Loops: while, for, and accumulation

Loops

• A loop repeats a block of code many times.
• Loops let you count, add up numbers, and do work without copying lines.
• In this lesson you learn the while loop and the for loop.

The while loop

• A while loop runs its block again and again while a condition is true.
• You need three parts: start a counter, test it, and change it inside the loop.
• If you never change the counter, the loop runs forever. Be careful.

public class Main {
    public static void main(String[] args) {
        int i = 1;            // start
        while (i <= 3) {      // test
            System.out.println("Line " + i);
            i = i + 1;        // change
        }
    }
}

The for loop

• A for loop puts the start, test, and change on one line.
• The shape is for (start; test; change) { ... }.
• It does the same job as the while above, but it is shorter and clearer.

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

Accumulation

• Accumulation means building up an answer step by step inside a loop.
• Use a variable that starts at 0 and grows: sum = sum + i;.
• i++ is a short way to write i = i + 1. sum += i is short for sum = sum + i.

public class Main {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1; i <= 5; i++) {
            sum += i;       // add each number
        }
        System.out.println(sum);   // 15
    }
}

Counting with a condition

• You can count how many times something is true.
• Start a counter at 0, and add 1 only when the test passes.
• Here we count even numbers from 1 to 10.

public class Main {
    public static void main(String[] args) {
        int count = 0;
        for (int i = 1; i <= 10; i++) {
            if (i % 2 == 0) {     // % gives the remainder
                count++;
            }
        }
        System.out.println(count);   // 5
    }
}

Off-by-one care

• The most common loop bug is doing one too many or one too few steps.
• i <= n includes n. i < n stops before n.
• To loop from 1 to n (and include n), use for (int i = 1; i <= n; i++).

public class Main {
    public static void main(String[] args) {
        int n = 4;
        // i goes 1, 2, 3, 4 — four times, because of <=
        for (int i = 1; i <= n; i++) {
            System.out.println(i);
        }
    }
}

Common mistakes

• for (int i = 0; i < n; i++) runs n times; <= runs one extra.
• Do not put a ; right after for(...).

Now you try

• Each task pre-fills the class skeleton — write your code inside main, or complete the method shown.
• Press Run to compile and run, then Check answer.
• Your code compiles and runs on the server, so even the first run is fast.