Counting occurences of letters in a string

#include <iostream>
using namespace std;

// Function to increment the letter count in the counters array
char add_letter( char letter, int counters[] ) {
  // Only deal with lower case letters
  char lower_case_letter = tolower(letter);

  // Check if character is a letter
  if ( lower_case_letter >= 'a' && lower_case_letter <= 'z' ) {
    ++counters[ lower_case_letter - 'a' ];
  }

  return letter;
}

int main() {
  // One integer for each  letter
  int counters[26] = { 0 };

  // Counting the number of occurences of each letter in this sentence
  char *sentence = "the quick brown fox jumps over the lazy dog";

  // Pass each character to the add_letter
  // function until null-terminator is reached
  while ( add_letter(*sentence++, counters) );

  // Display results
  for (int i = 0; i < 26; ++i) {
    if ( counters[i] ) {
      cout << char(i + 'a') << ": " << counters[i] << '\n';
    }
  }

  cin.ignore();
}