Increment and Decrement Operators

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Introduction Increment and decrement are arithmetic operators which quickly add or subtract one from a variable. These operators work on basic data types such as int and long. These operators can appear as an expression by themselves or as part of a complex expression. Increment and decrement can be used to add or subtract one from a variable.

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The Increment and Decrement Operators

The increment and decrement operators increase or decrease the value of a variable by 1. The increment operator is ++ and decrement is --. These operators can be used on int, long, float and double data types. They are primarily used on integer value variables such as int and long. Increasing or decreasing the value of a variable can be done by using the increment or decrement operators.

Usage

A shorter way of writing x = x + 1 is to use the increment operator to write x++. You can also decrease the value in a variable by using the decrement opertor. For example, a shorter way of writing y = y - 1 is to write y--. The below code shows how these shorthand operators work.

  x = 1;
  x++; //is a shorter way of writing x = x + 1
  x--; //is a shorter way of writing x = x - 1
  //note: at this point the value of x is 1. 

These shorthand notations do not change any logic in your programming but they do reduce the nubmer of keystrokes. Plus since they're often used in programs you'll need to be able to understand them. Also, when we start to use loops you'll find it often very helpful to use this shorter notation.

REVIEW! What will the following code output?

  int marbles= 2;
  System.out.println("You have " + marbles + " marbles.");
  marbles ++;
  System.out.println("Now you have " + marbles + " marbles.");
  marbles --;
  marbles --;
  marbles --;
  System.out.println("Now you have " + marbles + " marbles.");
  marbles --;
  System.out.println("Now you have " + marbles + " marbles.");

REVIEW OUTPUT

  You have 2 marbles.
  Now you have 3 marbles.
  Now you have 0 marbles.
  Now you have -1 marbles.

NOTE: The expression marbles++ is pronounced "marbles plus plus" and marbles-- is pronounced "marbles minus minus". Does it make sense to you that the programming language C++ ("C Plus Plus") is based on the programming language C?

Combined Assignment Operators

Now what if you want to use a shortcut for increasing a variable but you don't want to increase the variable by 1? You can use a Combined Assignment Operator. For example x = x+ y can also be written as x+=y. Or a = a+ 2 can be written as a+=2. The below table shows you common combined assignment operators.

Operator Meaning x += y x = x + y x -= y x = x - y x *= y x = x * y x /= y x = x / y x %= y x = x % y

Postfix and Prefix Modes

The operators ++ and x-- update the variable at different times depending on whether they are placed before or after the variable. If the operator is placed before a variable, it is known as preincrement or predecrement. Pre - increment/decrement means the variable is increased or decreased before we use it for anything else. If the operator is placed after a variable, it is known as postincrement or postdecrement. Post - increment/decrement means the variable is used by the expression and then modified after.

In simple expressions it doesn't make a difference if you write x++ or ++x. Also, in simple expressions x-- is the same as --x.

But in more complex expressions the placement of the operator changes the result of the expression. The following code snippet demonstrates the difference between these four operators:

  int number = 5;
  int y; 

  //the next four simple expressions show how the placement of the operators sometimes works the same way
  ++number; // preincrement, the variable 'number' stores the value 6
  number++; // postincrement, the variable 'number' now stores the value 7
  --number; // predecrement, the variable 'number' stores the value 6
  number--; // postdecrement, the variable 'number' stores the value 5

  //now, watch how the value of y changes depending on the placement of the operators
  y = number++;  //the value of y is equal to the value in x BEFORE the increment takes place. So y = 5 and number = 6;
  y = ++number;  //this time the value of number is increased before the value is assigned to y. So y = 7 and number = 7.

Note that the regular rules for arithmetic in Java apply here. Be careful not to unintentionally increment or decrement beyond the range a variable can hold. Strange things can happen when this occurs.

Summary

This section shows ways to speed up your typing by using less characters. You can always write expressions the long way, but you may prefer to use increment, decrement or combined assignment operators to save key strokes. You also need to know these shorthand strategies so you can read and understand expressions in programs that you read.

The increment and decrement operators are used to add or subtract one from a variable. The operators can be used as part of a complex expression or as a simple expression. The most common use for the increment operator is as a counter, such as in a loop.

Review

1. What will be printed by the following code?

   int x=2;
   int y;
   x++;   
   System.out.print(x);
   x+=2;
   System.out.print(x);
   y=x++;
   System.out.print(y);
   System.out.print(x);

2. Note the code below puts the expressions x++ and x+=2 in the println statement. Does output change?

   int x=2;
   int y;   
   System.out.print(x++);
   System.out.print(x+=2);
   y=x++;
   System.out.print(y);
   System.out.print(x);

ANSWERS: 1) 3556 2) Yes, the value of x is printed first (as 2) then increased to 3. But the combined operator expression x+=2