C Programming

Validation Of User Input In C

Basic Rules

• Don't pass invalid data onwards.
• Validate data at input time.
• Always give the user meaningful feedback
• Tell the user what you expect to read as input

/* example one, a simple continue statement */
#include <stdio.h>
#include <ctype.h>

main()
{
	int     valid_input;    /* when 1, data is valid and loop is exited */
	char    user_input;     /* handles user input, single character menu choice */

	valid_input = 0;
	while( valid_input == 0 ) {
		printf("Continue (Y/N)?\n");
		scanf("  %c", &user_input );
		user_input = toupper( user_input );
		if((user_input == 'Y') || (user_input == 'N') )  valid_input = 1;
		else  printf("\007Error: Invalid choice\n");
	}
}


	Sample Program Output
	Continue (Y/N)?
	b
	Error: Invalid Choice
	Continue (Y/N)?
	N

/* example two, getting and validating choices */
#include <stdio.h>
#include <ctype.h>

main()
{
	int     exit_flag = 0, valid_choice;
	char    menu_choice;
	
	while( exit_flag == 0 ) {
		valid_choice = 0;
		while( valid_choice == 0 ) {
			printf("\nC = Copy File\nE = Exit\nM = Move File\n");
			printf("Enter choice:\n");
			scanf("   %c", &menu_choice );
			if((menu_choice=='C') || (menu_choice=='E') || (menu_choice=='M'))
				valid_choice = 1;
			else
				printf("\007Error. Invalid menu choice selected.\n");
		}
		switch( menu_choice ) {
			case 'C' : ....................();    break;
			case 'E' : exit_flag = 1;  break;
			case 'M' : ....................();  break;
			default : printf("Error--- Should not occur.\n"); break;
		}
	}
}


	Sample Program Output
	C = Copy File
	E = Exit
	M = Move File
	Enter choice:
	X
	Error. Invalid menu choice selected.
	C = Copy File
	E = Exit
	M = Move File
	Enter choice:
	E

Other validation examples