Check if a Number is Prime in C – Function Program with Return Value

Introduction

Determining whether a number is prime is a common problem in computer science. In this article, you will learn how to write a function in C to check if a number is prime. The function will return 1 if the number is prime, otherwise 0.

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C Program to Check If a Number is Prime Using Function

#include <stdio.h>

int isPrime(int num) {
    if (num <= 1)
        return 0;
    
    for (int i = 2; i <= num / 2; i++) {
        if (num % i == 0)
            return 0;
    }
    return 1;
}

int main() {
    int number;

    printf("Enter a number: ");
    scanf("%d", &number);

    if (isPrime(number))
        printf("%d is a prime number.\n", number);
    else
        printf("%d is not a prime number.\n", number);

    return 0;
}

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Explanation

• The function isPrime() checks if a number is divisible by any number from 2 to num / 2.
• If any such divisor exists, the number is not prime (return 0).
• If no divisor is found, the number is prime (return 1).
• The main() function takes user input and prints the result using the return value from the function.

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Sample Output

Enter a number: 7  
7 is a prime number.

Enter a number: 12  
12 is not a prime number.

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Use Cases

• Checking prime numbers for cryptography, number theory, or competitive programming.
• Helps in understanding functions, conditionals, and loops in C programming.