25. While Loops¶

In these notes you will learn:

• How to create an infinite loop using a while-loop.
• How to write and trace while-loops that perform simple task such as incrementing variables.

25.1. Introduction¶

Suppose you want to print the numbers from 1 to 1000 on the screen. You could, of course, do something like this:

```println(1);
println(2);
println(3);

// ...

println(1000);```

But that’s not a very good solution because it takes so many lines of code. A much better way is to use a while-loop, which is a way to repeat a block of code zero or more times.

25.2. Infinite Loops¶

The simplest kind of loop is a loop that never stops, i.e. an infinite loop. We can use a while-loop to create an infinite loop like this:

```while (true) {             // infinite loop
println("Hello!");
}```

It prints Hello! forever:

```Hello!
Hello!
Hello!
Hello!
...```

Note

In practice, infinite loops are often a sign of a bug in your program, and fixing an infinite loop ususally requires adding a stopping condition to it.

25.3. Loops that Stop¶

Now suppose we want to print the numbers 1 to 10 on the screen using a while- loop. Somehow we need the loop to stop after it has called println 10 times. The standard technique for doing this is to use a variable, say i, to keep track of how many times the block of code in the while-loop has been executed. For example:

```int i = 1;

while (true) {  // infinite loop
println(i);
++i;  // adds 1 to i (same as i += 1)
}

println("done");```

This loop prints this:

```1
2
3
4
...
```

Again, this is an infinite loop. But it illustrates the important idea of incrementing a variable inside a loop.

How do we get a while-loop to stop? It turns out that the condition at the top of the while-loop decides if the code block underneath it should be executed or skipped (thus terminating the loop). So far our while-loop condition has always been true:

```while (true) {  // condition for this loop is "true": always executes
// ...                                             the code block underneath
}```

We can write any boolean expression (i.e. an expression that evaluates to either true or false) as the condition for a while-loop. For example:

```int i = 1;

while (i <= 10) {
println(i);
++i;
}

println("done");```

Now the while-loop condition is i <= 10, which means that the block of code under the while-loop will only be executed when i <= 10 is true. If i <= 10 is false, then the loop stops and jumps down to the first statement after the loop (i.e. println("done")).

And that’s really all there is to while-loops: the condition at the top decides whether or not to execute the block of code underneath. When the condition is false the loop is over, i.e. it is terminated.

25.4. Tracing a while-loop¶

Look again at this loop:

```int i = 1;

while (i <= 10) {
println(i);
++i;
}

println("done");```

What does it print? The best way to understand loops is to “play computer” and trace the code by hand, keeping track of the value of i as you go. Tracing it by hand, you do something like this:

• i is initially set to 1.
• The first time the loop condition is checked, i is 1, and so i <= 10 is true. Thus println(i) is called, causing 1 to be printed. Then 1 is added to i, making it equal to 2.
• After the code block is executed, the loop condition is checked again. Since i is 2, i <= 10 is true, and so 2 is printed and i is incremented to 3.
• After the code block is executed, the loop condition is checked again. Since i is 3, i <= 10 is true, and so 3 is printed and i is incremented to 3.

You can see how the pattern continues. Eventually, i gets the value 10. The loop condition i <= 10 is true, and so 10 is printed and i is incremented to 11.

But now when the loop condition is checked, i <= 10 is false because i is 11. A false loop condition causes the program to skip over the block of code directly underneath it and instead execute whatever statements follow the loop.

So overall, this code prints the numbers from 1 to 10:

``` 1
2
3
4
5
6
7
8
9
10```

Tracing loops is a valauble skill that you should practice. Loops in real programs can get tricky, and tracing a loop by hand is often the best way to understand what it does.

25.5. Example: Adding Objects to an ArrayList¶

In previous notes we saw code like this for adding a lot of bouncing ball objects to an ArrayList:

```// ...

ArrayList<BouncingBall> ballList;  // initially null

void setup() {
size(500, 500);

// create the initially empty ArrayList of BouncingBall objects
ballList = new ArrayList<BouncingBall>();

}

void draw() {
background(255);

// render and update all the balls
for(BouncingBall b : ballList) {
b.render();
b.update();
}
}```

The problem here is that adding 10 ball’s requires calling ballList.add(randomBouncingBall()) 10 times in a row. While this might be fine for 10 or so objects, if you wanted a couple of hundred balls then all these add statements become so numerous that they are extremely hard to manage.

A better approach is to use a while-loop:

```// ...

int NUM_BALLS = 10;

void setup() {

// ...

int i = 0;
while (i < NUM_BALLS) {
i += 1;
}

}

// ...```

The while-loop not only greatly shortens the code, but also makes it trivial to change the number of balls by setting NUM_BALLS.

25.6. Questions¶

1. What does each of the following code fragments print?

```int a = 5;                // fragment 1
while (a <= 10) {
println(a);
++a;
}

int a = 5;                // fragment 2
while (a < 10) {
println(a);
++a;

int a = 5;                // fragment 3
while (a <= 10) {
++a;
println(a);
}

int a = 5;                // fragment 4
while (a < 10) {
++a;
println(a);
}

int a = 5;                // fragment 5
while (a != 10) {
println(a);
++a;
}

int a = 5;                // fragment 6
while (a != 10) {
++a;
println(a);
++a;
}```
2. What does each of the following code fragments print?

```int k = 5;                // fragment 1
while (k > 1) {
println(k);
--k;
}

int k = -1;               // fragment 2
while (k > -5) {
println(k);
--k;
}

int k = -1;               // fragment 3
while (k > -5) {
--k;
println(k);
}```
3. What does each of the following code fragments print?

```int b = 0;                // fragment 1
while (b < 10) {
println(b);
b += 2;
}

int b = 0;                // fragment 2
while (b <= 10) {
println(b);
b += 2;
}

int b = -10;              // fragment 3
while (b < 0) {
println(b);
b += 3;
}

int b = 10;               // fragment 4
while (b >= 0) {
println(b);
b -= 2;
}

int b = 0;                // fragment 5
while (b < 10) {
println(b);
b *= 2;
}

int b = 1;                // fragment 6
while (b < 10) {
println(b);
b *= 2;
}```
4. Write a file loop that prints the following sequences of numbers. There should be exactly one println statement in the body of your while loop that prints one number of the sequence on each iteration.

1. 1, 2, 3, 4, 5
2. 5, 4, 3, 2, 1
3. -1, -2, -3, -4, -5
4. -5, -4, -2, -1
5. 1, 3, 5, 7, 9
6. 9, 7, 5, 3, 1
7. -5, 0, 5, 10, 15
8. 1, 1, 2, 2, 3, 3, 4, 4 (remember that there should be exactly one println statement in the body of your while-loop that prints exactly one number of the sequence)
9. 1, -1, 2, -2, 3, -3, 4, -4 (remember that there should be exactly one println statement in the body of your while-loop that prints exactly one number of the sequence)
10. -1, 1, -2, 2, -3, 3, -4, 4 (remember that there should be exactly one println statement in the body of your while-loop that prints exactly one number of the sequence)
5. Explain why the following is not an infinite loop:

```int n = 1;
while (n > 0) {
println("loop");
++n;
}```