stringLetter.c

#include <ctype.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include "string_letter.h"

/*
 * Function: GetLetterCount
 * ----------------------------
 *   Count appearance of each character in existing string.
 *   pString: String to be counted.
 *   returns: Array of letter counts.
 */
int* GetLetterCount(char* pString) {
    /* Student code here */

        char* string = malloc(sizeof (pString) * sizeof (int));
        int c = 0, count[26] = {0}, *pt;
        for (c = 0; pString[c] != '\0'; c++) {
            if (isupper(pString[c]) != 0)
                pString[c] = tolower(pString[c]);
        }
        for (c = 0; c < strlen(pString); c++)
              string[c] = pString[c];
        c = 0;
        while (string[c] != '\0') {
            if (string[c] >= 'a' && string[c] <= 'z')
                count[string[c] - 'a']++;
            c++;
        }
        pt = count;
        for (c = 0; c < 26; c++) {
            if (count[c] != 0)
                printf("%c : %d\n", c + 'a', count[c]);
        }
        return pt;
}

int main() {
    char input[100];
    printf("Enter a string: ");
    gets(input);
    int* result = GetLetterCount(input);
}

stringLetter.h

#ifndef STRING_LETTER_H
#define STRING_LETTER_H

int* GetLetterCount(char* string);

#endif  /* STRING_LETTER_H */