function that compares two characters of a string

char *compareChars(char *str) {
  char *result = (char *) malloc(sizeof(char) * strlen(str));
  int i;
  for (i = 0; i < strlen(str) - 1; i++) {
    if (str[i] < str[i + 1])
      result[i] = '<';
    if (str[i] > str[i + 1])
      result[i] = '>';
    if (str[i] == str[i + 1])
      result[i] = '=';
  }
  result[strlen(str) - 1] = '\0';
  return result;
}