C Exercises: Check a number is a prime number or not

Last update on March 20 2025 12:37:22 (UTC/GMT +8 hours)

12. Prime Check Recursion Variants

Write a program in C to check if a number is a prime number or not using recursion.

Pictorial Presentation:

Sample Solution:

C Code:

#include<stdio.h>

int checkForPrime(int);
int i;

int main()
{

    int n1,primeNo;

	printf("\n\n Recursion : Check a number is prime number or not :\n");
	printf("--------------------------------------------------------\n");
	
    printf(" Input any positive number : ");
    scanf("%d",&n1);
    
    i = n1/2;

    primeNo = checkForPrime(n1);//call the function checkForPrime

   if(primeNo==1)
        printf(" The number %d is a prime number. \n\n",n1);
   else
      printf(" The number %d is not a prime number. \n\n",n1);
   return 0;
}

int checkForPrime(int n1)
{
    if(i==1)
    {
        return 1;
    }
    else if(n1 %i==0)
    {
         return 0;
    }     
    else
       {
         i = i -1; 
         checkForPrime(n1);//calling the function checkForPrime itself recursively
      }
}

Sample Output:

 Recursion : Check a number is prime number or not :                                                          
--------------------------------------------------------                                                      
 Input any positive number : 7                                                                                
 The number 7 is a prime number.  

Explanation:

int checkForPrime(int n1)
{
    if(i==1)
    {
        return 1;
    }
    else if(n1 %i==0)
    {
         return 0;
    }     
    else
       {
         i = i -1; 
         checkForPrime(n1);//calling the function checkForPrime itself recursively
      }
}

The function 'checkForPrime()' takes an integer n1 as input and checks if the number is divisible by i (which is initially set to some value outside the function).

• If i is equal to 1, the function returns 1, indicating that the number is prime.
• If n1 is divisible by i, the function returns 0, indicating that the number is not prime.
• Otherwise, the function decrements the value of i and calls itself recursively with the same value of n1.
• The function continues to call itself recursively until i reaches 1 or n1 is found to be divisible by i.

Time complexity and space complexity:

The time complexity of this function is O(n) where n is the input number since the function checks all the numbers from 2 to n-1 to determine if the number is prime or not.

The space complexity is O(1) since the function uses only constant space for storing the value of i.

Flowchart:

For more Practice: Solve these Related Problems:

• Write a C program to check if a number is composite using recursion.
• Write a C program to list all prime numbers within a range using recursion.
• Write a C program to find the next prime number after a given integer using recursion.
• Write a C program to check for primality recursively while caching previous results.

C Programming Code Editor:

Click to Open Editor

Previous: Write a program in C to convert a decimal number to binary using recursion.
Next: Write a program in C to find the LCM of two numbers using recursion.