Count letters

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define LETTERS_AMOUNT ('z' - 'a')
size_t *map_chars(char string[1000], size_t len);

int main(void) {
  printf("Input string: ");
  char string[1000];
  scanf("%s", string);

  size_t *letters = map_chars(string, strlen(string));
  for (size_t i = 0; i < LETTERS_AMOUNT; i++) {
    if (letters[i]) {
      printf("%c: %zu\n", i + 'a', letters[i]);
    }
  }
}

size_t *map_chars(char string[1000], size_t len) {
  size_t *letters = (size_t *)malloc(LETTERS_AMOUNT * sizeof(size_t));

  for (size_t i = 0; i < LETTERS_AMOUNT; i++) {
    letters[i] = 0;
  }

  for (size_t i = 0; i < len; i++) {
    letters[string[i] - 'a']++;
  }

  return letters;
}