The C++Course provides a general introduction to programming in C++. It is based on A.B. Downey's book, How to Think Like a Computer Scientist. Click here for details.

Looping and Counting

The following program counts the number of times the letter 'a' appears in a string:

  pstring fruit = "banana";
  int length = fruit.length();
  int count = 0;

  int index = 0;
  while (index < length) {
    if (fruit[index] == 'a') {
      count = count + 1;
    index = index + 1;
  cout << count << endl;

This program demonstrates a common idiom, called a counter. The variable count is initialized to zero and then incremented each time we find an 'a'. (To increment is to increase by one; it is the opposite of decrement, and unrelated to excrement, which is a noun.) When we exit the loop, count contains the result: the total number of a's.

As an exercise, encapsulate this code in a function named countLetters, and generalize it so that it accepts the string and the letter as arguments.

As a second exercise, rewrite this function so that instead of traversing the string, it uses the version of find we wrote in the previous section.

Last Update: 2005-12-05