The accumulator pattern

Using an accumulator

A common pattern in a loop is to initialize a variable outside the loop, and update it during each iteration (or some of the iterations). Here is an example you may have seen before, a static method that counts the number of times the character 'p' occurs in a string:

  public static int countPs(String s)
  {
    int total = 0;
    for (int i = 0; i < s.length(); i += 1)
    {
      // if the character is a 'p', add 1 to the total
      char c = s.charAt(i);
      if (c == 'p')
      {
        total += 1;
      }
    }
    return total;
  }

Here is a less obvious example. How would you create the reverse of a string? The idea is to build the result string one character at a time, in a loop. In pseudocode we want something like:

start with an empty result string
for each character c in the string, starting from the end
    append c to the end of the result string


  public static String reverse(String s)
  {
    String result = "";  // start with empty string
    for (int i = s.length() - 1; i >= 0; i = i - 1)
    {
      result += s.charAt(i); // add on characters one at a time
    }
    return result;
  }

Note that this is another example of the accumulator pattern, where the "accumulator" variable is a string and the operation is concatenation.