The program to check prime number is written using Dev-C++ compiler installed on a Windows 7 64-bit system. You can use any other standard compiler to compile the program and it will still work.This program is intended for beginners learning C programming language. To help you understand the program, you will find following section helpful – problem definition, flowchart, program code and verified output.

### 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

### Flowchart – Program to check a Prime number

### Program Code

```
/*Program to Check if the number is Prime or not */
#include <stdio.h>
#include <conio.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;
}
```

### Output

The output of the above program is given below.