Input Validation using Loops in C

In C, input validation ensures that user input meets the required constraints, such as valid numerical ranges or specific formats. We can use loops like while and do-while to repeatedly prompt the user until valid input is provided. This prevents incorrect values from causing unexpected behavior in a program.

Examples of Input Validation Using Loops

1. Validating a Positive Integer Input

In this example, we prompt the user to enter a positive integer. If the user enters a negative number or zero, we keep prompting until they enter a valid positive integer.

main.c

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#include <stdio.h>

int main() {
    int num;

    // Loop to validate positive integer input
    do {
        printf("Enter a positive integer: ");
        scanf("%d", &num);

        if (num <= 0) {
            printf("Invalid input! Please enter a positive integer.\n");
        }
    } while (num <= 0);

    printf("You entered: %d\n", num);
    return 0;
}

Explanation:

  1. We declare an integer variable num to store the user input.
  2. The do-while loop ensures the user is prompted at least once.
  3. Inside the loop, we use scanf("%d", &num) to read an integer input.
  4. If the input is less than or equal to 0, we display an error message.
  5. The loop continues until a valid positive integer is entered.
  6. Once a valid number is provided, we print it using printf().

Output:

Enter a positive integer: -5
Invalid input! Please enter a positive integer.
Enter a positive integer: 0
Invalid input! Please enter a positive integer.
Enter a positive integer: 7
You entered: 7

2. Validating an Integer Within a Range

In this example, we ensure the user enters an integer within a specific range, say 1 to 100. If the input is outside this range, we prompt them again.

main.c

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#include <stdio.h>

int main() {
    int num;

    // Loop to validate input within range 1 to 100
    do {
        printf("Enter a number between 1 and 100: ");
        scanf("%d", &num);

        if (num < 1 || num > 100) {
            printf("Invalid input! Please enter a number between 1 and 100.\n");
        }
    } while (num < 1 || num > 100);

    printf("Valid input received: %d\n", num);
    return 0;
}

Explanation:

  1. We declare an integer variable num to store user input.
  2. The do-while loop runs at least once to prompt the user.
  3. We use scanf("%d", &num) to get an integer input.
  4. If the number is not in the range 1 to 100, we display an error message.
  5. The loop continues until a valid number in the specified range is entered.
  6. Once a valid number is provided, we display it using printf().

Output:

Enter a number between 1 and 100: 150
Invalid input! Please enter a number between 1 and 100.
Enter a number between 1 and 100: 0
Invalid input! Please enter a number between 1 and 100.
Enter a number between 1 and 100: 42
Valid input received: 42

3. Validating Character Input (Yes or No)

In this example, we validate character input to accept only Y or N (case insensitive). If the input is invalid, the user is prompted again.

main.c

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#include <stdio.h>
#include <ctype.h>

int main() {
    char choice;

    // Loop to validate character input (Y/N)
    do {
        printf("Enter Y or N: ");
        scanf(" %c", &choice);

        choice = toupper(choice); // Convert to uppercase for case insensitivity

        if (choice != 'Y' && choice != 'N') {
            printf("Invalid input! Please enter Y or N.\n");
        }
    } while (choice != 'Y' && choice != 'N');

    printf("You selected: %c\n", choice);
    return 0;
}

Explanation:

  1. We declare a char variable choice to store user input.
  2. The do-while loop ensures the prompt appears at least once.
  3. We use scanf(" %c", &choice) to read a character input.
  4. The function toupper(choice) converts lowercase letters to uppercase.
  5. If the input is not 'Y' or 'N', an error message is displayed.
  6. The loop repeats until a valid character is entered.

Output:

Enter Y or N: x
Invalid input! Please enter Y or N.
Enter Y or N: Y
You selected: Y

Conclusion

We explored different ways to implement input validation in C using loops:

  1. Ensuring a positive integer input.
  2. Validating input within a specific range.
  3. Restricting character input to specific values.

Using loops for input validation enhances the reliability of programs by preventing incorrect data entry.