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Prime Number Program in C

By Dinesh Thakur

In this tutorial, We will check a given positive integer N. The task is to write a C program to check if the number is prime or not.

We’ll be covering the following topics in this tutorial:

  • What is Prime number in C?
  • Algorithm for Prime Number
  • Pseudocode for Prime Number
  • C Program to Check Prime Number

What is Prime number in C?

Prime number in C: A Prime number is a positive integer that is greater than one and divided only by one and itself. For example, {2, 3, 5, 7, 11, 13, 17, 19, 23} are the prime numbers.

Note: Zero (0) and 1 not considered as prime numbers. Two (2) is the only one even prime number because all the numbers can be divided by 2.

To check whether the number is prime or not iterate through each of the numbers starting from 2 to sqrt(N) with a for loop and check if it divides N. When we find any number that divides, then we return false. If we didn’t see any number between 2 and sqrt(N) which divides N, then it implies that N is prime and we’ll return True. For example

3 = 1 * 3
Some prime number are − 1, 2, 3, 5 , 7, 11 etc.

Algorithm for Prime Number

START
 Step 1 → Enter variable V
 Step 2 → Divide the Number by V-1.
 Step 3 → If V is divisible by (V-1 to 2) it is not prime
 Step 4 → Else it is prime
STOP

Pseudocode for Prime Number

procedure prime_number : number
 FOR loop = 2 to number - 1
   check if number is divisible by loop
  IF divisible
     RETURN "NOT PRIME"
  END IF
 END FOR
 RETURN "PRIME"
end procedure

C Program to Check Prime Number

//prime number program in c
#include<stdio.h>
int main() {
    int number, loop, flag = 0;
    number = 19;
    for (loop = 2; loop <= number / 2; ++loop) {
       // condition for non-prime
       if (number % loop == 0) {
           flag = 1;
           break;
       }
    }
    if (number == 1) {
        printf("1 is neither prime nor composite.");
    }
    else {
        if (flag == 0)
           printf("%d is a prime number.", number);
        else
           printf("%d is not a prime number.", number);
    }
return 0;
}
 prime number program in c

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About Dinesh Thakur
Dinesh ThakurDinesh Thakur holds an B.C.A, MCDBA, MCSD certifications. Dinesh authors the hugely popular Computer Notes blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps.

Dinesh Thakur is a Freelance Writer who helps different clients from all over the globe. Dinesh has written over 500+ blogs, 30+ eBooks, and 10000+ Posts for all types of clients.


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