C Program: Detect duplicate numbers using while loop

C While Loop: Exercise-4 with Solution

Write a C program that prompts the user to input a series of numbers until they input a duplicate number. Use a while loop to check for duplicates.

Sample Solution:

C Code:

#include <stdio.h>

int main() {
    int userInput;          // Variable to store user input
    int previousNumbers[100]; // Array to store previous entered numbers
    int hasDuplicate = 0;   // Flag to track if a duplicate is found
    int index = 0;          // Index to keep track of the current position in the array

    printf("Input numbers (stop if you input a duplicate):\n");

    // Start a while loop to continue until a duplicate is entered
    while (!hasDuplicate) {
        printf("Input a number: ");
        int readStatus = scanf("%d", &userInput);  // Read the user input

        // Check if the input is a valid integer
        if (readStatus != 1) {
            printf("Invalid input. Please enter a valid integer.\n");

            // Clear the input buffer
            while (getchar() != '\n');

        // Check if the current input is equal to any of the previous numbers
        for (int i = 0; i < index; i++) {
            if (userInput == previousNumbers[i]) {
                hasDuplicate = 1;  // Set the flag to indicate a duplicate is found
                printf("Duplicate number entered. Program will stop.\n");
                break;  // Exit the loop when a duplicate is found

        previousNumbers[index++] = userInput;  // Store the current input in the array

    return 0;  // Indicate successful program execution

Sample Output:

Input numbers (stop if you input a duplicate):
Input a number: 2
Input a number: 3
Input a number: 1
Input a number: 4
Input a number: 5
Input a number: 3
Duplicate number entered. Program will stop.


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

  • #include <stdio.h>: This line includes the standard input-output library, allowing the use of functions like "printf()", "scanf()", and "getchar()".
  • int userInput;: Declares an integer variable 'userInput' to store the user's input.
  • int previousNumbers[100];: Declares an array 'previousNumbers' to store the previously entered numbers. The array size is set to 100.
  • int hasDuplicate = 0;: Initializes a flag 'hasDuplicate' to 0. This flag tracks whether a duplicate has been found.
  • int index = 0;: Initializes an index variable 'index' to keep track of the current position in the 'previousNumbers' array.
  • printf("Input numbers (stop if you input a duplicate):\n");: Prints a message prompting the user to input numbers, indicating that they should stop if they input a duplicate.
  • while (!hasDuplicate) {: Starts a "while" loop that continues until a duplicate is entered.
  • printf("Input a number: ");: Prompts the user to input a number.
  • int readStatus = scanf("%d", &userInput);: Reads the user's input as an integer and stores it in the variable userInput. The "scanf()" function returns the number of successfully read items.
  • if (readStatus != 1) { ... }: Checks if the input is not a valid integer. If true, it prints an error message, clears the input buffer, and continues to the next iteration of the loop.
  • for (int i = 0; i < index; i++) { ... }: Iterates through the previously entered numbers stored in the array previousNumbers to check for duplicates.
  • if (userInput == previousNumbers[i]) { ... }: Checks if the current input is equal to any of the previously entered numbers. If true, it sets the 'hasDuplicate' flag to 1, prints a message, and breaks out of the loop.
  • previousNumbers[index++] = userInput;: Stores the current input in the previousNumbers array and increments the index for the next iteration.
  • }: Closes the while loop.
  • return 0;: Indicates successful program execution.
  • }: Closes the "main()" function.


Flowchart: Detect duplicate numbers using while loop.

C Programming Code Editor:

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