# C Program to Check if a Number is Prime or Not

This program receives an input number from user and check if the number is prime number or not. The number is divided by 2, 3 and 5 to see if remainder of the division is 0. If remainder is zero, then the number is not prime otherwise, it is prime number.

We compiled the program using Dev-C++ compiler version 4.9.9.2 installed on a Windows 7 64-bit system. You may use other compilers such as Turbo C++ 3.0 after modifying the program source code. This is necessary to avoid any error in the program.

We recommend you to learn following C programming concepts before you begin with the example program.

## Problem Definition

Prime numbers are very important topic in mathematics. It is usually easy to find the small prime numbers, but very difficult to identify a prime number, when the numbers grow larger.We want to write a program to test whether a given number is a prime number or not.

What is a Prime number ?

Any positive number greater than 1 which is divisible only by itself and number one is called a Prime number, all other numbers are Composite numbers.

For example

`63 = 3 x 21 `

The number 63 has two factors 2 and 21, therefore it is a composite number.

`73 = 73 x 1 `

The factors of number 73 are 73 itself and 1, therefore, 73 is a prime number.

How do we process the given input number?

Here are the steps to process the input number in the program.

Step 1 – Get the number, N

Step 2 – Check if the number, N is divisible by 2, 3, or 5

Step 3 – If the number, N is divisible, then the remainder = = 0

Step 4 – If number is divisible, then check = 0, check = 1

Step 5 – If check = = 1, number is a prime number

Step 6 – Print the output

Step 7 – End the program

## Program Code – Prime Number Check

`/*Program to Check if the number is Prime or not */    #include <stdio.h>    #include <stdlib.h>    main()  {        int i,check,num; int a,b,c;        /* Get the number */        printf("Enter a number:");        scanf("%d",&num);        /* check if the number is divisible by 2,3 or 5*/        for( i=1; i<=5;i++)      {            if(num % i == 0)          {                check = 0;            }          else          {                check = 1;            }        }        /*print the result */        if(check == 1)      {            for(i=0;i<35;i++)            printf("_");printf("\n\n");            printf("The Number %d is a Prime Number\n\n",num);            for(i=0;i<35;i++)            printf("_");printf("\n\n");        }      else      {            for(i=0;i<35;i++)            printf("_");printf("\n\n");            printf("The Number %d is not a Prime Number\n\n",num);            for(i=0;i<35;i++)            printf("_");printf("\n\n");        }        getch();      return 0;  }`

The for loop in the above source code continuously divides the given number to verify if remainder becomes a zero. If not then variable check is 1.

`check = 1;`

The program performs a second test in which if the value of variable check is 0, it prints “Number is not a prime number” and if the value of variable check is 1, prints “The Number is a prime number”.

## Output

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