Kotlin
Java

Loops
Java

Created By: Geoffrey Challen
/ Updated: 2021-08-30

Today's lesson presents the final core computer capability: the ability to repeat something multiple times. By combining this with our ability to store and manipulate data and make decisions, we can truly solve any problem. So let's get started!

while

Let's meet our first loop. It's the simplest one in Java, and repeats a block of code while a condition is true:

Walk through the while loop and how it works. break and continue are presented later.

The while loop declaration looks quite similar to the if statements we've already seen. First the keyword while, followed by a conditional expression inclosed in parantheses, followed by a block. However, unlike the if statement which only executes once, the while statement continues to execute the block as long as the condition evaluates to true. This can cause problems!

Show an example of an unterminated while loop.

for

While while is the simplest loop, probably the most common loop you'll encounter is called a for loop. It's more complicated than a while loop, but designed to capture a common loop programming pattern. We frequently want to use a loop to increment a variable starting at zero and ending when it reaches some value. (We'll see why tomorrow.)

This is so common that Java provides a second loop that captures this pattern. Let's go through it together:

Walk carefully through the for loop declaration.

Variants

The for loop syntax is quite flexible and can produce lots of different sequences of values in the index variable:

We can also omit one, two, or all three of the parts of a for loop declaration:

However, by far the most common for loop you'll see is the start at 0, increment by 1, and end when the value is strictly less than some limit:

Loop Variable Scope

There is one important difference between this while loop:

and this for loop:

Let's try and figure out what it is!

Show the difference in loop variable scope between the while and for loop examples.

for v. while

Let's explore the connection between the two types of loop using a diagram.

Use a diagram to connect the operation of the for and while loops.

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Solution Walkthrough

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Solution Walkthrough