C Program to count prime numbers and display them

Write a C Program to count prime numbers and display them in descending order using recursion. Here’s simple Program to count prime numbers and display them in descending order using recursion in C Programming Language.

Recursion : :

Recursion is the process of repeating items in a self-similar way. In programming languages, if a program allows you to call a function inside the same function, then it is called a recursive call of the function.

The C programming language supports recursion, i.e., a function to call itself. But while using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go into an infinite loop.

Recursive functions are very useful to solve many mathematical problems, such as calculating the factorial of a number, generating Fibonacci series, etc.

Below is the source code for C Program to count prime numbers and display them in descending order using recursion which is successfully compiled and run on Windows System to produce desired output as shown below :

SOURCE CODE : :

/* C Program to count prime numbers and display them in descending order using recursion */


#include<stdio.h>
#include<math.h>

int countPrimes(int a, int b);
int isprime(int n);

int main()
{
        int a,b;
        printf("Enter values of a and b :");
        scanf("%d %d",&a,&b);
        printf("\n\nTotal prime numbers = %d\n",countPrimes(a,b));

        return 0;
}


countPrimes(int a, int b)
{
        if(a>b)
                return 0;
        if(isprime(b))
        {
                printf("%d  ",b);
                return 1 + countPrimes(a,b-1);
        }
        else
                return countPrimes(a,b-1);
}

int isprime( int n)
{
        int i;
        for(i=2; i<=sqrt(n); i++)
                if(n%i == 0)
                    return 0;
        return 1;
}

OUTPUT : :

***************OUTPUT***************


***************FIRST RUN************

Enter values of a and b :
2
100

97  89  83  79  73  71  67  61  59  53  47  43  41  37  31  29  23  19
17  13  11  7  5  3  2

Total prime numbers = 25



***************SECOND RUN************


Enter values of a and b :
5
50

47  43  41  37  31  29  23  19  17  13  11  7  5

Total prime numbers = 13

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