C code to check whether the number is prime or not

C code to check whether the number is prime or not

In this tutorial, we will discuss the C code to check whether the number is prime or not

In this post, we are going to learn how to check whether the given number is prime or not  using 5 ways in C language

This is done using for loop, while loop, do-while loop function and recursion

 

A prime number is a number which is greater than(positive) one and divisible by only two numbers: 1 and it self. when any number is divisible by any other number it is not a prime number.

So, prime numbers can,t be divided by other numbers than itself or 1 Eg 2,3,5,7,11,13,17 ………..

Code to check whether the number is prime or not using for loop

Program 1

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int num,i,count=0;//variable declaration
    printf("Enter a positive integer\n");
    scanf("%d",&num);//Takes input from the user

    for(i=1; i<=num; ++i){
        if(num%i==0){
            count++;

        }
    }

    if(num==1){
        printf("1 is neither prime or composite");
    }
    else{
         if(count==2){
        printf("%d is a prime number",num);
    }
    else{
        printf("%d is not a prime number",num);
    }
    }
    getch();
    return 0;
}

When the above code is executed, it produces the following result

Case 1

Enter a positive integer
17
17 is a prime number

Case 2

Enter a positive integer
24
24 is not a prime number

Case 3

Enter a positive integer
1
1 is neither prime or composite

 

In this program,

• integer variable num,i  are declared.
• integer variable count=0 is declared and initialized
• The program asks input from the user
• Then the user enters the input values for num.
• The program will read the input using scanf() function and store to the variable num
• Create a for loop of i from 1 to n and increase the value of i after every iteration by 1
• Calculate mod of every value of i and check whether the result is equal to zero or not
• when the if statement returns true then the count value is increased by 1 until the test condition becomes false
• The given numbers are tested whether it is prime or not using the count value
• if the value of the count is equal to 0 then the number is prime else  the number is composite
• Then,the program is displayed the output using printf() function.

 

Code to check whether the number is prime or not using while loop

Program 2

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int num,i,count=0;
    printf("Enter a positive integer\n");
    scanf("%d",&num);
i=1;
    while(i<=num){
        if(num%i==0){
            count++;

        }
         ++i;
    }

    if(num==1){
        printf("1 is neither prime or composite");
    }
    else{
         if(count==2){
        printf("%d is a prime number",num);
    }
    else{
        printf("%d is not a prime number",num);
    }
    }
    getch();
    return 0;
}

When the above code is executed, it produces the following result

Case 1

Enter a positive integer
47
47 is a prime number

Case 2

Enter a positive integer
39
39 is not a prime number

Case 3

Enter a positive integer
1
1 is neither the a prime or composite

 

In this program,

• integer variable num,i  are declared.
• integer variable count=0 is declared and initialized
• The program asks input from the user
• Then the user enters the input values for num.
• The program will read the input using scanf() function and store to the variable num
• Create a while loop of i from 1 to n and increase the value of i after every iteration by 1
• Calculate mod of every value of i and check whether the result is equal to zero or not
• when the if statement returns true then the count value is increased by 1 until the test condition becomes false
• The given numbers are tested whether it is prime or not using the count value
• if the value of the count is equal to 0 then the number is prime else  the number is composite
• Then,the program is displayed the output using printf() function.

Code to check whether the number is prime or not using do-while loop

Program 3

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int num,i,count=0;
    printf("Enter a positive integer\n");
    scanf("%d",&num);
i=1;
   do{
        if(num%i==0){
            count++;

        }
         ++i;
    } while(i<=num);

    if(num==1){
        printf("1 is neither prime or composite");
    }
    else{
         if(count==2){
        printf("%d is a prime number",num);
    }
    else{
        printf("%d is not a prime number",num);
    }
    }
    getch();
    return 0;
}

When the above code is executed, it produces the following result

Case 1

Enter a positive integer
83
83 is a prime number

Case 2

Enter a positive integer
91
91 is not a prime number

Case 3

Enter a positive integer
1
1 is neither the a prime or composite

In this program,

• integer variable num,i  are declared.
• integer variable count=0 is declared and initialized
• The program asks input from the user
• Then the user enters the input values for num.
• The program will read the input using scanf() function and store to the variable num
• Create a do-while loop of i from 1 to n and increase the value of i after every iteration by 1
• Calculate mod of every value of i and check whether the result is equal to zero or not
• when the if statement returns true then the count value is increased by 1 until the test condition becomes false
• The given numbers are tested whether it is prime or not using the count value
• if the value of the count is equal to 0 then the number is prime else  the number is composite
• Then,the program is displayed the output using printf() function.

 

Code to check whether the number is prime or not using function

Program 4

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int num,result=0;
    printf("Enter a number\n");
    scanf("%d",&num);
    result=primeNum(num);
    if(result==0)
         printf("%d is a prime number",num);
    else
        printf("%d is not a prime number",num);
               getch();
    return 0;
}
    int primeNum(int n)
    {
        int i;
        for(i=2; i<=n/2; i++)
        {
            if(n%2!=0)
                continue;
            else
                return 1;
        }
        return 0;
    }

When the above code is executed, it produces the following result

Case 1

Enter a positive integer
29
29 is a prime number

Case 2

Enter a positive integer
50
50 is not a prime number

 

C code to check whether the number is prime or not -using recursion

Program 5

#include <stdio.h>
#include <stdlib.h>
int isPrime(int,int);//function prototype
int main()
{
    int num,result;
    printf("Enter a positive number\n");
    scanf("%d",&num);
    result=isPrime(num,num/2);
    if(result==1)
         printf("%d is a prime number",num);
    else
        printf("%d is a composite number",num);
               getch();
    return 0;
}
    int isPrime(int n,int i)
    {
            if(i==1)
                return 1;
            else{
                    if(n%i==0)
                return 0;
            else
                isPrime(n,i-1);

            }
    }

When the above code is executed, it produces the following result

Case 1

Enter a positive integer
19
19 is a prime number

Case 2

Enter a positive integer
24
24 is a composite number

 

Suggested post

for loop in C language

while loop in C language

do-while loop in C language

if statements in C language

function in C language

 

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