February 9, 2022, Learn eTutorial

1995

**A number is called a prime number if it has only 2 divisors which are 1 and that number itself**. All other numbers other than prime numbers are composite numbers. For a better understanding of this prime number check C program example, we always recommend you to learn the basic topics of C programming listed below:

To check whether the number given is **prime** or **composite** using a C program, we must understand about prime number concept and composite numbers.

The **Prime number is an integer with no other positive divisors other than '1' and that number itself**. For example, '**7**' is a prime number because '**7**' has only **2** divisors that are '**1'** and '**7**' only. **Natural numbers other than the prime number are called the composite number**. For example, the number '**4**', and '**6**', which have more than two divisors are **composite** numbers.

Note: the number '**1**' is not a prime number as it has only one divisor.

Here we write a C program for finding out whether a number is **Prime** or not by using **Mod** Operator.

First, we are checking the number is greater than **1**; if not, the C program exits, displaying the number is not Prime. else we enter a '`for loop`

' from **1** to that number, in that loop we check "**if( ( num % j ) == 0)**" if it is true we prints number is not Prime. Else the given integer is prime.

**STEP 1:** Include the Header files to use the built-in functions in the C program.

**STEP 2:** Declare and Initialize the variables as an `integer`

Data type.

**STEP 3:** Accept the number from the user using `printf`

and `scanf`

built-in functions.

**STEP 4:** Check the number is less than one using an '`if`

' condition; if so, display not a Natural number and Exit the program.

**STEP 5:** Open a '`for loop`

' from **2** to half of the number and Increment the '**j**' by **one**. ( we have Divisors up to half of the number only]

**STEP 6:** Check the number Mod '**j**' is zero; if so, we Assign a Flag variable as one and Break the loop.

**STEP 7:** Check the Flag using an '`if`

' condition as if the Flag is Zero, print its a Prime number else it's a Composite number.

` ````
#include <stdio.h>
#include <stdlib.h>
void main()
{
int num, j, flag;
printf("Enter a number\n"); /* take the input number from user */
scanf("%d", & num);
if ( num <= 1)
{
printf("%d is not a prime numbers\n", num); /* check whether number is less than 1 */
exit(1);
}
flag = 0;
for ( j=2; j<= num/2; j++)
{
if( ( num % j ) == 0) /* do the calculation using mod operator and check the given number is prime or not */
{
flag = 1;
break;
}
}
if(flag == 0)
printf("%d is a prime number\n",num);
else
printf("%d is not a composite number\n", num); /* prints the output of the program */
}
```

Output RUN 1 Enter a number 34 34 is not a composite number RUN 2 Enter a number 29 29 is a prime number