Example: Character Frequency - Recursive in C

// Character Frequency - Recursive
#include <stdio.h>

#define MAX_CHARS 256 // Assuming ASCII characters

void calculateFrequencyRecursive(const char *str, int *freq) {
    // Base case: If the current character is the null terminator, stop recursion
    if (*str == '\0') {
        return;
    }

    // Recursive step: Increment frequency for the current character
    // Type casting *str to unsigned char ensures it's treated as a positive index
    freq[(unsigned char)*str]++;

    // Call the function for the rest of the string (str + 1)
    calculateFrequencyRecursive(str + 1, freq);
}

int main() {
    char inputString[] = "Hello Recursion!";
    int frequency[MAX_CHARS];

    // Initialize frequency array to all zeros before the first recursive call
    for (int i = 0; i < MAX_CHARS; i++) {
        frequency[i] = 0;
    }

    calculateFrequencyRecursive(inputString, frequency);

    printf("Character frequencies for \"%s\":\n", inputString);
    for (int i = 0; i < MAX_CHARS; i++) {
        if (frequency[i] > 0) {
            printf("Character '%c': %d\n", (char)i, frequency[i]);
        }
    }

    return 0;
}