C Program To Count Digit k in Number n using Recursion

Lets write a C program to count the number of occurrences of digit k in user input positive integer number n, using recursion.

For Example:

If user inputs n = 12235, and k = 2, then our C program should find how many times digit 2 is present in number 12235.

Note: (num % 10) fetches last digit in num. If num = 1234; (num%10) fetches the last digit from right, which is 4.

(num/10) removes the last number from right. If num = 1234; (num/10) removes last digit i.e., 4 and the result of (num/10) will be 123.

Related Read:
C Program To Count Digit k in Number n

Video Tutorial: C Program To Count Digit k in Number n using Recursion

Source Code: C Program To Count Digit k in Number n using Recursion

#include<stdio.h>

int occurrence(int, int);

int main()
{
    int n, k;

    printf("Enter a positive integer number\n");
    scanf("%d", &n);

    printf("Which digits occurrence you want to check for?\n");
    scanf("%d", &k);

    printf("\n%d has appeared %d times in %d.\n", k, occurrence(n, k), n);

    return 0;
}

int occurrence(int num, int k)
{
    if(num == 0)
        return 0;
    else if(k == (num%10))
        return(1 + occurrence(num/10, k));
    else
        return(occurrence(num/10, k));
}

Output:
Enter a positive integer number
12555
Which digits occurrence you want to check for?
5

5 has appeared 3 times in 12555.

Source Code: C Program To Count Digit k in Number n using Recursion and Ternary or Conditional Operator

#include<stdio.h>

int occurrence(int, int);

int main()
{
    int n, k;

    printf("Enter a positive integer number\n");
    scanf("%d", &n);

    printf("Which digits occurrence you want to check for?\n");
    scanf("%d", &k);

    printf("\n%d has appeared %d times in %d.\n", k, occurrence(n, k), n);

    return 0;
}

int occurrence(int num, int k)
{
    return( (num == 0)? 0 :
            (k == (num%10)) ?
            (1 + occurrence(num/10, k)) :
            (occurrence(num/10, k)));
}

Output:
Enter a positive integer number
155555061
Which digits occurrence you want to check for?
5

5 has appeared 5 times in 155555061.

To know more about Ternary or Conditional Operator visit:
Ternary Operator / Conditional Operator In C.

Logic To Count Digit k in Number n using Recursion

First we write the base condition i.e., if num is 0, then our function returns 0 to the calling function. Else if num%10 is equal to the value of k, then we return (1 + occurrence(num/10, k)). That way we count or increment by 1 for each time we found a digit which is equal to k, and then we reduce the num by one digit from right by doing num/10. k value will remain same throughout the program execution.

If num%10 is not equal to k, then we simply divide the num by 10, and reduce the value of num by one digit from right.

Example:

Lets assume that user input value for n and k:
n = 1223; k = 2;

So, 2 appeared 2 times in 1223.

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