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Increment and Decrement Operators

Incrementing and decrementing are such common operations that C++ provides special operators for them. The ++ operator adds one to the current value of an int, char or double, and -- subtracts one. Neither operator works on pstrings, and neither should be used on bools.

Technically, it is legal to increment a variable and use it in an expression at the same time. For example, you might see something like:

  cout << i++ << endl;

Looking at this, it is not clear whether the increment will take effect before or after the value is displayed. Because expressions like this tend to be confusing, I would discourage you from using them. In fact, to discourage you even more, I'm not going to tell you what the result is. If you really want to know, you can try it.

Using the increment operators, we can rewrite the letter-counter:

  int index = 0;
  while (index < length) {
    if (fruit[index] == 'a') {

It is a common error to write something like

  index = index++;             // WRONG!!

Unfortunately, this is syntactically legal, so the compiler will not warn you. The effect of this statement is to leave the value of index unchanged. This is often a difficult bug to track down.

Remember, you can write index = index +1;, or you can write index++;, but you shouldn't mix them.

Last Update: 2005-12-05