Lesson 9: Loops

In the previous lesson, we discussed ways to minimize errors in our programs. One way software engineers do this is to first plan their code by writing algorithms.

Discuss: Use the Hold That Thought strategy to discuss the prompt.

• What do you know or remember about algorithms?

Discussion Goal: Students recall what they learned about algorithms in Computer Science Principles (CSP), including the components of an algorithm such as sequencing, selection, and iteration.

Do This: Define algorithm.

The Painter class has more Boolean methods that give us information about the current state of a Painter object. Let's take a look at what these methods do.

Do This: Review the lesson objectives.

Do This: Direct students to Level 1 on Code Studio to predict the program's outcome, then run the program to compare their predictions to the actual outcome.

Discuss: Click through the animated slide to display the prompts.

• What do you notice about the code in this program?
• What do you wonder about the code in this program?

Discussion Goal: Students identify the methods used in the program and share ideas about what each method does. Students may wonder if they can repeat the block of code instead of only checking the condition once.

Do This: Click through the animated slide to identify the methods on the Painter class UML diagram and explain their purpose and functionality.

In some cases, it would be useful to repeat a set of instructions as long as a condition is true instead of just checking that condition once. We can do this using while loops.

Display: Show the video – While Loops.

Discuss: Click through the animated slide to display the prompts. Use the Retrieve-Pair-Share strategy to discuss the prompts.

• How could we use while loops with these new methods?
• What new behaviors could we add to the PainterPlus class using while loops and these new methods?

Discussion Goal: Students share ideas of using while loops with these new methods, such as repeatedly checking a condition and executing a block of code as long as the condition is true. Students suggest using while loops and these methods to continue moving while facing a specific direction or as long as the Painter has paint in their paint bucket.

Do This: Click through the animated slide to demonstrate using a while loop with the hasPaint() method.

Do This: Click through the animated slide to demonstrate using a while loop with the isFacingNorth() method.

Discuss: Use the Hold That Thought strategy to discuss the prompt. Click through the animated slide to explain that the condition is initially false.

• Why does this code segment not execute?

Discussion Goal: Students identify that the condition is initially false, so the while loop does not execute.

Step through each line of the code to help students identify the error. Ask students if this is an example of a syntax error or a logic error.

Discuss: Use the Hold That Thought strategy to discuss the prompt. Click through the animated slide to define infinite loop.

• What is causing this outcome?

Discussion Goal: Students realize that the condition never becomes false, resulting in an infinite loop.

In these examples, we saw that if the condition is initially false, the body of the loop does not execute at all. On the other hand, if the condition is always true, the loop is an infinite loop.

How do we plan an algorithm before writing Java code? Software engineers use pseudocode to plan their algorithms then translate their pseudocode to Java.

Do This: Click through the animated slide to define pseudocode.

Distribute: Give each student a copy of the Writing Algorithms handout.

Group: Place students in pairs or small groups of three.

Do This: Direct students to write pseudocode for the methods and add them to the PainterPlus UML diagram.

Do This: Direct students to Level 2 on Code Studio to complete Levels 2, 3, and 4. Students complete a Check for Understanding on Level 2, then continue to complete Levels 3 and 4 to implement their algorithms in the PainterPlus class.

This is a good time to commit our code and save our new version of the PainterPlus class to the Backpack. Anytime we make changes to our programs, it is helpful to commit, or save, our work as a new version in case we need to revert to a previous version.

Do This: Play the music clip to cue committing their code and saving the new version of the PainterPlus class to the Backpack.

When we write new code, getting feedback from our peers is helpful to make sure we have met the requirements of the problem efficiently.

Do This: Define efficient.

Do This: Click through the animated slide to have students participate in the Code Review Call and Response.

Do This: Direct students to complete a code review on Level 5.

We saw some new types of errors today! Let's discuss these errors we encountered and update our Debugging Wall with the strategies we used to debug them.

Discuss: Click through the animated slide to display the prompts. Use the Hold That Thought strategy to discuss the prompts.

• What errors did you encounter while writing your code?
• How did you find and fix the error?

Discussion Goal: Students share examples of errors they encountered, including loops where the condition is not initially true or where the condition never becomes false. Students suggest strategies for finding and fixing these errors.

Do This: Have students choose a strategy as a class and add it to the Debugging Wall.

Do This: Review the concepts covered in this lesson.

Display: Key Vocabulary

Check For Understanding Question(s) and solutions can be found in each lesson on Code Studio. You can use these questions as an exit ticket.

To assign questions from the AP Classroom Question Bank that align with this lesson, create a custom quiz in AP Classroom by searching the Question Bank for the Essential Knowledge statements listed at the top of this lesson plan. You can find instructions and video demonstrations to do this on AP Central.