4.5. Which Loop to Use

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4.4. The for Loop

Often you write a loop in the following common form:

 i = initialValue;  // Initialize loop control variable    while   (i < endValue) {  // Loop body  ...   i++;  // Adjust loop control variable  } 

A for loop can be used to simplify the preceding loop:

   for   (i = initialValue; i < endValue; i++) {  // Loop body  ... } 

In general, the syntax of a for loop is as shown below:

   for   (initial-action; loop-continuation-condition;      action-after-each-iteration) {  // Loop body;  Statement(s); } 

The flow chart of the for loop is shown in Figure 4.5(a).

Figure 4.5. A for loop performs an initial action once, then repeatedly executes the statements in the loop body, and performs an action after an iteration when the loop-continuation-condition evaluates to true .

The for loop statement starts with the keyword for , followed by a pair of parentheses enclosing initial-action , loop-continuation-condition , and action-after-each-iteration , and followed by the loop body enclosed inside braces. initial-action , loop-continuation-condition , and action-after-each-iteration are separated by semicolons.

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A for loop generally uses a variable to control how many times the loop body is executed and when the loop terminates. This variable is referred to as a control variable . The initial-action often initializes a control variable, the action-after-each-iteration usually increments or decrements the control variable, and the loop-continuation-condition tests whether the control variable has reached a termination value. For example, the following for loop prints "Welcome to Java!" a hundred times:

   int   i;   for   (i =     ; i <   100   ; i++) {   System.out.println(   "Welcome to Java!"   ); } 

The flow chart of the statement is shown in Figure 4.5(b). The for loop initializes i to , then repeatedly executes the println statement and evaluates i++ while i is less than 100 .

The initial-action , i=0 , initializes the control variable, i . The loop-continuation-condition , i< 100 is a Boolean expression. The expression is evaluated at the beginning of each iteration. If this condition is true , execute the loop body. If it is false , the loop terminates and the program control turns to the line following the loop.

The action-after-each-iteration , i++ , is a statement that adjusts the control variable. This statement is executed after each iteration. It increments the control variable. Eventually, the value of the control variable should force the loop-continuation-condition to become false. Otherwise the loop is infinite.

   for   (   int   i =     ; i <   100   ; i++) {   System.out.println(   "Welcome to Java!"   ); } 

If there is only one statement in the loop body, as in this example, the braces can be omitted.

Tip

The control variable must always be declared inside the control structure of the loop or before the loop. If the loop control variable is used only in the loop, and not elsewhere, it is good programming practice to declare it in the initial-action of the for loop. If the variable is declared inside the loop control structure, it cannot be referenced outside the loop. For example, you cannot reference i outside the for loop in the preceding code, because it is declared inside the for loop.

Note

The initial-action in a for loop can be a list of zero or more comma-separated variable declaration statements or assignment expressions. For example,

   for   (    int   i =     , j =      ; (i + j <   10   ); i++, j++) {  // Do something  } 

The action-after-each-iteration in a for loop can be a list of zero or more comma-separated statements. For example,

   for   (   int   i =   1   ; i <   100   ;  System.out.println(i), i++  ); 

This example is correct, but it is not a good example, because it makes the code difficult to read. Normally, you declare and initialize a control variable as initial action, and increment or decrement the control variable as an action after each iteration.

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Note

If the loop-continuation-condition in a for loop is omitted, it is implicitly true . Thus the statement given below in (a), which is an infinite loop, is correct. Nevertheless, it is better to use the equivalent loop in (b) to avoid confusion: