C program to check if a number is prime

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In this example, you will learn to check if the integer entered by the user is a prime number.

To understand this example, you should understand the followingC programming languageTopic:

• C if ... else statements

• C for loop

• C break and continue statements

A prime number is a positive integer that can only be1divisible by itself. For example:2,3,5,7,11,13,17

Program to check prime numbers

#include <stdio.h>
int main() {
　　　　int n, i, flag = 0;
　　　　printf("Enter a positive integer: ");
　　　　scanf("%d", &n);
　　　　for (i =　2; i <= n　/　2;　++i) {
　　　　　　　　//　Condition for non-prime
　　　　　　　　if (n % i == 0) {
　　　　　　　　　　　　flag =　1;
　　　　　　　　　　　　break;
　　　　　　　　}
　　　　}
　　　　if (n ==　1) {
　　　　　　　　printf("1neither prime nor composite.");
　　　　}
　　　　else {
　　　　　　　　if (flag == 0)
　　　　　　　　　　　　printf("%d is a prime number.", n);
　　　　　　　　else
　　　　　　　　　　　　printf("%d is not a prime number.", n);
　　　　}
　　　　return 0;
}

Output the result

Enter a positive integer:　29
29　is a prime number.

In the program, the for loop starts from iteration i = 2to i < n/2.

In each iteration, check if n can be completely divided by i:

if (n % i == 0) {
　　　
}

If n can be divided by i, n is not a prime number. In this example, set the flag to1, and use the break statement to terminate the loop.

After the loop, if n is a prime number, the flag is still 0. However, if n is not a prime number, the flag is1.

Visit this page to learn howPrint all prime numbers between two intervals.

Comprehensive Collection of C Programming Examples