C Program: User input validation with While Loop for minimum 8-Character username

Last update on March 18 2025 13:38:50 (UTC/GMT +8 hours)

7. Username Validation Using a While Loop

Write a C program that prompts the user to input a username. Use a while loop to keep asking for a username until a valid one is entered (e.g., at least 8 characters long).

Sample Solution:

C Code:

#include <stdio.h>
#include <string.h>

int main() {
    char username[20];  // Array to store the user input for the username

    // Start a while loop to keep asking for a valid username
    while (1) {
        // Prompt the user to input a username
        printf("Input a username (at least 8 characters): ");
        scanf("%s", username);  // Read the user input and store it in the array

        // Check if the entered username is at least 8 characters long
        if (strlen(username) >= 8) {
            break;  // Exit the loop if the username is valid
        } else {
            // Print an error message if the username is too short
            printf("Error: Username must be at least 8 characters long.\n");
        }

        // Clear the input buffer to handle any remaining characters
        while (getchar() != '\n');
    }

    // Print a message indicating a valid username was entered
    printf("Valid username entered: %s\n", username);

    return 0;  // Indicate successful program execution
}

Sample Output:

Input a username (at least 8 characters): Sdk34%k
Error: Username must be at least 8 characters long.
Input a username (at least 8 characters): Akdsui234#$
Valid username entered: Akdsui234#$

Explanation:

Here are key parts of the above code step by step:

#include <stdio.h>: Include the standard input-output library for functions like "printf()" and "scanf()".
#include <string.h>: Include the string manipulation library for functions like strlen.
int main() {: Start the main function.
char username[20];: Declare a character array 'username' to store the user input for the username.
while (1) {: Start an infinite while loop.
printf("Enter a username (at least 8 characters): ");: Prompt the user to enter a username.
scanf("%s", username);: Read the user input and store it in the 'username' array.
if (strlen(username) >= 8) {: Check if the entered username is at least 8 characters long using 'strlen'.
break;: Exit the loop if the username is valid.
} else { printf("Error: Username must be at least 8 characters long.\n"); }: Print an error message if the username is too short.
while (getchar() != '\n');: Clear the input buffer to handle any remaining characters.
}: Close the while loop.
printf("Valid username entered: %s\n", username);: Print a message indicating a valid username was entered.
return 0;: Indicate successful program execution.
}: Close the main function.

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to prompt the user to enter a username until it is at least 8 characters long and contains at least one digit.
Write a C program to validate a username ensuring it starts with a letter and is at least 8 characters long using a while loop.
Write a C program to prompt for a username that has no spaces and includes both uppercase and lowercase letters before accepting it.
Write a C program to accept a username and ensure it contains at least one special character and is between 8 and 16 characters long.

C Programming Code Editor:

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