for Loops

A loop repeats a block. A for loop puts three pieces on one line: initialize a counter, test whether another iteration should happen, and update the counter after the body. In for (int day = 1; day <= 3; day++), day starts at 1, the body runs while day is at most 3, and day++ increases it by one afterward.

Trace the condition before each iteration, including the final false check. The counter reaching 4 is what stops this loop; the body does not run with day=4. Choose meaningful starting and stopping values instead of changing < to <= until the example happens to print the desired answer.

Keep a running total outside the loop

An accumulator stores a result built across iterations. Initialize it before the loop, update it inside the body and report it afterward. A variable declared inside the body is created again on each iteration. Putting a total there would forget earlier work. Keep the current day’s amount separate from the accumulated amount so you can explain both.

public class Main {
    public static void main(String[] args) {
        for (int day = 1; day <= 3; day++) {
            System.out.println("Day " + day);
        }
    }
}

Day 1
Day 2
Day 3

List day before and after each update. The increment is not a print; no Day 4 line appears. Change the condition to day < 3 and explain which boundary value is lost.

public class Main {
    public static void main(String[] args) {
        int total = 0;

        for (int day = 1; day <= 3; day++) {
            int minutes = day * 10;
            total = total + minutes;
        }

        System.out.println(total);
    }
}

60

The amounts are 10, 20 and 30; the cumulative values are 10, 30 and 60. Assigning total=minutes would keep only 30. Resetting total inside the loop would have the same losing-history problem.

public class Main {
    public static void main(String[] args) {
        int count = 0;

        for (int index = 0; index < 0; index++) {
            count++;
        }

        System.out.println(count);
    }
}

0

The condition is checked before the first body execution. A zero-iteration loop is a normal case, not an error. Later, this behavior will make empty arrays and lists easy to handle.

For the int counters and totals here, count++ means increase count by one, and total += value is a shorter way to write total = total + value. Both update an existing variable. You can use the longer assignment until the shorter form feels familiar; trace the same old and new values either way. Similarly, total -= value means total = total - value. The pricing checkpoint will use that subtraction form.

Your turn

public class Main {
    public static void main(String[] args) {
        int count = 0;
        int total = 0;
        // Visit each value and update only for even values.
    }
}

public class Main {
    public static void main(String[] args) {
        int count = 0;
        int total = 0;

        for (int value = 1; value <= 6; value++) {
            if (value % 2 == 0) {
                count++;
                total += value;
            }
        }

        System.out.println(count);
        System.out.println(total);
    }
}

3
12

Change it and explain

Change the upper bound to 1, 2 and 7 and predict both answers. If a loop ran 100 times, would your count always be 100? Explain which condition decides that. Try printing current value, count and total inside the body to inspect a trace, then remove the extra debugging output from the final report.

Avoid incrementing the loop variable both in the header and inside the body. It skips values and makes the number of iterations harder to reason about. Declare it in the header when it is only needed for this loop.