C Program Accept data from the keyboard until correct data is entered

As a good programming practice, we display a message to prompt the user before accepting data from the keyboard. This enables the user to enter the required data correctly. However, the user may still enter incorrect data. Such data may cause the programs to print incorrect results.

One way to handle this problem is to print an error message and stop the program execution when incorrect data is encountered. However, the user will usually expect the program to discard incorrect data and allow him/her to reenter correct data. We can implement code to achieve this using a do…while loop. Consider the program segment given below.

do

printf (“Enter marks (0..100): “);

scanf(“%d”, &marks);

while (marks< 0 || marks> 100);

This loop prints a message and accepts marks from the keyboard as long as the marks entered are incorrect (either less than zero or greater than l 00). The control leaves the loop only when the user enters valid marks (in the range 0 to 100). The output is shown below, where the user has entered two incorrect values (120 and -20) before entering the correct value (90).

Example:

#include <stdio.h>

void main()

{

    int marks;

    clrscr();

    do

    {

         printf (“Enter marks (0..100): “);

         scanf(“%d”, &marks);

     }  while (marks< 0 || marks> 100);

    printf (“The Marks = %d \n”,marks);

    getch();

}