Functions

What is a function?

A function is a way to "teach" the computer how to do something new.

We can combine several existing steps into one "new" step.

Then, we can re-use that new step, or capability, whenever we want.

For example, say that we want to return to the origin from a given point in the sketch.

Say that we've drawn the following:

... using this code:

// Draw a flagpole
turtle.penUp()
turtle.diagonal(dx: 150, dy: 0)
turtle.penDown()
turtle.diagonal(dx: 0, dy: 300)
turtle.addArc(radius: 5, angle: 360)

We can find out where we are, and our current heading, by using print statements:

We could manually go back to the origin using these commands:

Having to do that manually, however, is tedious and time-consuming.

Instead, we can teach the computer how to do this for us.

Functions are usually defined or described at the top of a program.

Begin by writing a comment to describe the purpose of the function – note the special syntax:

/**
 Return the turtle to the origin from any point on the canvas.
 */ 

Then, use the auto-complete feature to begin writing a function – type func then press the Return key to accept the suggestion:

Next, fill in the function name:

Finally, we describe what we want to happen, inside the body of the function, between the { and } brackets:

To use the function, we must invoke it. Here is what that looks like – we just type the name of the function:

// Go back to origin
goToHome()

Here is the complete program described in this example:

// Create a turtle to draw things for you
// NOTE: Be sure to keep this line of code at the top of your program.
var turtle = Tortoise()

/**
 Return the turtle to the origin from any point on the canvas.
 */ 
func goToHome() {
    // Save current position
    let currentX = turtle.currentPosition().x
    let currentY = turtle.currentPosition().y
    
    // Return to origin
    turtle.diagonal(dx: -1 * currentX, dy: -1 * currentY)
}

// Add your code here to make the turtle draw shapes.
// Begin by typing:
//
// turtle.
//
// ... then see what the turtle can do for you!

// Draw a flagpole
turtle.penUp()
turtle.diagonal(dx: 150, dy: 0)
turtle.penDown()
turtle.diagonal(dx: 0, dy: 300)
turtle.addArc(radius: 5, angle: 360)

// Where are we?
print(turtle.currentPosition())
print(turtle.currentHeading())

// Go back to origin
goToHome()

// Now where are we?
print("--")
print(turtle.currentPosition())
print(turtle.currentHeading())

// Add the shape drawn by the turtle to the sketch
// NOTE: Be sure to keep this line of code at the end of your program.
addShape(drawnBy: turtle)