C Program to Check Prime Number or Not

In this tutorial, we will learn about how to create a program in C that will check whether the given number (by user at run-time) is a Prime number or not. We will also learn how to print all prime and composite numbers present in between any two number say 1 and 20, that is all prime and composite number between 1 to 20.

At last, we have created a program that will ask from user to enter any two number to display all the prime and composite numbers present in between the given two number. Let's first take a look at the program that will check whether the number entered by user is a prime or not.

How to Check for Prime Number ?

If number is divisible by any number from 2 to one less than the number (i.e. n-1). Let's suppose that user has supplied a number say 37 as input, then if 37 is divisible by any number from 2 to 36, then the number is not a prime number, otherwise it will be a prime number.

Note - Prime number is a number that can not be divided by any number except 1 and the number itself.

Check Prime Number or Not in C

To check whether the input number is a prime number or not in C programming, you have to ask from user to enter a number and start checking for prime number as shown in the program given below.

#include<stdio.h>
#include<conio.h>
int main()
{
    int num, i, count=0;
    printf("Enter a number: ");
    scanf("%d", &num);
    for(i=2; i<num; i++)
    {
        if(num%i == 0)
        {
            count++;
            break;
        }
    }
    if(count==0)
        printf("\nIt's a prime number");
    else
        printf("\nIt's not a prime number");
    getch();
    return 0;
}

As the above program was written under Code::Blocks IDE, therefore after successful build and run, here is the sample. This is the first snapshot of the sample run:

Supply any number say 37 and press ENTER key to check whether it is a Prime number or not. Here is the second snapshot of the sample run:

Program Explained

• Receive any number as input say 37
• Create a for loop that runs from 2 to one less than the number, that is 37-1 or 36
• Inside the for loop, check whether current value of loop variable say i divides the number say 37 (given by user at run-time) without leaving any remainder or not
• If it divides, then program flow goes inside the if block, and increment the value of any variable say count here and then use break keyword to exit from the loop. As if any number except 1 and the number itself, divides the number means the number is not a prime number
• Never forgot to initialize 0 to the count variable as its initial value at start of the program
• Now after exiting from the loop whether using break keyword or normally, check whether count variable holds its original value or not, if it holds (that is 0), it means the program flow never goes inside the if block, and it means any number from 2 to 36 does not completely divides the number, therefore the given number is a prime number
• Otherwise, if count variable holds the value other than 0, then it means that the given number is not a prime number

Print all Prime & Composite Numbers

Now let's create a program that will find and print all prime and composite numbers present in between 1 to 20:

#include<stdio.h>
#include<conio.h>
int main()
{
    int i, j, count=0;
    printf("\t\tBetween 1 to 20:\n\n");
    printf("Prime Numbers\t\tComposite Numbers\n");
    for(i=1; i<=20; i++)
    {
        for(j=2; j<i; j++)
        {
            if(i%j == 0)
            {
                count++;
		break;
            }
        }
        if(count==0)
            printf("%d", i);
        else
            printf("\t\t\t%d", i);
        count=0;
        printf("\n");
    }
    getch();
    return 0;
}

After successful build and run, here is the sample run. This is the first screenshot:

Program Explained

• Create a for loop that runs from 1 to 20
• For each and every value of loop variable say i (1, 2, 3, 4, ..., 20), check whether it is a prime or not
• Here we have create a two column, first is for prime and second is for composite numbers
• If the number is found as prime number, then print it without adding any horizontal tab (\t)
• And if the number is found as composite number, then print it after two or three horizontal tab to show it in second column
• Now come to the point, inside the for loop, we have created another for loop with another loop variable say j, here we have run the loop from 2 to one less than the outer loop variable's current value (i-1). That is, if the current value of i is 17, then check whether any number from 2 to 16 divides the number 17 totally (without leaving any remainder) or not, if it divides, then increment the count variable and use break keyword to exit from the inner for loop
• Now check whether count holds it original or initial value or not. If it holds, then print the number as prime number in first column, that is without adding horizontal tab before the number
• And if count holds any value other than 0, then print the number as composite number in second column, that is after some horizontal tab
• Never forgot to initialize 0 again to the count variable before going back to the first for loop to again check and do the same steps as told above
• In this way all the prime and composite numbers gets printed as output

Let's create the same program but this time the program given below displays prime number in one line and composite number in second line. This program will not display the prime and composite number in table form. Here is the program:

#include<stdio.h>
#include<conio.h>
int main()
{
    int i, j, count=0, arr[20], k=0;
    printf("\t\tBetween 1 to 20:\n\n");
    printf("Prime Numbers: ");
    for(i=1; i<=20; i++)
    {
        for(j=2; j<i; j++)
        {
            if(i%j == 0)
            {
                count++;
		break;
            }
        }
        if(count==0)
            printf("%d ", i);
        else
        {
            arr[k] = i;
            k++;
        }
        count=0;
    }
    printf("\nComposite Numbers: ");
    for(i=0; i<k; i++)
        printf("%d ", arr[i]);
    getch();
    return 0;
}

Here is the sample:

In the above program, we have checked whether the given number is a prime number or not as told in above steps, if it is, then print it out, otherwise initialize the current value to any array. This array will hold up its element as all the composite numbers present in between 1 and 20.

Allow User to Define Range

The question is, Write a program in C to print all composite and prime numbers between given range:

#include<stdio.h>
#include<conio.h>
int main()
{
    int i, j, count=0, arr[100], k=0, rangeStart, rangeEnd;
    printf("Enter Starting Number: ");
    scanf("%d", &rangeStart);
    printf("Enter Ending Number: ");
    scanf("%d", &rangeEnd);
    printf("\t\tBetween %d to %d:\n\n", rangeStart, rangeEnd);
    printf("Prime Numbers: ");
    for(i=rangeStart; i<=rangeEnd; i++)
    {
        for(j=2; j<i; j++)
        {
            if(i%j == 0)
            {
                count++;
		break;
            }
        }
        if(count==0)
            printf("%d ", i);
        else
        {
            arr[k] = i;
            k++;
        }
        count=0;
    }
    printf("\nComposite Numbers: ");
    for(i=0; i<k; i++)
        printf("%d ", arr[i]);
    getch();
    return 0;
}

The snapshot given below is the sample run of above program:

Now enter the range. Let's suppose, user has supplied 10 as starting number and 40 as ending number. Therefore here is the output you will see on your output screen after pressing ENTER key:

Same Program in Other Languages

• C++ Check Prime or Not
• Java Check Prime or Not
• Python Check Prime or Not

C Online Test