Activity: Boolean Statements and Expressions

In the previous lessons, we saw how ||logic:if|| and ||logic:else|| statements allow us to write code that can handle different situations by making comparisons between different numbers.

Comparison statements are centered around Boolean Logic, with the conditions evaluating to either ||logic:true|| or ||logic:false||.

The values of these expressions are stored and used as boolean variables. Whenever we see the term boolean, it means we have something that can evaluate to ||logic:true|| or ||logic:false||.

In these activities students will work with:

  • Boolean flags and values (||logic:true|| and ||logic:false||)
  • ||logic:and||
  • ||logic:or||
  • ||logic:not||

Concept: Boolean Flags

Boolean values are regularly used to help maintain the state of a given piece of code. It is common to describe boolean variables as “boolean flags” - these often are used to turn on and off different behaviors that might be useful.

For example, ||sprites:stay in screen|| is a flag that we have set that forces the sprite to stay within the bounds of the screen when set to ||logic:true||.

Link to Video

Example #1: Are you hungry?

  1. Review the code below
  2. Create the sample code and run the code
  3. Identify what needs to be done in game to ||game:splash|| “You’re hungry!”, and what needs to be done to ||game:splash|| “You’re not hungry!”
let isHungry = false
controller.B.onEvent(ControllerButtonEvent.Pressed, function () {
    if (isHungry) {
        game.splash("You're hungry!")
    } else {
        game.splash("You're not hungry!")
    }
})
controller.A.onEvent(ControllerButtonEvent.Pressed, function () {
    isHungry = true
})
game.splash("Press A if you are hungry!")
isHungry = false

In this example, ||variables:isHungry|| is a flag that will change the behavior of the ||controller:on B button pressed|| event. When it is off (set to false), the game assumes that you are not hungry; when you turn it on (set it to true), though, it tells the game that you are hungry, and it reacts to this information.

Student Task #1: Off and On

  1. Start with the code from example #1a
  2. Modify the ||controller:on A button pressed|| event so that it switches the value of ||variables:isHungry|| from ||logic:true|| to ||logic:false|| or from ||logic:false|| to ||logic:true||, by using an ||logic:if else|| block

Concept: ||logic:not||

When we have a boolean value, we have seen that we can write code that runs when it is true using a simple ||logic:if|| statement. However, we can also run code for when the boolean is false. This is done with the ||logic:not|| block.

Link to Video

Example #2: ||logic:not||

Example #2

let mySprite: Sprite = null
let isHungry = false
controller.A.onEvent(ControllerButtonEvent.Pressed, function () {
    isHungry = true
})
controller.B.onEvent(ControllerButtonEvent.Pressed, function () {
    isHungry = false
})
isHungry = true
mySprite = sprites.create(img`
. . . . . . 5 . 5 . . . . . . . 
. . . . . f 5 5 5 f f . . . . . 
. . . . f 1 5 2 5 1 6 f . . . . 
. . . f 1 6 6 6 6 6 1 6 f . . . 
. . . f 6 6 f f f f 6 1 f . . . 
. . . f 6 f f d d f f 6 f . . . 
. . f 6 f d f d d f d f 6 f . . 
. . f 6 f d 3 d d 3 d f 6 f . . 
. . f 6 6 f d d d d f 6 6 f . . 
. f 6 6 f 3 f f f f 3 f 6 6 f . 
. . f f d 3 5 3 3 5 3 d f f . . 
. . f d d f 3 5 5 3 f d d f . . 
. . . f f 3 3 3 3 3 3 f f . . . 
. . . f 3 3 5 3 3 5 3 3 f . . . 
. . . f f f f f f f f f f . . . 
. . . . . f f . . f f . . . . . 
`, SpriteKind.Player)
game.onUpdateInterval(5000, function () {
    if (!(isHungry)) {
        mySprite.say("I'm not hungry", 2000)
    }
})
  1. Review the code example above
  2. Examine its use of the ||logic:not|| block
  3. Notice how the ||sprites:sprite|| will only say something if the boolean is not ||logic:true||

Student Task #2: ||logic:not|| Left (is Right)

  1. Start a new project
  2. Create a ||sprites:sprite|| and use the ||controller:move mySprite with buttons|| block to move the sprite around the screen
  3. Make a variable named ||variables:isLeft|| and set it equal to ||logic:false||
  4. In the ||game:on game update|| block, assign ||variables:isLeft|| to a ||logic:0 < 0|| comparison. Replace the first 0 with the ||sprites:sprites|| ||sprites:x|| position, and the second 0 with 80
  5. Make it so that if the player presses the ||controller:A|| button, if ||variables:isLeft|| is ||logic:false||, then the sprite should ||sprites:say|| something

Concept: Alternating Booleans

We can also use the ||logic:not|| block when assigning a boolean. So if we wanted to, we could assign a boolean to be the opposite of another boolean. Setting a boolean to the opposite value of itself, ‘switches’ the variable state from ||logic:true|| to ||logic:false|| (or false to true).

Link to Video

Example #3: Alternating Booleans

let projectile: Sprite = null
let left = false
controller.A.onEvent(ControllerButtonEvent.Pressed, function () {
    left = !(left)
})
left = false
game.onUpdateInterval(500, function () {
    if (left) {
        projectile = sprites.createProjectile(img`
. . . . . . . . . . . . . . . . 
. . . . . . . . . . . . . . . . 
. . . . . . . . . . . . . . . . 
. . . . . 9 9 . . . . . . . . . 
. . . . . 9 . 9 9 . . . . . . . 
. . . . . 9 . . . 9 9 . . . . . 
. . . . . 9 . . . . . 9 9 . . . 
. . . . . 9 . . . . . . . 9 9 . 
. . . . . 9 . . . . . . . . . 9 
. . . . . 9 . . . . . . . 9 9 . 
. . . . . 9 . . . . . 9 9 . . . 
. . . . . 9 . . . 9 9 . . . . . 
. . . . . 9 . 9 9 . . . . . . . 
. . . . . 9 9 . . . . . . . . . 
. . . . . . . . . . . . . . . . 
. . . . . . . . . . . . . . . . 
`, Math.randomRange(50, 100), 0, SpriteKind.Player)
    } else {
        projectile = sprites.createProjectile(img`
. . . . . . . . . . . . . . . . 
. . . . . . . . . . . . . . . . 
. . . . . . . . . 7 7 . . . . . 
. . . . . . . 7 7 . 7 . . . . . 
. . . . . 7 7 . . . 7 . . . . . 
. . . 7 7 . . . . . 7 . . . . . 
. 7 7 . . . . . . . 7 . . . . . 
7 . . . . . . . . . 7 . . . . . 
. 7 7 . . . . . . . 7 . . . . . 
. . . 7 7 . . . . . 7 . . . . . 
. . . . . 7 7 . . . 7 . . . . . 
. . . . . . . 7 7 . 7 . . . . . 
. . . . . . . . . 7 7 . . . . . 
. . . . . . . . . . . . . . . . 
. . . . . . . . . . . . . . . . 
. . . . . . . . . . . . . . . . 
`, Math.randomRange(-100, -50), 0, SpriteKind.Player)
    }
    projectile.y = Math.randomRange(0, scene.screenHeight())
})
  1. Review the code example above
  2. Examine its use of the ||logic:not|| block
  3. Notice how the ||logic:not|| block is used to alternate the value of the boolean ||variables:left||

Student Task #3: Alternating Booleans

  1. Create a new project
  2. Add create a new ||sprites:sprite||
  3. Create a new boolean ||variables:pizza|| and set it equal to ||logic:true||
  4. Make this boolean alternate values when the player presses the ||controller:B|| button
  5. When the player presses the ||controller:A||, if ||variables:pizza|| is ||logic:true||, then fire a pizza ||sprites:projectile||, otherwise fire a burger ||sprites:projectile||

Concept: ||logic:and|| and ||logic:or||

Beyond just using a single boolean value, there are several ways in which we can combine these values. The most commonly used are ||logic:and|| and ||logic:or||. These are used similarly to their usage in the English language; for example, the statement

If I have food and I am hungry, I will make lunch

means that if both conditions (there being food, and being hungry) are true, then lunch will be made. If either is false, though, no lunch will be made.

Alternatively, the statement

If I need to buy milk or I need to buy eggs, I will go to the store

means that if either condition is true, I will go to the store. If I don’t need to buy milk and I also don’t need to buy eggs, then I will not end up going to the store.

Link to Video

Example #4a: ||logic:and||

  1. Review the code below
  2. Create the sample code and run the code
  3. Identify which combinations of true and false evaluate to true, and which evaluate to false
if (true && true)   game.splash("true and true is true")
else                game.splash("true and true is false")

if (true && false)  game.splash("true and false is true")
else                game.splash("true and false is false")

if (false && false) game.splash("false and false is true")
else                game.splash("false and false is false")

Example #4b: ||logic:or||

  1. Review the code below
  2. Create the sample code and run the code
  3. Identify which combinations of true or false evaluate to true, and which evaluate to false
if (true || true)   game.splash("true or true is true")
else                game.splash("true or true is false")

if (true || false)  game.splash("true or false is true")
else                game.splash("true or false is false")

if (false || false) game.splash("false or false is true")
else                game.splash("false or false is false")

Student Task #4: Using ||logic:and|| and ||logic:or||

  1. Create a new project
  2. Create a sprite, and use ||controller:move mySprite with buttons|| to make it move when the directional keys are pressed
  3. In the ||game:on game update|| block, add an ||logic:if|| statement that makes the sprite ||sprites:say|| “hello” for 200 milliseconds when the ||sprites:sprite x|| value is less than 30 ||logic:and|| ||sprites:sprite y|| is less than 30.
  4. In the ||game:on game update|| block, add another ||logic:if|| statement that makes the sprite |sprites:say|| “bye” for 200 ms when the ||sprites:sprite x|| is greater than 80 ||logic:or|| the ||sprites:sprite y|| is greater than 80
  5. Challenge: create the same behavior while using one or more ||logic:not|| blocks

What did we learn?

  1. What is a boolean flag? Describe a case in which you might you want to use one?
  2. What is the difference between an ||logic:and|| block and an ||logic:or|| block?
    • In what cases will they both evaluate to true?
    • In what cases will they both evaluate to false?
    • In what cases will they evaluate differently?

Teacher Material