intro.md

Intro to Ruby

Meet Mr. Turtle! Apart from being a legitimate turtle, he's a Ruby object. And he has some pretty serious artistic talent. Thanks to Mr. Turle will learn how to interact with objects in Ruby (yes! this means programming stuff!) and how to draw some clever shapes.

The very basic concept in Ruby is that everything is an object. We will learn what this means exactly in a little while, but for now it's important to keep in mind that everything is an object here.

Every object has some properties - talents and Mr. Turtle here draws. They all also have a way of communicating with us and are very prone to suggestions of what should they do. So, let's see how to persuade Turtle to do something for us - In the mean time we'll also learn basic concepts of computer programming.

First step

Mr. Turtle will cooperate eagerly and draw whatever you want, but only if you command him in a very precise and specific way. This applies to all programming activities - your programme will run great as far as you follow basic rules.

Here, the proper way to say something to Mr. Turtle is to put it inside the following block:

Turtle.start do
  // your code goes here
end

Everything in the block will be executed by the turtle. For the first example, let's just draw 100 pixels of a straight line. Type in the code into the editor and hit "Run" at the bottom.

Turtle.start do
  forward 100
end

Neat, huh?! But we can certainly do better than that. Square is a thing we're going for. We will need to make some turns in the process, let's see how this looks like.

Turtle.start do
  forward 100
  turnright 90
  forward 100
  turnright 90
  forward 100
  turnright 90
  forward 100
end

For the record - yes, Mr. turtle also turns left if you wish to.

Square was ok, but can you make a triangle?

Loop

Good job on your first triangle! You're definitely ready to take the next step.

Making a square wasn't so difficult but why repeat ourselves. We wrote

  forward 100
  turnright 90

4 times. Instead we can tell Mr. Turtle 'Could you 4 times do the following: go forward 100 pixels and turnright 90 degrees?' How would it look like in Mr. Turtle language?

Turtle.start do
  4.times do
    forward 100
    turnright 90
  end
end

Almost like in our plain English sentence, isn't it? Could you ask Mr. Turtle to draw octagon without repeating yourself?

Good job! Let's do greek-like pattern.

Turtle.start do
  5.times do
    forward 10
    turnleft 90
    forward 10
    turnright 90
    forward 10
    turnright 90
    forward 20
    turnright 90
    forward 20
    turnleft 90
    forward 20
    turnleft 90
  end
end

Variables

What if we want Mr. Turtle to draw line longer (for example 10 pixels) in every next step? We could count it in memory and write something like this:

Turtle.start do
    forward 10
    turnright 90
    forward 20
    turnright 90
    forward 30
    turnright 90
    // etc until we're done
  end
end

That's pretty exhausting. Looks like we're not repeating ourselves so we can't take it into the loop. That's tha moment when variables step in! 'i' would be our increase of the step and we will change it in every step starting from zero.

Turtle.start do
  i = 0
  40.times do
    forward 10
    forward i
    turnright 90
    i += 10
  end
end

Nice trick, a little code that is doing complexed snail. Or maybe it's just a shell of a snail. Could you draw him a body? Simple rectangular in the bottom would make it more similar to snail. Hint: to turn left use command: turnleft. And remeber to tell Mr. Turtle to turn to the right direction before the next step!

Conditionals

And some randomness.

Turtle.start do
  100.times do |i|
    ang = rand(360)
    turnright ang
    len = rand(40)
    forward len
    goto width/2, height/2 if(i % 10 == 0)
  end
end

Everything together

Turtle will draw the Earth.

Turtle.start do
  goto width/4, height/2
  360.times do |i|
    forward 2
    turnright 1
  end
  
  pencolor blue
  r = 120
  30.times do
    goto rand(width), rand(height)
    goto width/2, height/2
    penup
    turnright rand(360)
    forward r + rand(100)
    len = 5 + rand(30)
    pendown
    5.times do
      forward len
      turnleft 180-360/10
    end
  end
  
  goto width/2, height/2
end