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Engineering LibreTexts

15.5: The for Statement

  • Page ID
    15253
  • The loops we have written so far have several elements in common. They start by initializing a variable, they have a condition that depends on that variable, and inside the loop they do something to update that variable. This type of loop is so common that there is another statement, the for loop, that expresses it more concisely.

    For example, we could rewrite printTable like this:

    public static void printTable(int rows) {
        for (int i = 1; i <= rows; i = i + 1) {
            printRow(i, rows);
        }
    }
    

    for loops have three components in parentheses, separated by semicolons: the initializer, the condition, and the update.

    1. The initializer runs once at the very beginning of the loop.
    2. The condition is checked each time through the loop. If it is false, the loop ends. Otherwise, the body of the loop is executed (again).
    3. At the end of each iteration, the update runs, and we go back to step 2.

    The for loop is often easier to read because it puts all the loop-related statements at the top of the loop.

    There is one difference between for loops and while loops: if you declare a variable in the initializer, it only exists inside the for loop. For example, here is a version of printRow that uses a for loop:

    public static void printRow(int n, int cols) {
        for (int i = 1; i <= cols; i = i + 1) {
            System.out.printf("%4d", n * i);
        }
        System.out.println(i);  // compiler error
    }
    

    The last line tries to display i (for no reason other than demonstration) but it won’t work. If you need to use a loop variable outside the loop, you have to declare it outside the loop, like this:

    public static void printRow(int n, int cols) {
        int i;
        for (i = 1; i <= cols; i = i + 1) {
            System.out.printf("%4d", n * i);
        }
        System.out.println(i);
    }

    Assignments like i = i + 1 don’t often appear in for loops, because Java provides a more concise way to add and subtract by one. Specifically, ++ is the increment operator; it has the same effect as i = i + 1. And -- is the decrement operator; it has the same effect as i = i - 1.

    If you want to increment or decrement a variable by an amount other than 1, you can use += and -=. For example, i += 2 increments i by 2.

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