Lesson 11: Booleans and Conditionals
Overview
Students start by using booleans to compare the current value of a sprite property with a target value, using that comparison to determine when a sprite has reached a point on the screen, grown to a given size, or otherwise reached a value using the counter pattern. After using booleans directly to investigate the values or sprite properties, students add conditional if statements to write code that responds to those boolean comparisons.
Purpose
This lesson follows closely the booleans model that students first experienced in the Booleans Unplugged lesson. As before, we start with using booleans directly before using booleans to trigger if statements. In the following lesson we will introduce some boolean producing blocks, such as keyDown(), which can be used in place of simple boolean comparisons to write programs that respond to user input.
Assessment Opportunities
-
Use conditionals to react to changes in variables and sprite properties
See Level 11 in Code Studio.
Agenda
Warm Up (5 min)
Activity (40 min)
Wrap Up (5 min)
View on Code Studio
Objectives
Students will be able to:
- Use conditionals to react to changes in variables and sprite properties
Vocabulary
- Boolean Expression - in programming, an expression that evaluates to True or False.
- If-Statement - The common programming structure that implements "conditional statements".
Introduced Code
Teaching Guide
Warm Up (5 min)
Answering Boolean Questions
Goal: At the end of the Boolean Question game from the previous lesson, students began adding conditions to their boolean questions - meaning that if the answer to the question is true, something should happen. Before programming with conditionals, we want to make sure that students have a solid understanding of what booleans really are.
Prompt:
- How many different numbers are there in the world?
- How many different words or combination of letters and other characters are there?
- How many different boolean values are there?
Discuss: Students should realize that the first two questions (numbers and strings), are essentially infinite, but that booleans are limited to two states.
Remarks
As you begin programming today, you'll be using booleans to make programs that change their behavior depending on the answer to those boolean questions.
Activity (40 min)
Content Corner
Though seemingly simple, understanding how a boolean statement will evaluate can be difficult given that different programming languages have differing opinions on 'truthiness' and 'falsiness'. In fact, JavaScript (the language used in this course) has two different operators to test boolean equality == and ===.
The double equals operator (==) is pretty generous in determining truthiness, for example each of the following is considered true in JavaScript when using the == operator, but would be false using the === operator:
1 == true; "1" == true; 5 == "5"; null == undefined; "" == false;
We use the == operator in this course because it's more forgiving, but it's important to be aware that it can sometimes report back truth when you really didn't intend it to (in which case you might want to use the more strict === operator)
Booleans Plugged
Transition: Send students to Code Studio.
Wrap Up (5 min)
Adding Conditionals
Journal: Think back to all of the programs you've written so far; how might you use conditionals to improve one of your programs from past lessons? What condition would you check, and how would you respond to it?
- Lesson Overview
- Student Overview
- Make a Prediction
- 2
Student Instructions
Boolean Expressions
Decision-making on computers are based on statements called boolean expressions. A boolean expression is any expression that can only evaluate to either TRUE or FALSE. This can be particularly useful when used to compare the properties of two sprites. For example, you could figure out which one is larger or placed higher up. In the next few puzzles we'll use simple boolean expressions to compare sprite properties.
Predict
Read the code below. There are some new symbols in it which you haven't been introduced to. Take a guess at what they mean and try to answer the following question
Which result will be printed in the console by this program?
true
false
true
0
200
100
true
error
true
false
true
false
- Video: Booleans
- Teacher Overview
- Student Overview
Discussion Goals
Students should be able to explain that a Boolean expression is something that is either true or false, similar to a yes or no question. The more formal way to say this is that Boolean expressions evaluate to either true or false. That means that when the computer processes a Boolean expression, it checks to see whether the expression describes a situation that is true or false, and then uses the value of either true or false wherever the expression is found.
Some examples of Boolean expressions that evaluate to true are 3 > 1 and 4 <= 7, but press students to think of expressions that might be better represented by variables, such as studentAge < 70 or sizeOfClass > 2.
Some examples of Boolean expressions that evaluate to false are 4 == 7, schoolName == "Hogwarts", and currentYear < 1000.
Questions to Consider
- What is a Boolean expression?
- What’s an expression that would evaluate to true?
- What’s an expression that would evaluate to false?
Student Instructions
Boolean Expressions
The simplest Boolean expressions are questions that the computer can answer with true or false.
Another way to say this is that Boolean expressions evaluate to true or false.
Do This
Can you modify the values of the sprite properties so that each of the Boolean expressions evaluates to true?
- Read through the entire program to see how
sprite1andsprite2are being created and which properties are compared. - For each of the Boolean expressions, identify the sprite properties being compared.
- Change the code in the first 11 lines only so that each of the
console.log()statements printtrue.
Student Instructions
Student Instructions
Booleans
In the past few levels, we have been comparing values of sprites to find out whether something is true or false. Let's start putting that in the context of an animation.
Do This
The program draws a race car and a finish line. We are going to figure out when the race car crosses the finish line. The sprites have all been set up for you.
- Add a
console.logstatement inside the draw loop. ( Show me where ) - Add an Boolean expression inside the
console.logthat asks "Is the x position of the race car less than the x position of the finish line?" - Look at the output of the program as the car moves. When does the output change? Why?
Student Instructions
More Practice With Booleans
Let's use one more Boolean expression to check the condition of a sprite. You can use a console.log command to check whether its scale is 2 yet.
Do This
- Add a
console.logstatement. - Add a Boolean expression inside the
console.logthat checks whether thefruit.scaleis greater than 2.
- Booleans and Comparison Operators
- Student Overview
- Video: Conditional Statements
- Teacher Overview
- Student Overview
Discussion Goals
The broad point of this question is that programmers use if statements when they want the program to run differently in response to different situations. Encourage students to think of particular situations in which this would be the case. For example, they might want their characters to move faster when a "bonus" is in effect, or maybe they want more enemies to appear when the player reaches a certain level. Maybe they want the program to react in some way if a user presses a key or clicks the mouse, or they want a character to change animations if it touches a particular item.
Student Instructions
If Statement
The Boolean expressions you used earlier allow us to ask questions, but in order to respond to those questions, we need to use an if statement.
if statements usually go inside your draw loop because we want to check them each time the loop runs.
Do This
The race car program you wrote earlier is loaded here for you.
Assessment Opportunities
Use conditionals to control the flow of a program.
Extensive Evidence
The sprite's image changes from an apple to a pear when the sprite's scale reaches 2.
Convincing Evidence
There is a conditional in the draw loop that checks whether the sprite's scale has reached 2.
Limited Evidence
There is a conditional in the draw loop that checks the value of a Boolean statement.
No Evidence
There are no conditionals in the draw loop.
Student Instructions
Changing Fruit
Now that we know how to use if statements, you can do more than just check if the apple has reached a scale of 2. You turn it into a pear once it happens.
Do This
Use a conditional in the draw loop to check whether fruit.scale is greater than 2. If it is, change the fruit's animation to "pear".
Challenge: Can you also make the fruit stop growing once it turns into a pear?
- If Statements
- Student Overview
Student Instructions
Visible
Sometimes it's useful to hide the image for a while and then show it again. You can do this using the visible property. The visible property is a little different from other properties you have seen in the past: It is a Boolean value, either true (the sprite is visible) or false (the sprite is not visible). By default, visible is set to true.
Do This
Make the balloon pop when it hits the edge of the game area!
- Add a conditional that checks to see whether the balloon has hit the edge.
- Use a watcher on
balloon.scaleto help you out.
- Use a watcher on
- Create a pop sprite which uses the "pop" visual in the animation tab.
- Use the
visibleproperty to keep the "pop" sprite hidden at the beginning. - Inside the
ifadd two statements.
Student Instructions
Free Play
Use what you've learned to create whatever you like. When you're finished, you can click 
to send your creation to a friend, or 
to send it to your Projects Gallery.
Standards Alignment
View full course alignment
CSTA K-12 Computer Science Standards (2017)
AP - Algorithms & Programming
- 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-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.