[Chapter 6] 6.7 Iteration Statements

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| Chapter 6 Statements and Control Structures |

## 6.7 Iteration Statements

Iteration statements are used to specify the logic of a loop. Java has three varieties of iteration statement: `while`, `do`, and `for`.

References The do Statement; The for Statement; The while Statement

### The while Statement

A `while` statement evaluates a Boolean expression. If the expression is `true`, a given statement is repeatedly executed for as long as the expression continues to evaluate to `true`.

In Java, the expression in parentheses must produce a `boolean` value. This is different from C/C++, which allows any type of expression.

If the expression in parentheses evaluates to `true`, the statement contained in the `while` statement is executed and the expression in parentheses is evaluated again. This process continues until the expression evaluates to `false`.

If the expression in parentheses evaluates to `false`, the statement following the `while` statement is the next statement to be executed. The expression in parentheses is evaluated before the contained statement is executed, so it is possible for the contained statement not to be executed even once.

Here is an example of a `while` statement:

    while ( (c = in.read()) >= 0) {
        out.write(c);
    }

References Boolean Type; Expression 4; Statement 6

### The do Statement

A `do` statement executes a given statement and then evaluates a Boolean expression. If the expression evaluates to `true`, the statement is executed repeatedly as long as the expression continues to evaluate to `true`:

In Java, the expression in parentheses must produce a `boolean` value. This is unlike C/C++, which allows any type of expression.

The statement contained in the `do` statement is executed and then the expression in parentheses is evaluated. If the expression evaluates to `true`, the process is repeated.

If the expression evaluates to false, the statement following the `do` statement is the next statement to be executed. Because the expression is evaluated after the contained statement is executed, the statement is always executed at least once.

Here's an example of a `do` statement:

    do {
        c = in.read();
        out.write(c);
    } while (c != ';');

References Boolean Type; Expression 4; Statement 6

### The for Statement

A `for` statement is a more structured form of a `while` statement. A `for` statement performs an initialization step and then evaluates a Boolean expression. If the expression evaluates to `true`, a given statement is executed and an increment expression is evaluated repeatedly as long as the expression continues to evaluate to `true`:

Here is an example of a `for` statement:

    for (i = 0; i < a.length; i++) {
        a[i] = i;
    }

The initialization part of the `for` statement is executed first. If the initialization part contains nothing, no initialization is performed. The expression that follows must produce a `boolean` value. Before the body of the `for` statement is executed, the expression is evaluated. If the expression portion of the `for` statement is omitted, the default expression `true` is used. If the expression evaluates to `true`, the body of the `for` statement is executed and then the increment portion of the `for` statement is evaluated. Finally, the expression is evaluated again to determine if there should be another iteration. This process continues until the expression evaluates to `false`, at which point the statement following the `for` statement is the next statement to be executed. The `for` statement in the above example can be rewritten as a `while` statement as follows:

    i = 0;
    while (i < a.length) {
        a[i] = i;
        i++;
    }

One difference between comparable `for` and `while` loops is that a `continue` statement in the body of a `for` statement causes the increment portion of the statement to be evaluated. However, this may not be the case in a comparable `while` statement.

Here's a new version of our `for` example:

    for (i = 0; i < a.length; i++) {
        a[i] = i;
        continue;
    }

The added `continue` statement at the end of the `for` loop does not change the behavior of the loop. In particular, `i++` is still evaluated after each iteration through the body of the loop. Now let's add a `continue` statement at the equivalent place in our `while` example:

    i = 0;
    while (i < a.length) {
        a[i] = i;
        continue;
        i++;
    }

The `continue` statement in this `while` loop prevents the statement `i++` from being executed. The `continue` statement would have to be moved after the increment operation to match the logic of the `for` statement.

If the expression portion of a `for` statement is omitted, the default expression `true` is supplied. Take, for example, the following `for` statement:

    for ( FileInputStream in = new FileInputStream(fname);;) {
        c = in.read();
        if (c < 0)
            return;
        System.out.print((char)c);
    }

This example uses a local variable declaration in the initialization portion of the `for` statement. Local variable declarations in a `for` statement are subject to the same restrictions as local variable declarations in a block. In particular, a `for` statement cannot declare a local variable with the same name as a local variable or formal parameter that is defined in an enclosing block.

The above `for` statement is equivalent to the following `while` statement:

    {
        FileInputStream in = new FileInputStream(fname);
        while (true) {
            c = in.read();
            if (c < 0)
                return;
            System.out.print((char)c);
        }
    }

The enclosing block in the above example is provided to limit the scope of the local variable `in` to just the `while` statement.

The initialization portion of a `for` statement can also be empty. The following statement is a legal way of specifying an infinite loop:

    for (;;) {...}

This is equivalent to the following `while` statement:

    while (true) {...}

Unlike C/C++, there is no comma operator in Java. However, commas are explicitly allowed in the initialization portion of a `for` statement. For example, a `for` initialization can consist of multiple expressions separated by commas:

    i=2, j=5, k=44

When the initialization portion of a `for` statement contains local variable declarations, commas are also allowed because the syntax for declarations allows multiple variables, separated by commas, to be declared in one declaration. For example:

    int i=2, j=5, k=44

References Boolean Type; Expression 4; Statement 6; Local Variables; TopLevelExpression 6.4; The continue Statement; The while Statement

The switch Statement |  | The break Statement