High-level Programming Language (Pascal)

Program Input and Output

To be useful, a program needs some means to communicate with the user. They are input statements and output statements.

Input Statements

• are used to enable the user to input data
• to get raw data prepared by the user in order to start processing
• to get responses from the user when there are more than one action which the program can take
• it is a good practice to include some kinds of prompt whenever input is required
• there are two input statements in Pascal : read and readln (read as read line)
  • read and readln are predefined input procedures
  • can read different types of data values from the keyboard or data file and assign the data to the corresponding variable(s)
  • the data type to be read must be consistent with the variable(s) declared in variable declaration part (the only exception is that an integer can be assigned to a real-type variable, because an integer is a subset of real numbers)
  • general forms of the input statements

  read (<input list>)
  readln (<input list>)
  readln

  1. where <input list> is a single variable or a list of variables separated by commas
  2. only one string can be included in the input list of read or readln and it must be the last in the list
  3. during data input, data values must be separated by one or more space(s)
  4. execution of these input statements reads values from the standard file input and assigns them to the variables in the <input list>
  5. whenever executes a readln statement it pauses and waits for the required data
  6. after values have been read for all the variables, readln causes an advance to a new input line from which subsequent values will be read
  7. whereas read causes no such advance
  8. the last form of readln with no <input list> terminates input from the current line, so that subsequent input begins on a new line (i.e. program execution is suspended, providing an opportunity for the user to examine the output)
     
     
• type of argument of the input statements
  
  

  Type of Argument	Action of read on the argument
  char	Assign the next input character to its argument
  integer	Skip any blank, tab or carriage return, then read an integer that must end with a blank, tab or carriage return. The ending character is not read.
  real	Same as integer but a real number is read.
  string	Read all characters up to, but not including the carriage return.

Output Statements

• provides a method for easily displaying information
• to output messages or results of a program to the user
• there are two output statements in Pascal : write and writeln (read as write line)
  • write and writeln are predefined output procedures
  • general forms of the output statements

    write (<output list>)
    writeln (<output list>)
    writeln

1. where <output list> is a single expression or a list of expressions separated by commas and each of these expressions may be a constant, a variable or a formula
2. writeln statement will display program results and move the cursor to the beginning of next line
3. write statements will display program results without moving the cursor to the beginning of next line
4. the last form of writeln with no output list will cause the cursor to advance to a new line to display a blank line
   
   
• a list of examples
  
  

  Action	Statement	Output
  		12345678901234567
  Printing a number 　Positive integer 　Negative integer 　Real number	writeln (123); writeln (-123); writeln (123.0);	123 -123  1.230000000E+02
  Printing a character	writeln ('A');	A
  Printing a string	writeln ('Hello');	Hello
  Printing a list of data items	writeln ('A', 'B', 'C'); writeln ('Hello', 'John'); writeln (1, '+', 2, 'is ', 3);	ABC HelloJohn 1+2 is 3
  Printing an expression	writeln ('Sum = ', 2 + 6);	Sum = 8
  Printing Boolean	writeln ('3>2 is ', 3>2);	3>2 is TRUE

  
  
• The difference between write and writeln
  
  

  Statements	Output
  	123456789012345678901234567
  writeln ('Hello'); writeln ('Welcome to Pascal Programming');	Hello Welcome to Pascal Programming
  write ('Hello'); write ('Welcome to Pascal Programming');	HelloWelcome to Pascal Programming