Difference between revisions of "Increment and Decrement Operators"

Latest revision as of 15:23, 8 December 2011

COMP 1010 Home > Java Fundamentals

Introduction

This section outlines alternative syntax for increasing or decreasing the value of a variable. These methods help by allowing code to be written quicker and can even increase legibility. The increment and decrement operators increase or decrease a variable by 1. The combined assignment operator can be used for values other than 1.

See page 179 in the Gaddis text: http://tinyurl.com/gaddis179

The Increment and Decrement Operators

Then increment operator increases the value of a variable by 1 (equivalent to x = x+1). The decrement operator decreases the value of an operator by 1 (equivalent to x = x-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 integers such as int and long.

Usage

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 number of keystrokes. This shorthand notation is especially useful when using loops with counters.

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

If you want to use a shortcut for increasing a variable by a value other than 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.

Postfix and Prefix Modes

The operators ++ and -- 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. Preincrement/predecrement 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. Postincrement/postdecrement means the variable is used by the expression and then modified after.

In simple expressions it doesn't make a difference if whether you use postfix or prefix mode. In more complex expressions however, 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. 

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 does not change the output when placed in the print statement.