C Program to Check Prime Number

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What is Prime Number?

A positive integer that is divisible only by itself and 1.

Tips: It is recommended to use our online Prime Numbers calculator for better understanding.

Examples

The following table provides few examples of prime numbers.

NumberDivisorResult
13 1, 13 Prime Number
15 1, 3, 5, 15 Not a Prime Number
47 1, 47 Prime Number

Using for loop

In the following example, we will check whether the given number (7) is a Prime number or not using for loop.

Example

C Compiler
#include <stdio.h> int main() { int num = 7; int i = 1; int count = 0; for(i=1; i<=num; i++) { if(num % i == 0) count++; } if(count == 2) printf("%d is a prime number", num); else printf("%d is not a prime number", num); return 0; }

Using while loop

In the following example, we will check whether the given number (7) is a Prime number or not using while loop.

Example

C Compiler
#include <stdio.h> int main() { int num = 7; int i = 1; int count = 0; while(num >= i) { if(num % i == 0) count++; i++; } if(count == 2) printf("%d is a prime number", num); else printf("%d is not a prime number", num); return 0; }

Using do while loop

In the following example, we will check whether the given number (7) is a Prime number or not using do while loop.

Example

C Compiler
#include <stdio.h> int main() { int num = 7; int i = 1; int count = 0; do{ if(num % i == 0) count++; i++; }while(i <= num); if(count == 2) printf("%d is a prime number", num); else printf("%d is not a prime number", num); return 0; }

Prime Numbers between the Given Range

In the following example, we will find all the Prime numbers between 1 and 20.

Example

C Compiler
#include <stdio.h> int main() { int start = 1; int end = 20; int count = 0; int i = 1; printf("Prime numbers between 1 and 20: \n"); for(start=start; start<=end; start++) { for(i=1; i<=start; i++) { if(start % i == 0) count++; } if(count == 2) printf("%d ", start); count = 0; } return 0; }

Check Whether the Given Number is Prime or Composite

In the following example, we will check whether the given number is a Prime number or Composite number.

Example

C Compiler
#include <stdio.h> int main() { int num, i, count = 0; printf("Enter a (int) number: "); scanf("%d", &num); for(i=1; i<=num; i++) { if(num % i == 0) count++; } if(count == 2) printf("%d is a prime number", num); else printf("%d is a composite number", num); return 0; }

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