Random Numbers

What is a random number?

Have you ever asked a friend to pick a number between 1 and 10?

We can ask a computer to do this, as well. That is a random number.

While computers cannot easily provide truly random numbers (why that is so is another story) for our purposes, simulated randomness is good enough.

Applying random numbers

Say that we wanted to draw what looks a starfield.

We can begin by drawing a large rectangle to create a night sky:

// Set background
turtle.penColor = .white
turtle.fillColor = .black

// 1. Draw big rectangle to set "background" colour
// 
// Get to bottom left corner of rectangle 
turtle.penUp()
turtle.diagonal(dx: -1000, dy: -700)
// Draw the rectangle
turtle.penDown()
for i in 1 ... 2 {
    // Draw long edge
    turtle.forward(distance: 1000 * 2)
    turtle.left(angleInDegrees: 90)
    // Draw short edge
    turtle.forward(distance: 700 * 2)
    turtle.left(angleInDegrees: 90)
}

... to obtain results like this:

Next, we can draw a "star" by making a cross at the origin, with the following addition to the code we already wrote:

// Go back to the origin
turtle.penUp()
turtle.diagonal(dx: 1000, dy: 700)

// 2. Draw a star right at the origin
// Set fill colors
turtle.penColor = .yellow
turtle.fillColor = .clear
// Use random numbers to select height and width
let halfHeight = Double.random(in: 15...30)
let halfWidth = Double.random(in: 10...20)
// Draw the star in the shape of a cross
turtle.penDown()
turtle.left(angleInDegrees: 90)
for i in 1 ... 2 {
    // Draw half the height
    turtle.forward(distance: halfHeight)
    turtle.backward(distance: halfHeight)
    turtle.right(angleInDegrees: 90)
    // Draw half with width
    turtle.forward(distance: halfWidth)
    turtle.backward(distance: halfWidth)
    turtle.right(angleInDegrees: 90)
}

Try out this code to see the result.