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C program to delete prime numbers from an array

In this C program, we are going to learn how can we check and delete prime numbers from an array? We are declaring an array with some prime and non prime numbers and deleting prime numbers and then printing array elements with the numbers which are not prime.
Submitted by IncludeHelp, on March 09, 2018

Problem statement

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

Example

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

Output:
Array elements after removing prime numbers: 
100
200
    10
    20

Deleting prime numbers from 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[i]) – Here, i is a loop counter.
• Then, we are deleting the prime numbers, by checking elements using condition if(isPrime(arr[i])), if the condition is true, we are shifting other right side elements by one position left.
• Then, we are decreasing loop counter (i) by 1 to check the shifted element at the same place where prime number was exist.
• Then, we are decreasing the array size (len) by 1.
• Finally, we are printing the array elements – which are not primes.

C program to delete prime numbers from 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 i, j;  // loop counters
  // 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]);

  // delete prime numbers
  for (i = 0; i < len; i++) {
    if (isPrime(arr[i])) {
      // number is prime, then shift other
      // elements to the left
      for (j = i; j < len; j++) {
        arr[j] = arr[j + 1];
      }
      // decrease loop counter by 1,
      // to check shifted element
      i--;
      // decrease the length
      len--;
    }
  }

  // print elements after removing prime numbers
  printf("Array elements after removing prime numbers:\n");
  for (i = 0; i < len; i++) printf("%3d\n", arr[i]);

  printf("\n");

  return 0;
}

Output

Array elements after removing prime numbers: 
100
200
 10
 20

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