C Program to Check Prime Number


This is a program to check prime number in C.

Prime Number: A prime number is a number divisible only by 1 or itself.
Example: 2, 3, 5, 7, 11, 13, …., etc.

C Prime Number Problem Description

This program takes a number and checks whether a given number is prime or not.

C Prime Number Problem Solution

1. Take a number as input.
2. Check if the number is divisible by any of the natural numbers starting from 2.
3. If it is, then it is not a prime number. Otherwise, it is a prime number.
4. Exit.

Prime Number C Program/Source Code

Here is the source code of the C program to check if a number is a prime number or not. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

  1. /*
  2.  * C Program to find prime numbers and
  3.  * Output the given number with a suitable message.
  4.  */
  5. #include <stdio.h>
  6. #include <stdlib.h>
  8. void main()
  9. {
  10.     int num, j, flag;
  12.     printf("Enter a number \n");
  13.     scanf("%d", &num);
  15.     if (num <= 1)
  16.     {
  17.         printf("%d is not a prime numbers \n", num);
  18.         exit(1);
  19.     }
  20.     flag = 0;
  21. // To check prime number
  22.     for (j = 2; j <= num / 2; j++)
  23.     {
  24.         if ((num % j) == 0)
  25.         {
  26.             flag = 1;
  27.             break;
  28.         }
  29.     }
  30.     if (flag == 0)
  31.         printf("%d is a prime number \n", num);
  32.      else
  33.         printf("%d is not a prime number \n", num);
  34. }
Prime Number C Program Explanation

1. Take a number as input and store it in the variable num.
2. If the number is lesser than or equal to 1, then print the output as ” It is not a prime number”.
3. Initialize the variable flag to zero.
4. Using for loop, check if the input number is divisible by any of the natural numbers starting from 2.
5. If it is, then assigns the variable flag with 1.
6. Print the output as “It is a prime number”, if the variable flag ==0.
7. Otherwise print the output as “It is not a prime number” and exit.

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Prime Number in C Program Output
Enter a number
23 is a prime number
Enter a number
15 is not a prime number

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If you wish to look at other example programs on Mathematical Functions, go to C Programming Examples on Mathematical Functions. If you wish to look at programming examples on all topics, go to C Programming Examples.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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