# C Program to Find Prime Number Using Function

Previously we developed a C program to check whether the given number is a prime number or not? Now, we will do the same but using a function. In this post, we will write a C program to find the prime number using a function and find all prime numbers in a given range.

A natural number that has only two factors ( 1 and itself ) is called a prime number. For example- 5 is a prime number because it has only two factors 1 and 5. Similarly, 9 is not a prime number because it has more than 2 factors that are 1,3, and 9.

A function is a block of code that performs a specific task. For example, the main is a function and every program execution starts from the main function in C programming. The function is a small program that is used to do a particular task. In C a big program divided into several small subroutines/functions/procedures. Hence C is a function-oriented programming language.

The C program is made of one or more pre-defined/user-defined functions. Every program must have at least one function with the name main. The execution of the program always starts from the main function and ends with the main function. The main function can call other functions to do some special task.

## Function to check prime number in C

``````int checkPrime(int number)
{
int count = 0;
for(int i=2; i<=number/2; i++)
{
if(number%i == 0)
{
count=1;
break;
}
}
if(number == 1) count = 1;
return count;
}``````

We don’t have negative prime numbers. A positive integer is prime if it has only two divisors among the positive integers. Zero is neither positive nor negative, hence it, not a prime number. 1 is not considered prime numbers. 2 is the only even prime number.

-ve numbers:- not prime number
0:- not prime number
1:- not prime number

We know that prime number having only two factors, so if the number is divisible by any number between 2 to half of the number then the number is not a prime number.

A number will never divisible by the greater than half of the number. For example, half of the number 10 is 5 so, the number is never divisible by greater than 5 (except itself). Hence we should check only for the numbers 2 to half of the number. It will reduce the number of iteration of the loops, and the performance of the program will increase.

## C program to find prime number using a function

``````#include<stdio.h>
int checkPrime(int number)
{
int count = 0;

for(int i=2; i<=number/2; i++)
{
if(number%i == 0)
{
count=1;
break;
}
}

if(number == 1) count = 1;

return count;
}

int main()
{
int number ;

printf("Enter number: ");
scanf("%d",&number);

if(checkPrime(number) == 0)
printf("%d is a prime number.", number);
else
printf("%d is not a prime number.", number);

return 0;
}``````

Output for different test-cases:-

Enter number: 1
1 is not a prime number.

Enter number: 9
9 is not a prime number.

Enter number: 11
11 is a prime number.

## Display all prime numbers in a given range using a function

``````#include<stdio.h>
int checkPrime(int number)
{
int count = 0;

for(int i=2; i<=number/2; i++)
{
if(number%i == 0)
{
count=1;
break;
}
}

if(number == 1) count = 1;

return count;
}

int main()
{
int m, n;

printf("Enter min and max value of the range: ");
scanf("%d %d",&m, &n);

printf("Prime numbers from %d to %d are: ", m, n);
for(int i=m; i<=n; i++)
{
if(checkPrime(i) == 0)
printf("%d\t",i);
}

return 0;
}``````

Output for different test-cases:-

Enter min and max value of the range:` 1 20`
Prime numbers from 1 to 20 are: 2 3 5 7 11 13 17 19

Enter min and max value of the range: `20 50`
Prime numbers from 20 to 50 are: 23 29 31 37 41 43 47

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