C Programming

An introduction

A SIMPLE C PROGRAM
The following program is written in the C programming language.

#include <stdio.h>

main()

{

printf("Programming in C is easy.\n");

}

Sample Program Output

Programming in C is easy.

A NOTE ABOUT C PROGRAMS
In C, lowercase and uppercase characters are very important! All commands in C must be lowercase. The C programs starting point is identified by the word

main()

This informs the computer as to where the program actually starts. The brackets that follow the keyword main indicate that there are no arguments supplied to this program (this will be examined later on).

The two braces, { and }, signify the begin and end segments of the program. The purpose of the statement

#include <stdio.h>

is to allow the use of the printf statement to provide program output. Text to be displayed by printf() must be enclosed in double quotes. The program has only one statement

printf("Programming in C is easy.\n");

printf() is actually a function (procedure) in C that is used for printing variables and text. Where text appears in double quotes "", it is printed without modification. There are some exceptions however. This has to do with the \ and % characters. These characters are modifier's, and for the present the \ followed by the n character represents a newline character. Thus the program prints

Programming in C is easy.

and the cursor is set to the beginning of the next line. As we shall see later on, what follows the \ character will determine what is printed, ie, a tab, clear screen, clear line etc. Another important thing to remember is that all C statements are terminated by a semi-colon ;

Click here for a pascal comparison.

Summary of major points so far

program execution begins at main()

keywords are written in lower-case

statements are terminated with a semi-colon

text strings are enclosed in double quotes

C is case sensitive, use lower-case and try not to capitalise variable names

\n means position the cursor on the beginning of the next line

printf() can be used to display text to the screen

the curly braces {} define the beginning and end of a program block

DATA TYPES AND CONSTANTS
The four basic data types are

INTEGER
These are whole numbers, both positive and negative. Unsigned integers (positive values only) are supported. In addition, there are short and long integers.

The keyword used to define integers is,

int

An example of an integer value is 32. An example of declaring an integer variable called sum is,

int sum;

sum = 20;

FLOATING POINT
These are numbers which contain fractional parts, both positive and negative. The keyword used to define float variables is,

float

An example of a float value is 34.12. An example of declaring a float variable called money is,

float money;

money = 0.12;

DOUBLE
These are exponetional numbers, both positive and negative. The keyword used to define double variables is,

double

An example of a double value is 3.0E2. An example of declaring a double variable called big is,

double big;

big = 312E+7;

CHARACTER
These are single characters. The keyword used to define character variables is,

char

An example of a character value is the letter A. An example of declaring a character variable called letter is,

char letter;

letter = 'A';

Note the assignment of the character A to the variable letter is done by enclosing the value in single quotes. Remember the golden rule: Single character - Use single quotes.

Sample program illustrating each data type

#include < stdio.h >

main()

{

int sum;

float money;

char letter;

double pi;

sum = 10; /* assign integer value */

money = 2.21; /* assign float value */

letter = 'A'; /* assign character value */

pi = 2.01E6; /* assign a double value */

printf("value of sum = %d\n", sum );

printf("value of money = %f\n", money );

printf("value of letter = %c\n", letter );

printf("value of pi = %e\n", pi );

}

Sample program output

value of sum = 10

value of money = 2.210000

value of letter = A

value of pi = 2.010000e+06

PREPROCESSOR STATEMENTS
The define statement is used to make programs more readable. Consider the following examples,

#define TRUE 1 /* Don't use a semi-colon , # must be first character on line */

#define FALSE 0

#define NULL 0

#define AND &

#define OR |

#define EQUALS ==

game_over = TRUE;

while( list_pointer != NULL )

................

Note that preprocessor statements begin with a # symbol, and are NOT terminated by a semi-colon. Traditionally, preprocessor statements are listed at the beginning of the source file.

Preprocessor statements are handled by the compiler (or preprocessor) before the program is actually compiled. All # statements are processed first, and the symbols (like TRUE) which occur in the C program are replaced by their value (like 1). Once this substitution has taken place by the preprocessor, the program is then compiled.

In general, preprocessor constants are written in UPPERCASE.

Click here for more information of preprocessor statements, including macros.

Class Exercise C4
Use pre-processor statements to replace the following constants

0.312

Click here for answers

LITERAL SUBSTITUTION OF SYMBOLIC CONSTANTS USING #define
Lets now examine a few examples of using these symbolic constants in our programs. Consider the following program which defines a constant called TAX_RATE.

#include <stdio.h>

#define TAX_RATE 0.10

main()

{

float balance;

float tax;

balance = 72.10;

tax = balance * TAX_RATE;

printf("The tax on %.2f is %.2f\n", balance, tax );

}

The pre-processor first replaces all symbolic constants before the program is compiled, so after preprocessing the file (and before its compiled), it now looks like,

#include <stdio.h>

#define TAX_RATE 0.10

main()

{

float balance;

float tax;

balance = 72.10;

tax = balance * 0.10;

printf("The tax on %.2f is %.2f\n", balance, tax );

}

YOU CANNOT ASSIGN VALUES TO THE SYMBOLIC CONSTANTS
Considering the above program as an example, look at the changes we have made below. We have added a statement which tries to change the TAX_RATE to a new value.

#include <stdio.h>

#define TAX_RATE 0.10

main()

{

float balance;

float tax;

balance = 72.10;

TAX_RATE = 0.15;

tax = balance * TAX_RATE;

printf("The tax on %.2f is %.2f\n", balance, tax );

}

This is illegal. You cannot re-assign a new value to a symbolic constant.

ITS LITERAL SUBSTITUTION, SO BEWARE OF ERRORS
As shown above, the preprocessor performs literal substitution of symbolic constants. Lets modify the previous program slightly, and introduce an error to highlight a problem.

#include <stdio.h>

#define TAX_RATE 0.10;

main()

{

float balance;

float tax;

balance = 72.10;

tax = (balance * TAX_RATE )+ 10.02;

printf("The tax on %.2f is %.2f\n", balance, tax );

}

In this case, the error that has been introduced is that the #define is terminated with a semi-colon. The preprocessor performs the substitution and the offending line (which is flagged as an error by the compiler) looks like

tax = (balance * 0.10; )+ 10.02;

However, you do not see the output of the preprocessor. If you are using TURBO C, you will only see

tax = (balance * TAX_RATE )+ 10.02;

flagged as an error, and this actually looks okay (but its not! after substitution takes place).

MAKING PROGRAMS EASY TO MAINTAIN BY USING #define
The whole point of using #define in your programs is to make them easier to read and modify. Considering the above programs as examples, what changes would you need to make if the TAX_RATE was changed to 20%.

Obviously, the answer is once, where the #define statement which declares the symbolic constant and its value occurs. You would change it to read

#define TAX_RATE = 0.20

Without the use of symbolic constants, you would hard code the value 0.20 in your program, and this might occur several times (or tens of times).

This would make changes difficult, because you would need to search and replace every occurrence in the program. However, as the programs get larger, what would happen if you actually used the value 0.20 in a calculation that had nothing to do with the TAX_RATE!

SUMMARY OF #define

allow the use of symbolic constants in programs

in general, symbols are written in uppercase

are not terminated with a semi-colon

generally occur at the beginning of the file

each occurrence of the symbol is replaced by its value

makes programs readable and easy to maintain

ARITHMETIC OPERATORS
The symbols of the arithmetic operators are:-

Operation	Operator	Comment	Value of Sum before	Value of sum after
Multiply	*	sum = sum * 2;	4	8
Divide	/	sum = sum / 2;	4	2
Addition	+	sum = sum + 2;	4	6
Subtraction	-	sum = sum -2;	4	2
Increment	++	++sum;	4	5
Decrement	--	--sum;	4	3
Modulus	%	sum = sum % 3;	4	1

The following code fragment adds the variables loop and count together, leaving the result in the variable sum

sum = loop + count;

Note: If the modulus % sign is needed to be displayed as part of a text string, use two, ie %%

#include <stdio.h>

main()

{

int sum = 50;

float modulus;

modulus = sum % 10;

printf("The %% of %d by 10 is %f\n", sum, modulus);

}

Sample Program Output

The % of 50 by 10 is 0.000000

CLASS EXERCISE C5
What does the following change do to the printed output of the previous program?

printf("The %% of %d by 10 is %.2f\n", sum, modulus);

Answers

GOOD FORM
Perhaps we should say programming style or readability. The most common complaints we would have about beginning C programmers can be summarized as,

they have poor layout

their programs are hard to read

Your programs will be quicker to write and easier to debug if you get into the habit of actually formatting the layout correctly as you write it.

For instance, look at the program below

#include<stdio.h>

main()

{

int sum,loop,kettle,job;

char Whoknows;

sum=9;

loop=7;

whoKnows='A';

printf("Whoknows=%c,kettle=%d\n",whoknows,kettle);

}

It is our contention that the program is hard to read, and because of this, will be difficult to debug for errors by an inexperienced programmer. It also contains a few deliberate mistakes!

Okay then, lets rewrite the program using good form.

#include <stdio.h>

main()

{

int sum, loop, kettle = 0, job;

char whoknows;

sum = 9;

loop = 7;

whoknows = 'A';

printf( "Whoknows = %c, kettle = %d\n", whoknows, kettle );

}

We have also corrected the mistakes. The major differences are

the { and } braces directly line up underneath each other
This allows us to check ident levels and ensure that statements belong to the correct block of code. This becomes vital as programs become more complex

spaces are inserted for readability
We as humans write sentences using spaces between words. This helps our comprehension of what we read (if you dont believe me, try reading the following sentence. wishihadadollarforeverytimeimadeamistake. The insertion of spaces will also help us identify mistakes quicker.

good indentation
Indent levels (tab stops) are clearly used to block statements, here we clearly see and identify functions, and the statements which belong to each { } program body.

initialization of variables
The first example prints out the value of kettle, a variable that has no initial value. This is corrected in the second example.

KEYBOARD INPUT
There is a function in C which allows the programmer to accept input from a keyboard. The following program illustrates the use of this function,

#include <stdio.h>

main() /* program which introduces keyboard input */

{

int number;

printf("Type in a number \n");

scanf("%d", &number);

printf("The number you typed was %d\n", number);

}

Sample Program Output

Type in a number

The number you typed was 23

An integer called number is defined. A prompt to enter in a number is then printed using the statement

printf("Type in a number \n:");

The scanf routine, which accepts the response, has two arguments. The first ("%d") specifies what type of data type is expected (ie char, int, or float). List of formatters for scanf() found here.

The second argument (&number) specifies the variable into which the typed response will be placed. In this case the response will be placed into the memory location associated with the variable number.

This explains the special significance of the & character (which means the address of).

Sample program illustrating use of scanf() to read integers, characters and floats

#include < stdio.h >

main()

{

int sum;

char letter;

float money;

printf("Please enter an integer value ");

scanf("%d", &sum );

printf("Please enter a character ");

/* the leading space before the %c ignores space characters in the input */

scanf(" %c", &letter );

printf("Please enter a float variable ");

scanf("%f", &money );

printf("\nThe variables you entered were\n");

printf("value of sum = %d\n", sum );

printf("value of letter = %c\n", letter );

printf("value of money = %f\n", money );

}

Sample Program Output

Please enter an integer value

Please enter a character

Please enter a float variable

32.3

The variables you entered were

value of sum = 34

value of letter = W

value of money = 32.300000

This program illustrates several important points.

the c language provides no error checking for user input. The user is expected to enter the correct data type. For instance, if a user entered a character when an integer value was expected, the program may enter an infinite loop or abort abnormally.

its up to the programmer to validate data for correct type and range of values.

THE RELATIONAL OPERATORS
These allow the comparision of two or more variables.

Operator	Meaning
==	equal to
!=	not equal
<	less than
<=	less than or equal to
>	greater than
>=	greater than or equal to

In the next few screens, these will be used in for loops and if statements.

The operator

may be legal in Pascal, but is illegal in C.

ITERATION, FOR LOOPS
The basic format of the for statement is,

for( start condition; continue condition; re-evaulation )

program statement;

/* sample program using a for statement */

#include <stdio.h>

main() /* Program introduces the for statement, counts to ten */

{

int count;

for( count = 1; count <= 10; count = count + 1 )

printf("%d ", count );

printf("\n");

}

Sample Program Output

1 2 3 4 5 6 7 8 9 10

The program declares an integer variable count. The first part of the for statement

for( count = 1;

initialises the value of count to 1. The for loop continues whilst the condition

count <= 10;

evaluates as TRUE. As the variable count has just been initialised to 1, this condition is TRUE and so the program statement

printf("%d ", count );

is executed, which prints the value of count to the screen, followed by a space character.

Next, the remaining statement of the for is executed

count = count + 1 );

which adds one to the current value of count. Control now passes back to the conditional test,

count <= 10;

which evaluates as true, so the program statement

printf("%d ", count );

is executed. Count is incremented again, the condition re-evaluated etc, until count reaches a value of 11.

When this occurs, the conditional test

count <= 10;

evaluates as FALSE, and the for loop terminates, and program control passes to the statement

printf("\n");

which prints a newline, and then the program terminates, as there are no more statements left to execute.

/* sample program using a for statement */

#include <stdio.h>

main()

{

int n, t_number;

t_number = 0;

for( n = 1; n <= 200; n = n + 1 )

t_number = t_number + n;

printf("The 200th triangular_number is %d\n", t_number);

}

Sample Program Output

The 200th triangular_number is 20100

The above program uses a for loop to calculate the sum of the numbers from 1 to 200 inclusive (said to be the triangular number).

The following diagram shows the order of processing each part of a for

An example of using a for loop to print out characters

#include <stdio.h>

main()

{

char letter;

for( letter = 'A'; letter <= 'E'; letter = letter + 1 ) {

printf("%c ", letter);

}

Sample Program Output

A B C D E

An example of using a for loop to count numbers, using two initialisations

#include <stdio.h>

main()

{

int total, loop;

for( total = 0, loop = 1; loop <= 10; loop = loop + 1 ){

total = total + loop;

}

printf("Total = %d\n", total );

}

Sample Program Output

Total = 55

In the above example, the variable total is initialised to 0 as the first part of the for loop. The two statements,

for( total = 0, loop = 1;

are part of the initialisation. This illustrates that more than one statement is allowed, as long as they are separated by commas.

MAKING DECISIONS

SELECTION (IF STATEMENTS)
The if statements allows branching (decision making) depending upon the value or state of variables. This allows statements to be executed or skipped, depending upon decisions. The basic format is,

if( expression )

program statement;

Example;

if( students < 65 )

++student_count;

In the above example, the variable student_count is incremented by one only if the value of the integer variable students is less than 65.

The following program uses an if statement to validate the users input to be in the range 1-10.

#include <stdio.h>

main()

{

int number;

int valid = 0;

while( valid == 0 ) {

printf("Enter a number between 1 and 10 -->");

scanf("%d", &number);

/* assume number is valid */

valid = 1;

if( number < 1 ) {

printf("Number is below 1. Please re-enter\n");

valid = 0;

}

if( number > 10 ) {

printf("Number is above 10. Please re-enter\n");

valid = 0;

}

printf("The number is %d\n", number );

}

Sample Program Output

Enter a number between 1 and 10 --> -78

Number is below 1. Please re-enter

Enter a number between 1 and 10 --> 4

The number is 4

EXERCISE C10
Write a C program that allows the user to enter in 5 grades, ie, marks between 0 - 100. The program must calculate the average mark, and state the number of marks less than 65.

Answer

Consider the following program which determines whether a character entered from the keyboard is within the range A to Z.

#include <stdio.h>

main()

{

char letter;

printf("Enter a character -->");

scanf(" %c", &letter );

if( letter >= 'A' ) {

if( letter <= 'Z' )

printf("The character is within A to Z\n");

}

Sample Program Output

Enter a character --> C

The character is within A to Z

The program does not print any output if the character entered is not within the range A to Z. This can be addressed on the following pages with the if else construct.

Please note use of the leading space in the statement (before %c)

scanf(" %c", &letter );

This enables the skipping of leading TABS, Spaces, (collectively called whitespaces) and the ENTER KEY. If the leading space was not used, then the first entered character would be used, and scanf would not ignore the whitespace characters.

COMPARING float types FOR EQUALITY
Because of the way in which float types are stored, it makes it very difficult to compare float types for equality. Avoid trying to compare float variables for equality, or you may encounter unpredictable results.

switch() case:
The switch case statement is a better way of writing a program when a series of if elses occurs. The general format for this is,

switch ( expression ) {

case value1:

program statement;

......

break;

case valuen:

program statement;

.......

break;

default:

.......

break;

}

The keyword break must be included at the end of each case statement. The default clause is optional, and is executed if the cases are not met. The right brace at the end signifies the end of the case selections.

Rules for switch statements

values for 'case' must be integer or character constants

the order of the 'case' statements is unimportant

the default clause may occur first (convention places it last)

you cannot use expressions or ranges

#include <stdio.h>

main()

{

int menu, numb1, numb2, total;

printf("enter in two numbers -->");

scanf("%d %d", &numb1, &numb2 );

printf("enter in choice\n");

printf("1=addition\n");

printf("2=subtraction\n");

scanf("%d", &menu );

switch( menu ) {

case 1: total = numb1 + numb2; break;

case 2: total = numb1 - numb2; break;

default: printf("Invalid option selected\n");

}

if( menu == 1 )

printf("%d plus %d is %d\n", numb1, numb2, total );

else if( menu == 2 )

printf("%d minus %d is %d\n", numb1, numb2, total );

}

Sample Program Output

enter in two numbers --> 37 23

enter in choice

1=addition

2=subtraction

37 minus 23 is 14

The above program uses a switch statement to validate and select upon the users input choice, simulating a simple menu of choices.

EXERCISE C11
Rewrite the previous program, which accepted two numbers and an operator, using the switch case statement.

Answer

ACCEPTING SINGLE CHARACTERS FROM THE KEYBOARD

getchar
The following program illustrates this,

#include <stdio.h>

main()

{

int i;

int ch;

for( i = 1; i<= 5; ++i ) {

ch = getchar();

putchar(ch);

}

Sample Program Output

AACCddEEtt

The program reads five characters (one for each iteration of the for loop) from the keyboard. Note that getchar() gets a single character from the keyboard, and putchar() writes a single character (in this case, ch) to the console screen.

The file ctype.h provides routines for manipulating characters.