Activity: Assignment Operators

Assignment Operators are assignments that also perform an operator on the variable. They are used in JavaScript to modify the value.

Concept: ||variables:change by||

The ||variables:change by|| block was very commonly used to modify the value a variable was assigned to when programming in Blocks.

This can be accomplished in JavaScript by using the variable itself in the equation:

num = num + 5;

This works because the equation on the right side of the assignment operator is evaluated first, and the result of that equation is assigned to the variable on the right side.

For example, if ||variables:num|| stored 4 before the code above, the right hand side of the equation would first be evaluated to 9, and then ||variables:num|| would be reassigned 9 as it’s new value.

Modifying a value in this way is a very common task, so JavaScript and other languages introduce the += operator, which will add the value on the right side to the value currently stored on the left side.

Using the += operator, the example code above can also be written as

num += 5;

This process of condensing the expression extends to all operators, so…

• num -= 5; is equivalent to num = num - 5;
• num *= 5; is equivalent to num = num * 5;
• num /= 5; is equivalent to num = num / 5;

These are called Assignment Operators, like =.

Example #1: Seconds in an Hour

1. Review the code below
2. Observe how assignment operators are used to modify the ||variables:seconds|| variable
3. Identify the two values that are ||game:splashed||

let seconds: number = 1;
seconds *= 60;
game.splash(seconds + " seconds in a minute");
seconds *= 60;
game.splash(seconds + " seconds in an hour");

Student Task #1: Seconds in a Week

1. Start with the code from example #1
2. After the second ||game:game.splash||, use an assignment operator to make ||variables:seconds|| store the number of seconds in a day
3. ||game:Splash|| the new value of ||variables:seconds||. Make sure to include a description of the value to match the previous values that were ||game:splashed||
4. Use another assignment operator to make ||variables:seconds|| store the number of seconds in a week
5. ||game:Splash|| the new value of ||variables:seconds||, along with it’s corresponding description
6. Challenge: repeat this process for both the seconds in a month, and seconds in a year

Concept: Increment and Decrement

In JavaScript, it is very common to add or subtract one from a value: for example, to count the number of times a button was pressed, or the index in a loop.

In JavaScript, there are several Assignment Operators that can do this. For example, both num = num + 1; and num += 1; will result in ||variables:num|| being incremented by 1.

However, because this is such a common task, there is an even shorter way to write it. This is done using the Increment Operator, represented by two plus signs. num++; is equivalent to the two expressions above.

Similarly, the Decrement Operator is represented by two minus signs, meaning num = num - 1; is the same as both num -= 1; and num--;.

Example #2: Incrementing Values

1. Review the code below
2. Identify how the increment operator is used to change the value of ||variables:myNumber||

let myNumber = 5;
game.splash(myNumber + " is too low!");
myNumber++;
myNumber++;
game.splash(myNumber + " is too high!");

Student Task #2: Just Right

1. Start with the code from example #2
2. Use the decrement operator to lower of ||variables:myNumber|| by 1
3. ||game:Splash|| ||variables:myNumber|| with the description “ is just right!”

Concept: Appending Strings

The Assignment Operator ||math:+=|| can also be useful when adding to strings.

Values can be appended using the ||math:+=|| operator. For example, the following snippet:

let phrase: string = "hello";
phrase = phrase + " world";

can also be written:

let phrase: string = "hello";
phrase += " world";

This allows for strings to be “built” by adding pieces to them as necessary.

Example #3: Food Order

1. Review the code written below
2. Identify how the ||math:+=|| operator is used to build up an ||variables:order|| string

let order: string = "I would like to eat ";
let food: string = game.askForString("What would you like to eat?");
order += food;
order += ". I would also like to drink ";

let drink: string = game.askForString("What would you like to drink?");
order +=  drink;
order +=  ".";

game.splash(order);

Student Task #3: Dessert

1. Copy the code from the example above
2. Modify the code so that it also prompts the user “What dessert would you like”
3. Append to ||variables:order|| the sentence “ For dessert, I would like “ followed by the dessert the user requested
4. Append a “.” to end the sentence
5. Challenge: instead of storing the variables ||variables:food||, ||variables:drink||, and ||variables:dessert||, append the result of ||game:game.askForString|| directly to ||variables:order||

What did we learn?

1. Explain how the different increment, decrement, and assignment operators can be useful for modifying a number ||variables:variable||.

namespace SpriteKind {
    export const Asteroid = SpriteKind.create();
}

namespace asteroids {
    sprites.onCreated(SpriteKind.Asteroid, function (sprite: Sprite) {
        sprite.setFlag(SpriteFlag.AutoDestroy, true);
        setPosition(sprite, 10);
        setMotion(sprite);
    });

    game.onUpdateInterval(1500, function () {
        sprites.create(sprites.space.spaceAsteroid0, SpriteKind.Asteroid);
    });

    function setMotion(asteroid: Sprite) {
        asteroid.vx = randint(-8, 8);
        asteroid.vy = randint(35, 20);
    }

    function setPosition(sprite: Sprite, edge: number) {
        sprite.x = randint(edge, screen.width - edge);
        sprite.y = 0;
    }
}

let name: string = "Captain ";
name += game.askForString("What is your name?");

let intro: string = "Hello, ";
intro += name;
intro += "! This is my Space Game!";
game.splash(intro);

let x: number = screen.width / 2;
let y: number = screen.height - 20;