C Program to Check Whether a Number is Prime Number or Not

June 17, 2023

Prime Number Program in C

Let’s look at Prime Number Program in C. A number is considered a prime number when it satisfies the below conditions. 

  • A prime number is a number which can be divided by 1 and itself
  • A number which can not be divided by any other number other than 1 or itself is a prime number.
  • It should have only 2 factors. They are, 1 and the number itself.
prime or not in c

Check Whether a Number is Prime Number or Not in C

Given an integer input, the objective is to check whether or not the input integer is a prime or not. In order to do so we check if the number is divisible by 2, if so it’s not a prime. We also divide the numbers with the input until square root of the input, if any of them divides the number perfectly, it’s not a prime.  Here are some of the methods to Check Whether a Number is Prime or Not in C

  • Method 1: Simple iterative solution
  • Method 2: Optimization by break condition
  • Method 3: Optimization by n/2 iterations
  • Method 4: Optimization by √n
  • Method 5: Optimization by skipping even iteration
  • Method 6: Basic Recursion technique

We’ll discuss the above mentioned methods in detail in the upcoming sections.

While loop in C

Method 1: Simple iterative solution

This method uses the assumption that a prime number will exactly have 2 divisors 1 and itself.

Working

For an input num

  • Initialize count = 0
  • Run an iterative for loop in iteration of (i) b/w 1 -> num
    • Check if num is divisible i
    • If divisible increment count
  • If num == 0 or num == 1 : Not Prime
  • If count > 2 : Not prime
  • Else, it is Prime

Code

Run
#include <stdio.h>  

int main()
{
    int i, count = 0;
    int num = 19;
    
    // checking number of divisors b/w 1 & num
    for(i = 1; i <= num; i++) { if(num % i == 0) count += 1; } // 0 & 1 are not prime number if(num == 0 || num == 1) printf("%d is not prime", num); //if number of divisors are > 2 then not prime else prime 
    else if(count > 2) 
        printf("%d is not prime", num);
    else
    printf("%d is prime", num);

return 0;
}
// Time complexity O(N)
// Space complexity O(1)

Code

19 is Prime

Greatest of the Three numbers

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Reverse of a number

Method 2: Optimization by break condition

This method uses the assumption that –

  • All numbers are divisible by 1 and itself
  • So it is better to check for divisors between [2,n-1]
  • If any divisor is found, then break the loop and don’t check further as it’s not prime
  • 0, 1 and all negative numbers are not prime

Working

For an input num

  • Initialize a flat called as isPrime = 1 i.e. true
  • If num < 2
    • isPrime = 0
  • Run an iterative for loop in iteration of (i) b/w 2 -> num -1
    • Check if num is divisible i
    • If divisible isPrime = 0 and break loop
  • If isPrime  == 1 : Prime
  • Else, it is Not Prime

Code

Run 
#include <stdio.h>

int main()
{
    int i,num = 21;
    int isPrime= 1;
    
    
    // 0, 1 and negative numbers are not prime
    if(num < 2){
        isPrime = 0;
    }
    else{   
        // shouldn't have any divisors in b/w 2 & num-1 
        for(i=2; i < num; i++)
        {
            if(num % i == 0)
            {
                isPrime = 0;
                break;
            }
        }
    }

    if(isPrime)
        printf("%d is Prime", num);
    else
        printf("%d is not Prime", num);
 
    return 0;
}
//Time Complexity: O(N)
//Space Complexity O(1)

// This code is better than previous code as we
// 1. Break the loop as soon as we find first divisor
// 2. Loop runs b/w [2,n-1] rather than previous [1, n]
// 3. Condition for 0, 1, neg. nos checked earlier in the code

Output

21 is not prime

Method 3: Optimization by n/2 iterations

This method uses the assumption that –

  • Checking for divisors b/w [2, n-1] is wasteful
  • As no number will have any divisor beyond num/2+1
  • Example: 36 won’t have any divisors between [17, 35]

Working

For an input num

  • Initialize a flat called as isPrime = 1 i.e. true
  • If num < 2
    • isPrime = 0
  • Run an iterative for loop in iteration of (i) b/w 2 -> num/2
    • Check if num is divisible i
    • If divisible isPrime = 0 and break loop
  • If isPrime  == 1 : Prime
  • Else, it is Not Prime

Code

Run
 include <stdio.h>

int main()
{
    int i,num = 11;
    int isPrime= 1;
    
    
    // 0, 1 and negative numbers are not prime
    if(num < 2){
        isPrime = 0;
    }
    else{   
    // no need to run loop till num-1 as for any number x the numbers in
    // the range(num/2 + 1, num) won't be divisible anyways.
        int x = num/2;
        for(i=2; i<x; i++)
        {
            if(num % i == 0)
            {
                isPrime = 0;
                break;
            }
        }
    }

    if(isPrime)
        printf("%d is Prime", num);
    else
        printf("%d is not Prime", num);
 
    return 0;
}
//Time Complexity: O(N)
//Space Complexity O(1)

// This code is better than previous code as we
// 1. Loop runs b/w [2,num/2] rather than previous [2, n-1]
// Even though time complexity is same as previous code
// But, this works 2x faster than method 2

Output

11 is Prime

Method 4: Optimization by √n

This method uses the assumption that –

  • We can simply check all divisors between [2, √num]
  • Explanation given in code itself

Working

For an input num

  • Initialize a flat called as isPrime = 1 i.e. true
  • If num < 2
    • isPrime = 0
  • Run an iterative for loop in iteration of (i) b/w 2 -> √num
    • Check if num is divisible i
    • If divisible isPrime = 0 and break loop
  • If isPrime  == 1 : Prime
  • Else, it is Not Prime

Code

Run
#include <stdio.h> 
#include <math.h>

int main()
{
    int i,num = 17;
    int isPrime= 1;
    
    
    // 0, 1 and negative numbers are not prime
    if(num < 2){
        isPrime = 0;
    }
    else{
    // Number n is not a prime, it can be factored into two factors a & b:
    // n = a * b (Excluding 1 & n)

    /*Now a & b can't be both greater than the square root of n, since
    then the product a * b would be greater than sqrt(n) * sqrt(n) = n.
    So in any factorization of n, at least one of the factors must be
    smaller than the square root of n, and if we can't find any factors
    less than or equal to the square root, n must be a prime.
    */
        for(i=2; i <= sqrt(num); i++)
        {
            if(num % i == 0)
            {
                isPrime = 0;
                break;
            }
        }
    }

    if(isPrime)
        printf("%d is Prime", num);
    else
        printf("%d is not Prime", num);
 
    return 0;
}
//Time Complexity: O(√N)
//Space Complexity O(1)

// This code is better than previous code as we
// 1. Loop runs b/w [2,√num] rather than previous [2, n/2]

Output

17 is Prime

Method 5: Optimization by skipping even iteration

This method uses the assumption that –

  • We can simply check all divisors between [2, √num]
  • 2 is the only even prime number
  • We can skip all even iterations in the loop

Code

#include <stdio.h>
#include <math.h>

int isPrime(int n){
    // 0 and 1 are not prime numbers
    // negative numbers are not prime
    if (n <= 1)
        return 0;

    // special case as 2 is the only even number that is prime
    else if (n == 2)
        return 1;

    // Check if n is a multiple of 2 thus all these won't be prime
    else if (n % 2 == 0)
        return 0;

    // If not, then just check the odds
    for (int i = 3; i <= sqrt(n); i += 2)
    {
        if (n % i == 0)
            return 0;
    }
    
    return 1;
}

int main() {
    int num=23;
    
    if (isPrime(num))
        printf("%d is prime", num);
    else
        printf("%d is not prime", num);

    return 0;
}
//Time complexity : O(√N)
//Space complexity : O(1)

//This code is better than previous code
// 1. We skipping all even iterations b/w [3, √num]
// Works almost 2x faster than method 4

Output

23 is Prime

Method 6: Basic Recursion technique

This method uses recursion, the code below is self-explanatory-

  • Initialize i =2, Start with checkPrime(num, i)
  • In each recursion call check if ‘i’ is a divisor of num
    • If i is divisor return 0: means not prime
    • else increment the value of i to check for the next possible divisor in the next recursion
  • Keep doing this until recursion from 2 to n

Code

#include <stdio.h>

int checkPrime(int n, int i)
{
    // we check if i is divisor of n or not here
    
    // 0, 1 & neg. nos are not prime
    if (n < 2)
        return 0;
    
    // if this satisfies then its prime as we
    // have completed recursion from 2 to n
    if (i == n)
        return 1;

    // Base cases
    if (n % i == 0)
        return 0;
       
    // none of above condition matches
    // keep checking for next divisor in i+1 recursion call
    i += 1;
    
    return checkPrime(n, i);
}
int main()
{
    int i=2;
    int num = 19;
    
    if(checkPrime(num, i))
        printf("%d is Prime",num);
    else
        printf("%d is Not Prime",num);

    return 0;
}
// Time complexity: O(N)
// Space complexity: O(1)
// Auxilary Space complexity: O(N)
// Due to function call stack

Output

19 is Prime

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While loop in C

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