2.7 while Loops

1. When you need iteration

Iteration is one kind of repetition. An iteration statement changes the flow of control: it sends the program back to run the same segment of code again, for as long as the Boolean expression that controls the loop stays true. That can mean many times, once, or zero times 2.7.A.1. You need iteration when a task repeats the same steps many times, or when you do not know how many times the steps must repeat until the program is running: “add interest until the savings reach 1000”, “keep halving until the value is 1”.

Without a loop you would copy the same lines again and again, and you could not handle a number of repetitions that changes.

2. How a while loop runs

A while loop is a type of iterative statement. The Boolean expression is evaluated before each iteration of the loop body, including the first. When it is true, the body runs. This continues until the expression is false, and then the iteration ends 2.7.B.1.

int countdown = 3;
while (countdown > 0) {
    System.out.println(countdown);
    countdown--;
}
System.out.println("Liftoff!");

Output:

3
2
1
Liftoff!

Check	countdown	countdown > 0	Printed
1	3	true	3
2	2	true	2
3	1	true	1
4	0	false	(loop ends)

The Boolean expression was evaluated 4 times; the body ran 3 times. The variable that controls the loop (countdown) is called the loop control variable. A working loop has three parts: it is initialized before the loop, tested in the condition and updated in the body.

Watch out: the condition is only checked at the top. In the example below, steps passes 10 in the middle of the body, but the rest of the body still runs.

int steps = 8;
while (steps < 10) {
    steps += 3;
    System.out.println("steps = " + steps);
}

Output:

steps = 11

3. Zero iterations

If the Boolean expression is false the first time it is evaluated, the loop body does not run at all 2.7.A.3.

int lives = 0;
while (lives > 0) {
    System.out.println("Playing...");
    lives--;
}
System.out.println("Game over");

Output:

Game over

This matters when a loop depends on its starting values (initial conditions). Ask: “What must be true before the loop starts for it to work as intended?”

4. Infinite loops

If the Boolean expression of a loop is true every time it is checked, the loop never stops: this is an infinite loop 2.7.A.2. The code compiles, but the program never moves past the loop. Three common causes:

Forgetting the update:

int count = 1;
while (count <= 5) {
    System.out.println(count);
}

Updating in the wrong direction:

int count = 1;
while (count <= 5) {
    System.out.println(count);
    count--;
}

count moves away from 5 (1, 0, -1, -2…), so count <= 5 stays true. (Strictly, count would overflow after about two billion iterations, but for the AP® exam this loop is an infinite loop.)

Skipping over the stopping value with !=:

int n = 0;
while (n != 10) {
    n += 4;
}

n goes 0, 4, 8, 12, 16… and is never exactly 10. Writing n < 10 would stop the loop.

A stray semicolon also causes trouble: while (count <= 5); is a loop with an empty body, so count never changes.

5. Off by one errors

An off by one error happens when the loop runs one time too many or one time too few 2.7.A.4. Goal: print the lap numbers 1 to 5.

int lap = 1;
while (lap < 5) {
    System.out.print(lap + " ");
    lap++;
}

Output:

1 2 3 4 

One time too few: when lap is 5, lap < 5 is false. The fix is lap <= 5:

int lap = 1;
while (lap <= 5) {
    System.out.print(lap + " ");
    lap++;
}

Output:

1 2 3 4 5 

Starting at 0 with lap <= 5 would be one time too many (6 iterations). To check any loop, trace the first iteration and the last iteration.

6. Repeat until a goal is reached

while loops are a natural fit when you do not know the number of iterations in advance. A savings account earns 10 percent each year (integer dollars, rounded down). How many years until 100 dollars grows to at least 150?

int balance = 100;
int years = 0;
while (balance < 150) {
    balance = balance + balance / 10;
    years++;
}
System.out.println(years + " years, balance " + balance);

Output:

5 years, balance 160

years at check	balance	balance < 150
0	100	true
1	110	true
2	121	true
3	133	true
4	146	true
5	160	false

Notice balance / 10 is integer division: 121 / 10 is 12 and 133 / 10 is 13.

You can also end a loop early with a condition in the body. Here a boolean variable stops the search at the first multiple of 7 above 50:

int n = 51;
boolean found = false;
while (!found) {
    if (n % 7 == 0) {
        found = true;
    } else {
        n++;
    }
}
System.out.println("First multiple of 7 above 50: " + n);

Output:

First multiple of 7 above 50: 56

Watch out: if the n++ came before the if, the search would skip testing 51. The order of statements in the body decides whether an early exit happens too early, too late or not at all.