📓2.7: while Loops

While Loops

📖 This page is a condensed version of CSAwesome Topic 2.7

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Go to the public template repository for our class: BWL-CS Java Template
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Specify the repository name: CS2-Unit2PartB-Notes
For the description, write: Iteration/repetition (while loops, for loops)
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Type a brief commit message at the top of the file that opens, for example: updated Main.java
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While Loops

When you play a song, you can set it to loop, which means that when it reaches the end it starts over at the beginning. A loop in programming, also called iteration or repetition, is a way to repeat one or more statements. If you didn’t have loops to allow you to repeat code, your programs would get very long very quickly! Using a sequence of code, selection (ifs), and repetition (loops), the control structures in programming, you can construct an algorithm to solve almost any programming problem!

A while loop executes the body of the loop as long as (or while) a boolean condition is true. When the condition is false, we exit the loop and continue with the statements that are after the body of the while loop. If the condition is false the first time you check it, the body of the loop will not execute.

Notice the while statement looks a lot like an if statement, but it runs more than once. The curly braces ({}) are optional when there is just 1 statement following the condition, but required if there are more than 1 statement in the loop. In the AP exam, they will always use curly braces, which is a good practice to follow.

    // The statements in an if run one time if the condition is
    // is true and zero times if it is false.
    if (condition) {
        statements;
    }

    // The statements in a while loop run zero or more times,
    // determined by how many times the condition is true
    while (condition) {
        statements;
    }

Here’s what the flow of control looks like in a Java while loop. Notice that while the condition is true, the loop body is repeated.

Three Steps to Writing a Loop

The simplest loops are counter-controlled loops like below, where the loop control variable is a counter that controls how many times to repeat the loop. There are 3 steps to writing a loop using this loop control variable as seen below in a loop that counts from 1 to 10.

🔄 Remember these 3 steps to writing a loop:

Initialize the loop variable (before the while loop)
Test the loop variable (in the loop header)
Change the loop variable (in the while loop body at the end)

Tracing Loops

A really important skill to develop is the ability to trace the values of variables and how they change during each iteration of a loop.

You can create a tracing table that keeps track of the variable values each time through the loop as shown below. This is very helpful on the exam. Studies have shown that students who create tables like this do much better on code tracing problems on multiple choice exams.

A trace table shows the values of all of the variables each time through the loop. Iteration 0 means before the loop. When you are tracing through code, pretend to be the computer running the code line by line, repeating the code in the loop, and keeping track of the variable values and output.

Common Errors with Loops

One common error with loops is to accidentally create an ♾️ infinite loop ♾️. An infinite loop is one that never stops because the condition is always true.

Sometimes we will write an infinite loop on purpose like this:

   while (true) {
       System.out.println("This is a loop that never ends");
   }

But if we create an infinite loop by accident, our program may seem to get stuck. For example look at this loop:

   int i = 0;
   while (i < 10) {
       System.out.println(i);
   }

That loop looks a lot like loops earlier in this chapter but it is actually an infinite loop. Can you see why?

The problem in this loop—and a common way to accidentally create an infinite while loop—is that although it includes steps 1 and 2 (initializing the loop variable and testing it) it forgot step 3 and never changes the loop variable. The loop variable, i, starts at 0 and the loop loops as long as i < 10 which will always be true because there’s no code in the loop that changes i. The simple fix is to add a line that increments i:

   int i = 0;
   while (i < 10) {
       System.out.println(i);
       i++;
   }

Another common error with loops is an off-by-one error where the loop runs one too many or one too few times. This is usually a problem with step 2 (the test condition) and using the incorrect relational operator < or <=.

Input-Controlled Loops

You can use a while loop to repeat the body of the loop a certain number of times as shown above.

However, a while loop is typically used when you do NOT know how many times the loop will execute. It is often used for a input-controlled loop where the user’s input indicates when to stop.

For example, in the Magpie chatbot lab on replit.com below, the while loop stops when you type in “Bye”. The stopping value is often called the sentinel value for the loop. Notice that if you type in “Bye” right away, the loop will never run. If the loop condition evaluates to false initially, the loop body is not executed at all. Another way to stop the loop prematurely is to put in a return statement that makes it immediately return from the method.

💻 In-Class Activity: Guessing Game

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Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.