C Program to Determine Whether a Number is Prime or Not

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

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

How to Find the Prime Number

If a number is divisible by any number from 2 to one less than the number (i.e., n-1), let's suppose that the user has supplied a number, say 37, as input. 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: A prime number is a number that cannot be divided by any number except 1 and the number itself.

Check for the prime number in C

To check whether the input number is a prime number or not in C programming, you have to ask the user to enter a number and start checking for a 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 in the Code::Blocks IDE, here is the sample after a successful build and run. This is the first snapshot of the sample run:

c program to check prime or not

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

prime number program c

Program Explained

Print all prime and composite numbers in C

Now let's create a program that will find and print all prime and composite numbers present in the range of 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 a successful build and run, here is the sample run. This is the first screenshot:

c program print prime composite number

Program Explained

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

print all prime composite number c

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

Allow the user to specify a range

The question is, "Write a program in C to print all composite and prime numbers between the 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 a sample run of the above program:

find all prime composite number c

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

print all prime composite numbers c program

The same program in different languages

C Quiz


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