Lesson 12: Arrays and For Loops

Overview

Using a for loop to iterate over all of the elements in an array is a really useful construct in most programming languages. In this lesson, students learn the basics of how a for loop can be used to repeat code, and then combine it with what they've already learned about arrays to write programs that process all elements in an array. Students use for loops to go through each element in a list one at a time without having to write code for each element. Towards the end of the lesson students will apply this with the colorLed list on the board to create an app that changes all of the LEDs each time a button is clicked.

Purpose

As students start using arrays more frequently, a common pattern emerges wherein you want to run some code on each element of an array. While for loops are a generally useful structure for repeating code, they are a particularly useful for iterating over an array. In this lesson we build on the understanding of arrays that students have developed in the last two lessons by introducing the for loop, which combines a variable, the counter pattern, and a conditional all in a single construct.

Assessment Opportunities

  1. Modify the exit condition of a for loop to control how many times it repeats

    Code Studio: see rubric on bubble 13

  2. Use a for loop to iterate over an array

    Code Studio: see rubric on bubble 13

Agenda

Lesson Modifications

Warm Up (5 min)

Activity (40 min)

Wrap Up

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Objectives

Students will be able to:

  • Modify the exit condition of a for loop to control how many times it repeats
  • Use a for loop to iterate over an array

Vocabulary

  • Array - A data structure in JavaScript used to represent a list.
  • For Loop - Loops that have a predetermined beginning, end, and increment (step interval).

Introduced Code

Teaching Guide

Lesson Modifications

Attention, teachers! If you are teaching virtually or in a socially-distanced classroom, we have recommendations for alternate lessons that can be used for this unit on physical computing. Click Here to read our recommendations for Unit 6.

Warm Up (5 min)

Run Code on All Elements of an Array

Discussion Goal

You shouldn't expect students to know how exactly a computer would do this, but to start thinking about what is needed to keep track of where you are in an array (the index), how you would know you've reached the end of the array (the array length), and how you might incrementally increase where you are in the list (the counter pattern).

Discuss: If you had a list full of all of your ToDos and you wanted the computer to print out each one, how might you do it? Don't worry about the specific code, focus on the what information your program would need to know and keep track of. Would your approach still work if you added or removed a ToDo from your list?

Remarks

Doing something to every element of an array is a really common problem in Computer Science, and figuring out how to solve this problem will let us write more efficient programs. Today we're going to focus on three things to help us with this problem

  1. Using the index to do something to elements in an array
  2. Keeping track of a counter so we can move through the elements of an array by index
  3. Using the length of an array to know when we've reached the end

Activity (40 min)

Arrays and For Loops

Transition: Send students to Code Studio

Wrap Up

Journal

Prompt: Have students reflect on their development of the five practices of CS Discoveries (Problem Solving, Persistence, Creativity, Collaboration, Communication). Choose one of the following prompts as you deem appropriate.

  • Choose one of the five practices in which you believe you demonstrated growth in this lesson. Write something you did that exemplified this practice.

  • Choose one practice you think you can continue to grow in. What’s one thing you’d like to do better?

  • Choose one practice you thought was especially important for the activity we completed today. What made it so important?

  • Programs that Repeat
  • 2
  • 3
  • (click tabs to see student view)
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Student Instructions

Make a Prediction

Read through the code for this program and predict what will happen each time the button is clicked?

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Student Instructions

Knowing When to Stop

If you clicked the button too many times in the last level, you got an error. Whenever you're writing code that repeats, you should think about when to stop repeating.

Do This

This program is similar to the previous one, but there is a conditional inside the event handler. You'll need to complete the conditional so that we don't try to toggle an LED that doesn't exist.

Would your code work without changes for a board with more or fewer LEDs? If not, could you modify it so it would?

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Discussion Questions

For-loops are a very powerful construct, but this introduction intentionally only covers a small part of their functionality. As students are more comfortable with loops, more functionality will be introduced. For now, students should know that they can use a for loop to repeat a segment of code multiple times. The segment should go wherever in the program that code will be run. (This aspect of loops is highlighted to keep students from treating loops like functions and placing them at the bottom of the code.)

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Questions to Consider

  • What are loops used for in programming?
  • Where do loops go in your code?
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Student Instructions

Predict

What will print as a result of this loop?

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Student Instructions

Looping Over Arrays

One of the most powerful ways to use a for loop is to loop over an array, running code on each item in the array. We can do this by using the for loop counter variable (usually i) as the index of your array.

Do This

This program should loop over the array buttons and do two things to each button - change the background color to red, and change the height to 50 px.

  • Add a second setProperty block inside the loop
  • Change the target to buttons[i]
  • Set the "height" property to 50

expandable

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Student Instructions

Array Length

In the last program we told the loop to run three times with the code for (var i = 0; i < 3; i++). This works, but we could write smarter programs by using the length of the array to decide how many times to loop.

Do This

Change the exit condition of this for loop so that it runs while i < buttons.length.

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Student Instructions

Constructing a for loop from scratch

Now that you've had some practice modifying for loops to process arrays, let's see if you can do it yourself.

Do This

We've provided the design elements and an array to start with, but the rest is on you.

  • Add an event handler to respond to the "thumbsup_button" being clicked
  • Place a for loop inside the event handler
  • Modify the exit condition of the for loop so that it will run until it gets to the end of the array images
  • Inside your for loop change the current image to "icon://fa-thumbs-o-up"

Challenge: Can you add a second button that turns all of the images back to thumbs down?

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Student Instructions

Turning all the Color LEDs On

Now that you know how to use a for loop to process all of the elements in an array, you can turn on all of the Color LEDs much more easily than before.

Do This

Over the next few levels, you'll create an app that will control all of the Color LEDs on your board. The first step is to wire up the button that turns all of the LEDs on. (Don't worry about the rest of the comments in the workspace. You'll work on those in later levels.)

  • Add an event handler for "button_on"
  • Place a for loop in your event handler that repeats until it reaches in the end of the array colorLeds
  • Inside the for loop, call colorLeds[i].on() to turn on the current color LED
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Student Instructions

Turning all the Color LEDs Off

Now that you've got one button to turn the color LEDs on, you can make another turn them off.

Do This

Add an event handler to "button_off" with a for loop that turns each color LED off.

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Assessment Opportunities

Key Concepts:

Use a for loop to iterate over an array

Assessment Criteria:
Extensive Evidence

Convincing Evidence

Limited Evidence

No Evidence

Student Instructions

Add Some Color

Now that you can turn the LEDs on and off, it's time to add some color.

Do This

Add event handlers to "button_red" and "button_green" that turns all the LEDs those colors.

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Assessment Opportunities

Key Concepts:

Use a for loop to iterate over an array

Assessment Criteria:
Extensive Evidence

All five buttons work as described using for loops, and the loops use the length of the array as an exit condition, with no errors or unnecessary code.

Convincing Evidence

The on and off buttons work as described using for loops, and there are no errors in the code.

Limited Evidence

For loops are used to turn the LEDs on and off, but there may be errors that keep the buttons from working as described.

No Evidence

There are no for loops in the code.

Student Instructions

Make it Your Own

Think of a new button that you would like in this project. Don't forget about the other LED options, such as pulse and blink.

Do This

Add a new button to the project, and program it with a behavior of your choice.

Standards Alignment

View full course alignment

CSTA K-12 Computer Science Standards (2017)

AP - Algorithms & Programming
  • 2-AP-11 - Create clearly named variables that represent different data types and perform operations on their values.
  • 2-AP-12 - Design and iteratively develop programs that combine control structures, including nested loops and compound conditionals.
  • 2-AP-13 - Decompose problems and subproblems into parts to facilitate the design, implementation, and review of programs.
  • 2-AP-14 - Create procedures with parameters to organize code and make it easier to reuse.
  • 2-AP-16 - Incorporate existing code, media, and libraries into original programs, and give attribution.
  • 2-AP-17 - Systematically test and refine programs using a range of test cases.
  • 2-AP-19 - Document programs in order to make them easier to follow, test, and debug.
CS - Computing Systems
  • 2-CS-01 - Recommend improvements to the design of computing devices, based on an analysis of how users interact with the devices.
  • 2-CS-02 - Design projects that combine hardware and software components to collect and exchange data.
  • 2-CS-03 - Systematically identify and fix problems with computing devices and their components.