The Running-Value Pattern

Here is the correct answer to a bug you already met. In Chapter 4, Bug Hunt #4 reported an average of 98 ms that should have been 128. This computes it properly, casting one side to double so the fraction survives, exactly as Chapter 2 taught:

public class Arr2 {
    public static void main(String[] args) {
        int[] responseTimes = {120, 95, 210, 88};
        int total = 0;
        for (int i = 0; i < responseTimes.length; i++) {
            total += responseTimes[i];
        }
        double average = (double) total / responseTimes.length;
        System.out.println("Total  : " + total);
        System.out.println("Average: " + average + " ms");
    }
}

Total  : 513
Average: 128.25 ms

The same shape, three jobs

The pattern of walking an array while carrying a running value is one you will use constantly. Sum, count, and "find the largest" are all the same shape. Here is the largest:

public class Arr3 {
    public static void main(String[] args) {
        int[] loadTimes = {340, 180, 520, 260, 410};
        int slowest = loadTimes[0];
        for (int i = 1; i < loadTimes.length; i++) {
            if (loadTimes[i] > slowest) {
                slowest = loadTimes[i];
            }
        }
        System.out.println("Slowest page load: " + slowest + " ms");
    }
}

Slowest page load: 520 ms

Start by assuming the first item is the answer, then walk the rest and keep a better one whenever you find it. Notice the loop starts at i = 1, because position 0 is already your starting guess — a deliberate, correct use of starting at one, very different from the silent off-by-one bug of the last chapter, which skipped real data by accident.

Every running-value loop you write follows this same shape: start with a value that makes sense before you've seen anything (0 for a sum, the first item for a max), then walk the rest, updating as you go. Once you can see that shape, sum, count, and max stop being three things to memorize and become one thing to recognize.