Prime number is a integer which have no other positive divisors other than 1 and that number itself. natural numbers other than primary number is called composite number

For example 3 is a prime number, 3 has no other positive divisors other than 1 and 3.

In this c program we are finding out a number is prime or not by using mod operator. First we are checking the number is greater than 1, if not the c program gets exit displaying number is not prime. else we enters a for loop from 1 to that number, in that loop we check if( ( num % j ) == 0) if it is true we prints number is not prime. else the given integer is prime.

#include <stdio.h> #include <stdlib.h> #include <conio.h> void main() { int num, j, flag; clrscr(); printf("Enter a number\n"); /* take th input number fro user */ scanf("%d", &num); if ( num <= 1) { printf("%d is not a prime numbers\n", num); /* check whether number is less than 1 */ exit(1); } flag = 0; for ( j=2; j<= num/2; j++) { if( ( num % j ) == 0) /* do the calculation using mod operator and check the given number is prime or not */ { flag = 1; break; } } if(flag == 0) printf("%d is a prime number\n",num); else printf("%d is not a prime number\n", num); /* prints the output of the program */ }

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**OUTPUT of C program to check number is prime or not**

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Output

RUN 1

Enter a number

34

34 is not a prime number

RUN 2

Enter a number

29

29 is a prime number

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Output

RUN 1

Enter a number

34

34 is not a prime number

RUN 2

Enter a number

29

29 is a prime number