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C program to check prime numbers in an array

In this C program, we are going to learn to check prime numbers in an array. We will declare an array with some prime and non prime numbers, and then print the elements with 'prime' and 'Not prime' message.
Submitted by IncludeHelp, on March 09, 2018

Problem statement

Given an array of integer elements and we have to check which prime numbers using C program are.

Example

Input:
Array elements are: 
100, 200, 31, 13, 97, 10, 20, 11

Output:
100 - Not Prime
200 - Not Prime
    31 - Prime
    13 - Prime
    97 - Prime
    10 - Not Prime
    20 - Not Prime
    11 - Prime

Checking prime numbers in an array

• We are declaring an array (arr) with the elements: 100, 200, 31, 13, 97, 10, 20, 11
• To check prime numbers, we declare a function isPrime() that will return 1, if number is prime and return 0 if number is not prime.
• Then, in main() function - we are using a loop with 0 to len-1 (total number of array elements) and calling isPrime() by passing array elements one by one (arr[loop]) – Here, loop is a loop counter.
• Then we are checking a condition with print statement: (isPrime(arr[loop])?"Prime":"Not Prime") - This will return/print "Prime", if isPrime(arr[loop]) will return 1 or this will return/print "Not Prime", if isPrime(arr[loop]) will return 0.

C program to check prime numbers in an array

#include <stdio.h>

// function to check number is prime or not
// function will return 1 if number is prime
int isPrime(int num) {
  int i;  // loop counter
  // it will be 1 when number is not prime
  int flag = 0;
  // loop to check number is prime or not
  // we will check, if number is divisible
  // by any number from 2 to num/2, then it
  // will not be prime
  for (i = 2; i < num / 2; i++) {
    if (num % i == 0) {
      flag = 1;
      break;
    }
  }
  // flag is 1, if number is not prime
  if (flag == 1)
    return 0;
  else
    return 1;
}

int main() {
  int loop;  // loop counter
  // declaring array with prime and not prime numbers
  int arr[] = {100, 200, 31, 13, 97, 10, 20, 11};
  // calculate length of the array
  int len = sizeof(arr) / sizeof(arr[0]);

  // print array elements with message
  //"prime" or "Not prime"
  for (loop = 0; loop < len; loop++) {
    printf("%3d - %s\n", arr[loop],
           (isPrime(arr[loop]) ? "Prime" : "Not Prime"));
  }

  printf("\n");

  return 0;
}

Output

100 - Not Prime
200 - Not Prime
 31 - Prime
 13 - Prime
 97 - Prime
 10 - Not Prime
 20 - Not Prime
 11 - Prime

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