|------
.750 x 2 = 1.500 1
2 | 2 1
.500 x 2 = 1.000 1
|------
2 | 1 0
|------
| 0 1
Writing remainders in reverse writing whole parts in the same
order we get 101 order in which they are obtained we get 011
thus the binary equivalent of 5.375 would be 101.011. The
normalized form of this binary number is obtained by adjusting
the exponent until the decimal point is to the right of most
significant 1. In this case the result is 1.01011 x 22. The
IEEE format for floating point storage uses a sign bit, a
mantissa and an exponent for representing the power of 2. The
sign bit denotes the sign of the number: a 0 represents a
positive value and a 1 denotes a negative value. The mantissa
is represented in binary. Converting the floating-point number
to its normalized form results in a mantissa whose most
significant digit is always 1. The IEEE format takes advantage
of this by not storing this bit at all. The exponent is an
integer stored in unsigned binary format after adding a
positive integer bias. This ensures that the stored exponent
is always positive. The value of the bias is 127 for floats
and 1023 for doubles. Thus, 1.01011 x 22 is represented as
shown below:
--- ---------------
----------------------------------------------
| 0 | 100 0000 1 | 010 1100 0000 0000 0000 0000 |
--- ----------------
---------------------------------------------
sign bit exponent- mantissa stored in normalized form obtained
after adding a bias
127 to exponent 2
Data Structures
Which is the best sorting method?
Ans: There is no sorting method that is universally superior
to all others. The programmer must carefully examine the
problem and the desired results before deciding the particular
sorting method. Some of the sorting methods are given below:
Bubble sort : When a file containing records is to be sorted
then Bubble sort is the best sorting method when sorting by
address is used.
Bsort : It can be recommended if the input to the file is
known to be nearly sorted.
Meansort : It can be recommended only for input known to be
very nearly sorted.
Quick Sort : In the virtual memory environment, where pages of
data are constantly being swapped back and forth between
external and internal storage. In practical situations, quick
sort is often the fastest available because of its low
overhead and its average behavior.
Heap sort : Generally used for sorting of complete binary
tree. Simple insertion sort and straight selection sort : Both
are more efficient than bubble sort. Selection sort is
recommended for small files when records are large and for
reverse situation insertion sort is recommended. The heap sort
and quick sort are both more efficient than insertion or
selection for large number of data.
Shell sort : It is recommended for moderately sized files of
several hundred elements.
Radix sort : It is reasonably efficient if the number of
digits in the keys is not too large.
-------------------------------------------------------------------------------------------------
76.
Calculating Wasted Bytes On Disk
When a file gets stored on the disk, at a time DOS allocates
one cluster for it. A cluster is nothing but a group of
sectors. However, since all file sizes cannot be expected to
be a multiple of 512 bytes, when a file gets stored often part
of the cluster remains unoccupied. This space goes waste
unless the file size grows to occupy these wasted bytes. The
following program finds out how much space is wasted for all
files in all the directories of the current drive.
#include <dir.h>
#include <dos.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
unsigned bytes_per_cluster ;
unsigned long wasted_bytes ;
unsigned long num_files = 0 ;
main( )
{
int ptr = 0, flag = 0, first = 0 ;
struct ffblk f[50] ;
struct dfree free ;
/* get cluster information and calculate bytes per cluster */
getdfree ( 0, &free ) ;
bytes_per_cluster = free.df_bsec * free.df_sclus ;
chdir ( "\\" ) ;
/* check out files in root directory first */
cal_waste( ) ;
/* loop until all directories scanned */
while ( ptr != -1 )
{
/* should I do a findfirst or a findnext? */
if ( first == 0 )
flag = findfirst ( "*.*", &f[ptr], FA_DIREC ) ;
else
flag = findnext ( &f[ptr] ) ;
while ( flag == 0 )
{
/* make sure its a directory and skip over . & .. entries */
if ( f[ptr].ff_attrib == FA_DIREC && f[ptr].ff_name[0] != '.'
)
{
flag = chdir ( f[ptr].ff_name ) ; /* try changing directories
*/
if ( flag == 0 ) /* did change dir work? */
{
cal_waste( ) ;
first = 0 ; /* set for findfirst on next pass */
break ;
}
}
flag = findnext ( &f[ptr] ) ; /* search for more dirs */
}
if ( flag != 0 || ptr == 49 ) /* didn't find any more dirs */
{
ptr-- ;
chdir ( ".." ) ; /* go back one level */
first = 1 ; /* set to findnext on next pass */
}
else
ptr++ ;
}
printf ( "There are %lu bytes wasted in %lu files.\n",
wasted_bytes,
num_files ) ;
}
cal_waste( )
{
int flag = 0 ;
long full_cluster ;
struct ffblk ff ;
/* look for all file types */
flag = findfirst ( "*.*", &ff, FA_RDONLY | FA_HIDDEN |
FA_SYSTEM | FA_ARCH
) ;
while ( flag == 0 )
{
num_files++ ;
full_cluster = ff.ff_fsize / bytes_per_cluster *
bytes_per_cluster ;
wasted_bytes += bytes_per_cluster - ( ff.ff_fsize -
full_cluster ) ;
flag = findnext ( &ff ) ;
}
}
Data Structures
Polish Notation
The method of writing all operators either before their
operation, or after them, is called Polish notation, in honor
of its discoverer, the Polish mathematician Jan Lukasiewicz.
When the operators are written before their operands, it is
called the prefix form. When the operators come after their
operands. It is called the postfix form, or, sometimes reverse
Polish form or suffix form. In this context, it is customary
to use the coined phrase infix form to denote the usual custom
of writing binary operators between their operands. For
example, the expression A + B becomes +AB in prefix form and
AB+ in postfix form. In the expression A + B x C, the
multiplication is done first, so we convert it first,
obtaining first A + ( BCx ) and then ABCx+ in postfix form.
The prefix form of this expression is +A x BC. The prefix and
postfix forms are not related by taking mirror images or other
such simple transformation. Also all parentheses have been
omitted in the Polish forms.
-------------------------------------------------------------------------------------------------
77.
The Longjmp And Setjmp
The C programming language does not let you nest functions.
You cannot write a function definition inside another function
definition, as in:
int fun1( )
{
int fun2() /* such nesting of functions is not allowed */
{
.....
}
}
Because of this restriction it is not possible to hide
function names inside a hierarchy. As a result all the
functions that you declare within a program are visible to
each other. This of course is not a major drawback since one
can limit visibility by grouping functions within separate C
source files that belong to different logical units of the
program. C does, however, suffer in another way because of
this design decision. It provides no easy way to transfer
control out of a function except by returning to the
expression that called the function. For the vast majority of
function calls, that is a desirable limitation. You want the
discipline of nested function calls and returns to help you
understand flow of control through a program. Nevertheless, on
some occasions that discipline is too restrictive. The program
is sometimes easier to write, and to understand, if you can
jump out of one or more function invocations at a single
stroke. You want to bypass the normal function returns and
transfer control to somewhere in an earlier function
invocation.
For example, you may want to return to execute some code for
error recovery no matter where an error is detected in your
application. The setjmp and the longjmp functions provide the
tools to accomplish this. The setjmp function saves the
"state" or the "context" of the process and the longjmp uses
the saved context to revert to a previous point in the
program. What is the context of the process? In general, the
context of a process refers to information that enables you to
reconstruct exactly the way the process is at a particular
point in its flow of execution. In C program the relevant
information includes quantities such as values of SP, SS,
FLAGS, CS, IP, BP, DI, ES, SI and DS registers.
To save this information Turbo C uses the following structure,
which is defined, in the header file 'setjmp.h'.
typedef struct
{
unsigned j_sp ;
unsigned j_ss ;
unsigned j_flag ;
unsigned j_cs ;
unsigned j_ip ;
unsigned j_bp ;
unsigned j_di ;
unsigned j_es ;
unsigned j_si ;
unsigned j_ds ;
} jmp_buf[1] ;
This is a system-dependent data type because different systems
might require different amounts of information to capture the
context of a process. In Turbo C, jmp_buf is simply an array
of ten 2-byte integers. To understand the mechanics of setjmp
and longjmp, look at the following code
fragment.
#include "setjmp.h"
jmp_buf buf ;
main( )
{
if ( setjmp ( buf ) == 0 )
process( ) ;
else
handle_error( ) ; /* executed when longjmp is called */
}
process( )
{
int flag = 0 ;
/* some processing is done here */
/* if an error occurs during processing flag is set up */
if ( flag )
longjmp ( buf, 1 ) ;
}
Upon entry to setjmp the stack contains the address of the
buffer buf and the address of the if statement in the main
function, to which setjmp will return. The setjmp function
copies this return address as well as the current values of
registers, SP, SS, FLAGS, BP, DI, ES, SI and DS, into the
buffer buf. Then setjmp returns with a zero. In this case, the
if statement is satisfied and the process( ) function is
called. If something goes wrong in process( ) (indicated by
the flag variable), we call longjmp with two arguments: the
first is the buffer that contains the context to which we will
return. When the stack reverts back to this saved state, and
the return statement in longjmp is executed, it will be as if
we were returning from the call to setjmp, which originally
saved the buffer buf. The second argument to longjmp specifies
the return value to be used during this return. It should be
other than zero so that in the if statement we can tell
whether the return is induced by a longjmp.
The setjmp/longjmp combination enables you to jump
unconditionally from one C function to another without using
the conventional return statements. Essentially, setjmp marks
the destination of the jump and longjmp is a non-local goto
that executes the jump.
Data Structures
Comparison Trees...
The comparison trees also called decision tree or search tree
of an algorithm, is obtained by tracing through the actions of
the algorithm, representing each comparison of keys by a
vertex of the tree (which we draw as a circle). Inside the
circle we put the index of the key against which we are
comparing the target key. Branches (lines) drawn down from the
circle represent the possible outcomes of the comparison and
are labeled accordingly. When the algorithm terminates, we put
either F (for failure) or the location where the target is
found at the end of the appropriate branch, which we call a
leaf, and draw as a square. Leaves are also sometimes called
end vertices or external vertices of the tree. The remaining
vertices are called the internal vertices of the tree. The
comparison tree for sequential search is especially simple.
-------------------------------------------------------------------------------------------------
78.
Suppose we have a floating-point number with higher precision
say 12.126487687 and we wish it to be printed with only
precision up to two decimal places. How can I do this?
Ans. This can achieved through the use of suppression char '*'
in the format string of printf( ) which is shown in the
following program.
main( )
{
int p = 2 ;
float n = 12.126487687 ;
printf ( "%.*f",p, n ) ;
}
-------------------------------------------------------------------------------------------------
79.
Spawning
All programs that we execute from DOS prompt can be thought of
as children of COMMAND.COM. Thus, the program that we execute
is a child process, whereas COMMAND.COM running in memory is
its parent. The process of a parent process giving birth to a
child process is known as 'spawning'. If the spawned program
so desires, it may in turn spawn children of its own, which
then execute and return control to their parent. Who is the
parent of COMMAND.COM? COMMAND.COM itself. We can trace the
ancestors of our program using the field Parent Process ID (PID)
present at offset 0x16 in the Program Segment Prefix (PSP). To
trace this ancestry our program should first locate its PSP,
extract the parent process ID from it and then use this to
find PSP of the parent. This process can be repeated till we
reach COMMAND.COM (process ID of COMMAND.COM is its own PSP),
the father of all processes. Here is a program which achieves
this...
/* SPAWN.C */
#include "dos.h"
unsigned oldpsp, newpsp, far *eb_seg, i ;
char far *eb_ptr ;
main( )
{
oldpsp = _psp ;
while ( 1 )
{
printf ( "\n" ) ;
printname ( oldpsp ) ;
printf ( " spawned by " ) ;
newpsp = * ( ( unsigned far * ) MK_FP ( oldpsp, 0x16 ) ) ;
if ( * ( ( unsigned * ) MK_FP ( newpsp, 0x16 ) ) == newpsp )
break ;
else
oldpsp = newpsp ;
printname ( newpsp ) ;
}
printf ( "%-20s (%04X)", "COMMAND.COM", newpsp ) ;
}
printname ( unsigned lpsp )
{
char drive[5], dir[68], name[13], ext[5] ;
eb_seg = ( unsigned far * ) MK_FP ( lpsp, 0x2C ) ;
eb_ptr = MK_FP ( *eb_seg, 0 ) ;
i = 0 ;
while ( 1 )
{
if ( eb_ptr[i] == 0 )
{
if ( eb_ptr[i + 1] == 0 && eb_ptr[i + 2] == 1 )
{
i += 4 ;
break ;
}
}
i++ ;
}
fnsplit ( eb_ptr + i, drive, dir, name, ext ) ;
strcat ( name, ext ) ;
printf ( "%-20s (%04X)", name, oldpsp ) ;
}
On running the program from within TC the output obtained is
shown below. SPWAN.EXE (58A9) spawned by TC.EXE (0672) TC.EXE
(0672) spawned by COMMAND.COM (05B8). The program simply
copies its own process ID in the variable oldpsp and then uses
it to extract its own filename from its environment block.
This is done by the function printname( ). The value in oldpsp
is then used to retrieve the parent's PID in newpsp. From
there the program loops reporting the values of oldpsp, newpsp
and the corresponding file names until the program reaches
COMMAND.COM.
The printname( ) function first locates the environment block
of the program and then extracts the file name from the
environment block. The fnsplit( ) function has been used to
eliminate the path present prior to the file name. Do not run
the program from command line since it would give you only one
level of ancestry.
Data Structures
Choosing the data structures to be used for information
retrieval. For problems of information retrieval, consider the
size, number, and location of the records along with the type
and structure of the keys while choosing the data structures
to be used. For small records, high-speed internal memory will
be used, and binary search trees will likely prove adequate.
For information retrieval from disk files, methods employing
multiway branching, such as trees, B-trees , and hash tables,
will usually be superior. Tries are particularly suited to
applications where the keys are structured as a sequence of
symbols and where the set of keys is relatively dense in the
set of all possible keys. For other applications, methods that
treat the key as a single unit will often prove superior.
B-trees, together with various generalization and extensions,
can be usefully applied to many problems concerned with
external information retrieval.
-------------------------------------------------------------------------------------------------
80.
Variably Dimensioned Arrays
While dealing with Scientific or Engineering problems one is
often required to make use of multi-dimensioned array.
However, when it comes to passing multidimensional arrays to a
function C is found wanting. This is because the C compiler
wants to know the size of all but the first dimension of any
array passed to a function. For instance, we can define a
function compute ( int n, float x[] ), but not compute ( int
n, x[][]).
Thus, C can deal with variably dimensioned 1-D arrays, but
when an array has more than one dimension, the C compiler has
to know the size of the last dimensions expressed as a
constant. This problem has long been recognized, and some of
the solutions that are often used are:
Declare the arrays in the functions to be big enough to tackle
all possible situations. This can lead to a wastage of lot of
precious memory in most cases. Another solution is to
construct multiple-dimension array as an array of pointers.
For example, a matrix (2-D array) of floats can be declared as
a 1-D array of float pointers, with each element pointing to
an array of floats. The problem with this method is that the
calling function has to define all arrays in this fashion.
This means that any other computations done on the arrays must
take this special structure into account.
Another easy solution, though seldom used, exists. This is
based on the following method:
Pass the array to the function as though it is a pointer to an
array of floats (or the appropriate data type), no matter how
many dimensions the array actually has, along with the
dimensions of the array. Reference individual array elements
as offsets from this pointer.
Write your algorithm so that array elements are accessed in
storage order. The following program for multiplying two
matrices illustrates this
procedure.
# define M 3
# define N 2
# define P 4
float a[M][N], b[N][P], c[M][P] ;
void mulmat ( int, int, int, float*, float*, float* ) ;
main( )
{
int i, j ;
for ( i = 0 ; i < M ; i++ )
for ( j = 0 ; j < N ; j++ )
a[i][j] = i + j ;
for ( i = 0 ; i < N ; i++ )
for ( j = 0 ; j < P ; j++ )
b[i][j] = i + j ;
mulmat ( M, N, P, a, b, c ) ;
for ( i = 0 ; i < M ; i++ )
{
printf ( "\n" ) ;
for ( j = 0 ; j < P ; j++ )
printf ( "%f\t", c[i][j] ) ;
}
}
void mulmat ( int m, int n, int p, float *a, float *b, float
*c )
{
float *ptrtob, *ptrtoc ;
int i, j, k, nc ;
/* set all elements of matrix c to 0 */
for ( i = 0 ; i < m * p ; i++ )
*( c + i ) = 0 ;
for ( i = 0 ; i < m ; i++ )
{
ptrtob = b ;
for ( k = 0 ; k < n ; k++ )
{
ptrtoc = c ;
for ( j = 0 ; j < p ; j++ )
*ptrtoc++ += *a * *ptrtob++ ;
a++ ;
}
c += p ;
}
}
We know that C stores array elements in a row-major order.
Hence to ensure that the elements are accessed in the storage
order the above program uses a variation of the normal
matrix-multiplication procedure. The pseudo code for this is
given below:
for i = 1 to m
for j = 1 to p
c[i][j] = 0
end
for k = 1 to n
for j = 1 to p
c[i][j] = c[i][j] + a[i][k] * b[k][j]
end
end
end
------------------------------------------------------------------------------------------------
81.
Why is it not possible to scan strings from keyboard in case
of array of pointers to string?
Ans: When an array is declared, dimension of array should be
specified so that compiler can allocate memory for the array.
When array of pointers to strings is declared its elements
would contain garbage addresses. These addresses would be
passed to scanf( ). So strings can be received but they would
get stored at unkown locations. This is unsafe.
-------------------------------------------------------------------------------------------------
82.
Bit Arrays
If in a program a variable is to take only two values 1 and 0,
we really need only a single bit to store it. Similarly, if a
variable is to take values from 0 to 3, then two bits are
sufficient to store these values. And if a variable is to take
values from 0 through 7, then three bits will be enough, and
so on. Why waste an entire integer when one or two or three
bits will do? Because there aren't any one bit or two bit or
three bit data types available in C. However, when there are
several variables whose maximum values are small enough to
pack into a single memory location, we can use `bit fields' to
store several values in a single integer. Bit fields are
discussed in most standard C texts. They are usually used when
we want to store assorted information which can be
accommodated in 1, 2, 3 bits etc.
For example, the following data about an employee can be
easily stored using bit fields.
male or female
single, married, divorced or widowed
have one of the eight different hobbies
can choose from any of the fifteen different schemes proposed
by the company to pursue his/her hobby.
This means we need one bit to store gender, two to store
marital status, three for hobby, and four for scheme (with one
value used for those who are not desirous of availing any of
the schemes). We need ten bits altogether, which means we can
pack all this information into a single integer, since an
integer is 16 bits long.
At times we may need to store several True or False statuses.
In such cases instead of using bit fields using an array of
bits would be more sensible. On this array we may be required
to perform the following operations:
Set a bit (make it 1).
Clear a bit (make it 0).
Test the status of a bit in the array.
Reach the appropriate bit slot in the array.
Generate a bit mask for setting and clearing a bit.
We can implement these operations using macros given below:
#define CHARSIZE 8
#define MASK ( y ) ( 1 << y % CHARSIZE )
#define BITSLOT ( y ) ( y / CHARSIZE )
#define SET ( x, y ) ( x[BITSLOT( y )] |= MASK( y ) )
#define CLEAR ( x, y ) ( x[BITSLOT( y )] &= ~MASK( y ) )
#define TEST ( x, y ) ( x[BITSLOT( y )] & MASK( y ) )
#define NUMSLOTS ( n ) ( ( n + CHARSIZE - 1) / CHARSIZE )
Using these macros we can declare an array of 50 bits be
saying,
char arr[NUMSLOTS(50)] ;
To set the 20th bit we can say,
SET(arr, 20 ) ;
And if we are to test the status of 40th bit we may say,
if ( TEST ( arr, 40 ) )
Using bit arrays often results into saving a lot of precious
memory. For example, the following program which implements
the Sieve of Eratosthenes for generating prime numbers smaller
than 100 requires only 13 bytes. Had we implemented the same
logic using an array of integers we would have required an
array of 100 integers, that is 200 bytes.
#include <stdio.h>
#include <string.h>
#define MAX 100
main( )
{
char arr[NUMSLOTS( MAX )] ;
int i, j ;
memset ( arr, 0, NUMSLOTS( MAX ) ) ;
for ( i = 2 ; i < MAX ; i++ )
{
if ( !TEST ( arr, i ) )
{
printf ( "\n%d", i ) ;
for ( j = i + i ; j < MAX ; j += i )
SET ( arr, j ) ;
}
}
}
-------------------------------------------------------------------------------------------------
83.
Information Hiding in C
Though C language doesn't fully support encapsulation as C++
does, there is a simple technique through which we can
implement encapsulation in C. The technique that achieves this
is modular programming in C. Modular programming requires a
little extra work from the programmer, but pays for itself
during maintenance. To understand this technique let us take
the example of the popular stack data structure. There are
many methods of implementing a stack (array, linked list,
etc.). Information hiding teaches that users should be able to
push and pop the stack's elements without knowing about the
stack's implementation. A benefit of this sort of information
hiding is that users don't have to change their code even if
the implementation details change.
Consider the following scenario:
To be able to appreciate the benefits of modular programming
and thereby information hiding, would first show a traditional
implementation of the stack data structure using pointers and
a linked list of structures. The main( ) function calls the
push( ) and pop( ) functions.
#include <alloc.h>
typedef int element ;
void initialize_stack ( struct node ** ) ;
void push ( struct node **, element ) ;
element pop ( struct node * ) ;
int isempty ( struct node * ) ;
struct node
{
element data ;
struct node *next ;
} ;
void main( )
{
struct node *top ;
element num ;
initialize_stack ( &top ) ;
push ( &top, 10 ) ;
push ( &top, 20 ) ;
push ( &top, 30 ) ;
if ( isempty ( top ) )
printf ( "\nStack is empty" ) ;
else
{
num = pop ( top ) ;
printf ( "\n Popped %d", num ) ;
}
}
void initialize_stack ( struct node **p )
{
*p = NULL ;
}
void push ( struct node **p, element n )
{
struct node *r ;
r = ( struct node *) malloc ( sizeof ( struct node ) ) ;
r -> data = n ;
if ( *p == NULL )
r -> next = NULL ;
else
r -> next = *p ;
*p = r ;
}
element pop ( struct node *p )
{
element n ;
struct node *r ;
n = p -> data ;
r = p ;
p = p -> next ;
free ( r ) ;
return ( n ) ;
}
int isempty ( struct node *p )
{
if ( p == NULL )
return ( -1 ) ;
else
return ( 0 ) ;
}
Notice how the specific implementation of the data structure
is strewn throughout main( ). main( ) must see the definition
of the structure node to use the push( ), pop( ), and other
stack functions. Thus the implementation is not hidden, but is
mixed with the abstract operations.
Data Structures
Radix Sort
This sorting technique is based on the values of the actual
digits in the positional representations of the numbers being
sorted. Using the decimal base, for example, where the radix
is 10, the numbers can be partitioned into ten groups on the
sorter. For example, to sort a collection of numbers where
each number is a four-digit number, then, All the numbers are
first sorted according to the the digit at unit's place.
In the second pass, the numbers are sorted according to the
digit at tenth place. In the third pass, the numbers are
sorted according to the digit at hundredth place. In the forth
and last pass, the numbers are sorted according to the digit
at thousandth place.
During each pass, each number is taken in the order in which
it appears in partitions from unit's place onwards. When these
actions have been performed for each digit, starting with the
least significant and ending with most significant, the
numbers are sorted. This sorting method is called the radix
sort.
Let us take another example. Suppose we have a list of names.
To sort these names using radix sort method we will have to
classify them into 26 groups The list is first sorted on the
first letter of each name, i.e. the names are arranged in 26
classes, where the first class consists of those names that
begin with alphabet 'A', the second class consists of those
names that begin with alphabet 'B' and so on. During the
second pass each class is alphabetized according to the second
letter of the name, and so on.
-------------------------------------------------------------------------------------------------
84.
Exception Handling in C
Consider the following program:
#include <math.h>
void main( )
{
float i ;
i = pow ( -2, 3 ) ;
printf ( "%f", i ) ;
}
int matherr ( struct exception *a )
{
if ( a -> type == DOMAIN )
{
if ( !strcmp ( a -> name, "pow" ) )
{
a -> retval = pow ( - ( a -> arg1 ), a -> arg2 ) ;
return 1 ;
}
}
return 0 ;
}
If we pass a negative value in pow( ) function a run time
error occurs. If we wish to get the proper output even after
passing a negative value in the pow( ) function we must handle
the run time error. For this, we can define a function matherr(
) which is declared in the 'math.h' file. In this function we
can detect the run-time error and write our code to correct
the error. The elements of the exception structure receives
the function name and arguments of the function causing the
exception.
Data Structures
AVL Trees
For ideal searching in a binary search tree, the heights of
the left and right sub-trees of any node should be equal. But,
due to random insertions and deletions performed on a binary
search tree, it often turns out to be far from ideal. A close
approximation to an ideal binary search tree is achievable if
it can be ensured that the difference between the heights of
the left and the right sub trees of any node in the tree is at
most one. A binary search tree in which the difference of
heights of the right and left sub-trees of any node is less
than or equal to one is known as an AVL tree. AVL tree is also
called as Balanced Tree. The name "AVL Tree" is derived from
the names of its inventors who are Adelson-Veilskii and Landi.
A node in an AVL tree have a new field to store the "balance
factor" of a node which denotes the difference of height
between the left and the right sub-trees of the tree rooted at
that node. And it can assume one of the
three possible values {-1,0,1}.
-------------------------------------------------------------------------------------------------
85.
Unique combinations for a given number
How do I write a program which can generate all possible
combinations of numbers from 1 to one less than the given
number ?
main( )
{
long steps, fval, bstp, cnt1 ;
int num, unit, box[2][13], cnt2, cnt3, cnt4 ;
printf ( "Enter Number " ) ;
scanf ( "%d", &num ) ;
num = num < 1 ? 1 : num > 12 ? 12 : num ;
for ( steps = 1, cnt1 = 2 ; cnt1 <= num ; steps *= cnt1++ ) ;
for ( cnt1 = 1 ; cnt1 <= steps ; cnt1++ )
{
for ( cnt2 = 1 ; cnt2 <= num ; cnt2++ )
box[0][cnt2] = cnt2 ;
for ( fval = steps, bstp = cnt1, cnt2 = 1 ; cnt2 <= num ;
cnt2++ )
{
if ( bstp == 0 )
{
cnt4=num ;
while ( box[0][cnt4] == 0 )
cnt4-- ;
}
else
{
fval /= num - cnt2 + 1 ;
unit = ( bstp + fval - 1 ) / fval ;
bstp %= fval ;
for ( cnt4 = 0, cnt3 = 1 ; cnt3 <= unit ; cnt3++ )
while ( box[0][++cnt4] == 0 ) ;
}
box[1][cnt2] = box[0][cnt4] ;
box[0][cnt4] = 0 ;
}
printf ( "\nSeq.No.%ld:", cnt1 ) ;
for ( cnt2 = 1 ; cnt2 <= num ; cnt2++ )
printf ( " %d", box[1][cnt2] ) ;
}
}
This program computes the total number of steps. But instead
of entering into the loop of the first and last combination to
be generated it uses a loop of 1 to number of combinations.
For example, in case of input being 5 the number of possible
combinations would be factorial 5, i.e. 120. The program
suffers from the limitation that it cannot generate
combinations for input beyond 12 since a long int cannot
handle the resulting combinations.
Data Structures
Hashing...
Hashing or hash addressing is a searching technique. Usually,
search of an element is carried out via a sequence of
comparisons. Hashing differs from this as it is independent of
the number of elements n in the collection of data. Here, the
address or location of an element is obtained by computing
some arithmetic function. Hashing is usually used in file
management. The general idea is of using the key to determine
the address of a record. For this, a function fun( ) is
applied to each key, called the hash function. Some of the
popular hash functions are: 'Division' method, 'Midsquare'
method, and 'Folding' method. Two records cannot occupy the
same position. Such a situation is called a hash collision or
a hash clash. There are two basic methods of dealing with a
hash clash. The first technique, called rehashing, involves
using secondary hash function on the hash key of the item. The
rehash function is applied successively until an empty
position is found where the item can be inserted. If the hash
position of the item is found to be occupied during a search,
the rehash function is again used to locate the item. The
second technique, called chaining, builds a linked list of all
items whose keys hash to the same values. During search, this
short linked list is traversed sequentially for the desired
key. This technique involves adding an extra link field to
each table position.
-------------------------------------------------------------------------------------------------
86.
The following program demonstrates how to get input from the
user in graphics mode, echoed in the current colors and font
size and font style.
#define ON 1
#define OFF 0
#include <graphics.h>
main( )
{
char nameString[80], ageString[80] ;
int age, gd = DETECT, gm ;
initgraph ( &gd, &gm, "c:\\tc\\bgi" ) ;
setbkcolor ( BLUE ) ;
setcolor ( YELLOW ) ;
settextstyle ( GOTHIC_FONT, HORIZ_DIR, 0 ) ;
moveto ( 0, 0 ) ;
outtext ( "Enter your name: " ) ;
getGrString ( nameString ) ;
moveto ( 0, gety( ) + textheight ( "A" ) ) ;
outtext ( "Name: " ) ;
outtext ( nameString ) ;
moveto ( 0, gety( ) + textheight ( "A" ) ) ;
outtext ( "Press key to exit! " ) ;
getch( ) ;
closegraph( ) ;
restorecrtmode( ) ;
}
getGrString ( char *inputString )
{
int stringIndex = 0, oldColor ;
char ch, outString[2] ;
/* xVal will store the screen position for each char */
int xVal[255] ;
outString[1] = 0 ;
xVal[0] = getx( ) ;
do
{
cursor ( ON ) ;
ch = getch( ) ;
cursor ( OFF ) ;
if ( ch == 0 ) /* avoid dealing with all special keys */
getch( ) ;
else
{
if ( ch == 8 ) /* backspace */
{
oldColor = getcolor( ) ;
--stringIndex ;
if ( stringIndex < 0 )
stringIndex = 0 ;
/* move to ( old horz position, current vert position ) */
moveto ( xVal[stringIndex], gety( ) ) ;
setcolor ( getbkcolor( ) ) ;
outString[0] = inputString[stringIndex] ;
outtext ( outString ) ;
moveto ( xVal [stringIndex], gety( ) ) ;
setcolor ( oldColor ) ;
}
else
{
inputString[stringIndex] = ch ;
outString[0] = ch ;
outtext ( outString ) ;
++stringIndex ;
xVal[stringIndex] = getx( ) ;
}
}
} while ( ch != 13 && ch != 10 ) ;
inputString[stringIndex] = 0 ;
}
cursor ( int on )
{
int curX, oldColor ;
/* we'll use an underscore as a cursor */
char uBarStr[2] = { '_', 0 } ;
if ( !on )
{
oldColor = getcolor( ) ;
setcolor ( getbkcolor( ) ) ;
}
/* save horizontal position before drawing cursor */
curX = getx( ) ;
outtext ( uBarStr ) ;
moveto ( curX, gety( ) ) ;
/* if we changed the color to erase cursor, change it back */
if ( !on )
setcolor ( oldColor ) ;
}
The function getGrString( ) echoes graphically the user input
and stores it in a buffer, and the function cursor( ) handles
the cursor position.
System Utility
What is garbage collection?
Ans: Suppose some memory space becomes reusable because a node
is released from a linked list. Hence, we want the space to be
available for future use. One way to bring this about is to
immediately reinsert the space into the free-storage list.
However, this method may be too time-consuming for the
operating system. The operating system may periodically
collect all the deleted space onto the free-storage list. The
technique that does this collection is called Garbage
Collection. Garbage Collection usually takes place in two
steps: First the Garbage Collector runs through all lists,
tagging whose cells are currently in use, and then it runs
through the memory, collecting all untagged space onto the
free-storage list. The Garbage Collection may take place when
there is only some minimum amount of space or no space at all
left in the free-storage list, or when the CPU is idle and has
time to do the collection. Generally speaking, the Garbage
Collection is invisible to the programmer.
-------------------------------------------------------------------------------------------------
87.
How do I get the time elapsed between two function calls ?
Ans: The function difftime( ) finds the difference between two
times. It calculates the elapsed time in seconds and returns
the difference between two times as a double value.
#include <time.h>
#include <stdio.h>
#include <dos.h>
main( )
{
int a[] = { 2, -34, 56, 78, 112, 33, -7, 11, 45, 29, 6 } ;
int s ;
time_t t1, t2 ; // time_t defines the value used for time
function
s = sizeof ( a ) / 2 ;
t1 = time ( NULL ) ;
sel_sort ( a, s ) ; // sort array by selection sort
bub_sort ( a, s ) ; // sort array by bubble sort method
t2 = time ( NULL ) ;
printf ( "\nThe difference between two function calls is %f",
difftime (
t2, t1 ) ) ;
}
In the above program we have called difftime( ) function that
returns the time elapsed from t1 to t2.
-------------------------------------------------------------------------------------------------
88.
General
main( )
{
char *s ;
s = fun ( 128, 2 ) ;
printf ( "\n%s", s ) ;
}
fun ( unsigned int num, int base )
{
static char buff[33] ;
char *ptr ;
ptr = &buff [ sizeof ( buff ) - 1 ] ;
*ptr = '\0' ;
do
{
*--ptr = "0123456789abcdef"[ num % base ] ;
num /= base ;
} while ( num != 0 ) ;
return ptr ;
}
The above program would convert the number 128 to the base 2.
You can convert a number to a hexadecimal or octal form by
passing the number and the base, to the function fun( ).
Data Structures
What is a priority queue?
Ans: As we know in a stack, the latest element is deleted and
in a queue the oldest element is deleted. It may be required
to delete an element with the highest priority in the given
set of values and not only the oldest or the newest one. A
data structure that supports efficient insertions of a new
element and deletions of elements with the highest priority is
known as priority queue. There are two types of priority
queues: an ascending priority queue is a collection of items
into which items can be inserted arbitrarily and from which
only the smallest item can be removed. A descending order
priority queue is similar but allows only the largest item to
be deleted.
-------------------------------------------------------------------------------------------------
89.
What is the difference between const char *p, char const *p,
and char* const p ?
'const char *p' and 'char const *p' are the same, i.e. p
points to a constant character. On the other hand, 'char*
const p' means p is a constant pointer pointing to a character
which means we cannot change the pointer p but we can change
the character which p is pointing to.
-------------------------------------------------------------------------------------------------
90.
What's the difference between a null pointer, a NULL macro,
the ASCII NUL character and a null string?
Ans: A null pointer is a pointer which doesn't point anywhere.
A NULL macro is used to represent the null pointer in source
code. It has a value 0 associated with it. The ASCII NUL
character has all its bits as 0 but doesn't have any
relationship with the null pointer. The null string is just
another name for an empty string "".
System Utility
Sparse Matrix...
A sparse matrix is one where most of its elements are zero.
There is no precise definition as to know whether a matrix is
sparsed or not, but it is a concept which we all can recognize
intuitively. The natural method of representing matrices in
memory as two-dimensional arrays may not be suitable for
sparse matrices. That is one may save space by storing only
those entries which may be nonzero. If this is done, then the
matrix may be thought of as an ordered list of non-zero
elements only. Information about a non-zero element has three
parts:
an integer representing its row,
an integer representing its column and
the data associated with this element.
That is, each element of a matrix is uniquely characterized by
its row and column position, say i, j. We might store that
matrix as a list of 3-tuples of the form (i, j, data), as
shown below,
Although the non-zero elements may be stored in the array in
any order, keeping them ordered in some fashion may be
advantageous for further processing. Note that above array is
arranged in increasing order of the row number of non-zero
elements. Moreover, for elements in the same row number, the
array is arranged in order of increasing column number.
-------------------------------------------------------------------------------------------------
91.
Pointers
What does the error "Null Pointer Assignment" mean and what
causes this error?
Ans: The Null Pointer Assignment error is generated only in
small and medium memory models. This error occurs in programs
which attempt to change the bottom of the data segment. In
Borland's C or C++ compilers, Borland places four zero bytes
at the bottom of the data segment, followed by the Borland
copyright notice "Borland C++ - Copyright 1991 Borland Intl.".
In the small and medium memory models, a null pointer points
to DS:0000. Thus assigning a value to the memory referenced by
this pointer will overwrite the first zero byte in the data
segment. At program termination, the four zeros and the
copyright banner are checked. If either has been modified,
then the Null Pointer Assignment error is generated. Note that
the pointer may not truly be null, but may be a wild pointer
that references these key areas in the data segment.
Data Structures
How to build an expression trees ?
Ans: An expression tree is a binary tree which is built from
simple operands and operators of an (arithmetic or logical )
expression by placing simple operands as the leaves of a
binary tree and the operators as the interior nodes. If an
operator is binary , then it has two nonempty subtrees, that
are its left and right operands (either simple operands or sub
expressions). If an operator is unary, then only one of its
subtrees is nonempty, the one on the left or right according
as the operator is written on the right or left of its
operand. We traditionally write some unary operators to the
left of their operands, such as "-" ( unary negation) or the
standard functions like log( ), sin( ) etc. Others are written
on the right, such as the factorial function ()!. If the
operator is written on the left, then in the expression tree
we take its left subtree as empty. If it appears on the right,
then its right subtree will be empty. An example of an
expression tree is shown below for the expression ( -a < b )
or ( c + d ) .
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92.
Can we get the remainder of a floating point division ?
Ans : Yes. Although the % operator fails to work on float
numbers we can still get the remainder of floating point
division by using a function fmod( ). The fmod( ) function
divides the two float numbers passed to it as parameters and
returns the remainder as a floating-point value. Following
program shows fmod( ) function at work.
#include <math.h>
main( )
{
printf ( "%f", fmod ( 5.15, 3.0 ) ) ;
}
The above code snippet would give the output as 2.150000.
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93.
How to extract the integer part and a fractional part of a
floating point number?
Ans: C function modf( ) can be used to get the integer and
fractional part of a floating point.
#include "math.h"
main( )
{
double val, i, f ;
val = 5.15 ;
f = modf ( val, &i ) ;
printf ( "\nFor the value %f integer part = %f and fractional
part = %f",
val, i, f ) ;
}
The output of the above program will be:
For the value 5.150000 integer part = 5.000000 and fractional
part =
0.150000
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94.
How do I define a pointer to a function which returns a char
pointer?
Ans:
char * ( *p )( ) ;
or
typedef char * ( * ptrtofun )( ) ;
ptrtofun p ;
Here is a sample program which uses this definition.
main( )
{
typedef char * ( * ptrtofun ) ( ) ;
char * fun( ) ;
ptrtofun fptr ;
char *cptr ;
fptr = fun ;
cptr = (*fptr) ( ) ;
printf ( "\nReturned string is \"%s\"", cptr ) ;
}
char * fun( )
{
static char s[ ] = "Hello!" ;
printf ( "\n%s", s ) ;
return s ;
}
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95.
What's wrong with the following declaration: char* ptr1, ptr2
; get errors when I try to use ptr2 as a pointer.
Ans: char * applies only to ptr1 and not to ptr2. Hence ptr1
is getting declared as a char pointer, whereas, ptr2 is being
declared merely as a char. This can be rectified in two ways :
char *ptr1, *ptr2 ;
typedef char* CHARPTR ; CHARPTR ptr1, ptr2 ;
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96.
How to use scanf( ) to read the date in the form of dd-mm-yy?
Ans: To read the date in the form of dd-mm-yy one possible way
is,
int dd, mm, yy ;
char ch ; /* for char '-' */
printf ( "\nEnter the date in the form of dd-mm-yy : " ) ;
scanf( "%d%c%d%c%d", &dd, &ch, &mm, &ch, &yy ) ;
Another way is to use suppression character * is as follows:
int dd, mm, yy ;
scanf( "%d%*c%d%*c%d", &dd, &mm, &yy ) ;
The suppression character '*' suppresses the input read from
the standard input buffer for the assigned control character.
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97.
Why the output of sizeof ( 'a' ) is 2 and not 1 ?
Ans: Character constants in C are of type int, hence sizeof (
'a' ) is equivalent to sizeof ( int ), i.e. 2. Hence the
output comes out to be 2 bytes.
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98.
Can we use scanf( ) function to scan a multiple words string
through keyboard?
Ans: Yes. Although we usually use scanf( ) function to receive
a single word string and gets( ) to receive a multi-word
string from keyboard we can also use scanf( ) function for
scanning a multi-word string from keyboard. Following program
shows how to achieve this.
main( )
{
char buff[15] ;
scanf ( "%[^\n]s", buff ) ;
puts ( buff ) ;
}
In the scanf( ) function we can specify the delimiter in
brackets after the ^ character. We have specified '\n' as the
delimiter. Hence scanf( ) terminates only when the user hits
Enter key.
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99.
How to set the system date through a C program ?
Ans: We can set the system date using the setdate( ) function
as shown in the following program. The function assigns the
current time to a
structure date.
#include "stdio.h"
#include "dos.h"
main( )
{
struct date new_date ;
new_date.da_mon = 10 ;
new_date.da_day = 14 ;
new_date.da_year = 1993 ;
setdate ( &new_date ) ;
}
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100.
How can I write a general-purpose swap without using
templates?
Ans: Given below is the program which uses the stringizing
preprocessor directive ## for building a general purpose swap
macro which can swap two integers, two floats, two chars, etc.
#define swap( a, b, t ) ( g ## t = ( a ), ( a ) = ( b ), ( b )
= g ## t )
int gint;
char gchar;
float gfloat ;
main( )
{
int a = 10, b = 20 ;
char ch1 = 'a' , ch2 = 'b' ;
float f1 = 1.12, f2 = 3.14 ;
swap ( a, b, int ) ;
printf ( "\na = %d b = %d", a, b ) ;
swap ( ch1, ch2, char ) ;
printf ( "\nch1 = %c ch2 = %c", ch1, ch2 ) ;
swap ( f1, f2, float ) ;
printf ( "\nf1 = %4.2f f2 = %4.2f", f1, f2 ) ;
}
swap ( a, b, int ) would expand to,
( gint = ( a ), ( a ) = ( b ), ( b ) = gint )
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101.
What is a heap ?
Ans : Heap is a chunk of memory. When in a program memory is
allocated dynamically, the C run-time library gets the memory
from a collection of unused memory called the heap. The heap
resides in a program's data segment. Therefore, the amount of
heap space available to the program is fixed, and can vary
from one program to another.
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102.
How to obtain a path of the given file?
Ans: The function searchpath( ) searches for the specified
file in the subdirectories of the current path. Following
program shows how to make use of the searchpath( ) function.
#include "dir.h"
void main ( int argc, char *argv[] )
{
char *path ;
if ( path = searchpath ( argv[ 1 ] ) )
printf ( "Pathname : %s\n", path ) ;
else
printf ( "File not found\n" ) ;
}
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103.
Can we get the process identification number of the current
program?
Ans: Yes! The macro getpid( ) gives us the process
identification number of the program currently running. The
process id. uniquely identifies a program. Under DOS, the
getpid( ) returns the Program Segment Prefix as the process
id. Following program illustrates the use of this macro.
#include <stdio.h>
#include <process.h>
void main( )
{
printf ( "The process identification number of this program is
%X\n",
getpid( ) ) ;
}
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104.
How do I write a function that takes variable number of
arguments?
Ans: The following program demonstrates this.
#include <stdio.h>
#include <stdarg.h>
void main( )
{
int i = 10 ;
float f = 2.5 ;
char *str = "Hello!" ;
vfpf ( "%d %f %s\n", i, f, str ) ;
vfpf ( "%s %s", str, "Hi!" ) ;
}
void vfpf ( char *fmt, ... )
{
va_list argptr ;
va_start ( argptr, fmt ) ;
vfprintf ( stdout, fmt, argptr ) ;
va_end ( argptr ) ;
}
Here, the function vfpf( ) has called vfprintf( ) that take
variable argument lists. va_list is an array that holds
information required for the macros va_start and va_end. The
macros va_start and va_end provide a portable way to access
the variable argument lists. va_start would set up a pointer
argptr to point to the first of the variable arguments being
passed to the function. The macro va_end helps the called
function to perform a normal return.
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105.
Can we change the system date to some other date?
Ans: Yes, We can! The function stime( ) sets the system date
to the specified date. It also sets the system time. The time
and date is measured in seconds from the 00:00:00 GMT, January
1, 1970. The following program shows how to use this function.
#include <stdio.h>
#include <time.h>
void main( )
{
time_t tm ;
int d ;
tm = time ( NULL ) ;
printf ( "The System Date : %s", ctime ( &tm ) ) ;
printf ( "\nHow many days ahead you want to set the date : " )
;
scanf ( "%d", &d ) ;
tm += ( 24L * d ) * 60L * 60L ;
stime ( &tm ) ;
printf ( "\nNow the new date is : %s", ctime ( &tm ) ) ;
}
In this program we have used function ctime( ) in addition to
function stime( ). The ctime( ) function converts time value
to a 26-character long string that contains date and time.
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106.
How to use function strdup( ) in a program?
Ans : The string function strdup( ) copies the given string to
a new location. The function uses malloc( ) function to
allocate space required for the duplicated string. It takes
one argument a pointer to the string to be duplicated. The
total number of characters present in the given string plus
one bytes get allocated for the new string. As this function
uses malloc( ) to allocate memory, it is the programmer’s
responsibility to deallocate the memory using free( ).
#include <stdio.h>
#include <string.h>
#include <alloc.h>
void main( )
{
char *str1, *str2 = "double";
str1 = strdup ( str2 ) ;
printf ( "%s\n", str1 ) ;
free ( str1 ) ;
}
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107.
On including a file twice I get errors reporting redefinition
of function.
How can I avoid duplicate inclusion?
Ans: Redefinition errors can be avoided by using the following
macro definition. Include this definition in the header file.
#if !defined filename_h
#define filename_h
/* function definitions */
#endif
Replace filename_h with the actual header file name. For
example, if name of file to be included is 'goto.h' then
replace filename_h with 'goto_h'.
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108.
How to write a swap( ) function which swaps the values of the
variables using bitwise operators.
Ans: Here is the swap( ) function.
swap ( int *x, int *y )
{
*x ^= *y ;
*y ^= *x ;
*x ^= *y ;
}
The swap( ) function uses the bitwise XOR operator and does
not require any temporary variable for swapping.
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