Teaching Ruby to a 9-Year-Old Part 3: Let's Program a Robot

This is part of a series of posts titled "Teaching Ruby to a 9-Year-Old."  See the Introduction for more information.

Computers Need to be Taught

Computers are giant calculators, and they can do billions of math problems every second.  They're also good at doing the same thing over and over.  But remember, they speak Computer Talk, not English.  And they have to be taught to do everything.  This can be a lot of work, but it's worth it in the end.

With Ruby, you can tell the computer to make decisions and do things.  We need to be careful how we tell it, though, because of the way computers think.  Let's look at a couple examples.

When Mom and Dad tell you, "Go put on your shoes for school," what are they really telling you to do?  In our house, it means:

1. Put on your backpack
2. Grab your water bottle
3. Go out into the garage
4. Put on your shoes

You know this because Mom and Dad taught you that.  (Sometimes Mom and Dad will remind you, and they'll even tell you all of those steps one by one!)  Remember, computers are dumb and have to be told everything.  Everything!  

Programming a Robot

So let's pretend we have a robot, which is really just a computer shaped like a human, and let's teach it how to do things.

Let's say that the robot already knows what things are.  It just doesn't know how to do stuff yet.  In Ruby, we'll put a list of actions together and define that list as something called a function.  (Define means to explain what something means.  For example, I just defined the word define.  Get it?)

Defining "Put on shoes for school" in Ruby

Now we've taught our robot something:  "If I say put_on_shoes_for_school, then grab_backpack, grab_water_bottle, go_into_garage, place_shoes_on_feet, that's it."

Notice that there are no spaces in the actions.  Also notice that you say "that's it" by writing "end".

If you programmed this into your robot, and then you gave your robot a command:  "put_on_shoes_for_school", what do you think the robot would do?  Let's try it using Ruby!

Step 1 - Defining function put_shoes_on_for_school

Okay, this part worked great.  Ruby gave us a fist-bump.  Let's tell it to put_on_shoes_for_school now.

Uh oh!  Why did Ruby barf?

Oh no!  It looks like Ruby didn't like what you said.  What do you think happened?

Remember how we learned that computers have to be told how to do everything?  Let's look closer at what we told it to do.  When we said put_on_shoes_for_school, Ruby said, "Okay, I'll start by grab_backpack.  Oh no!  I don't know how to grab_backpack!  I'm going to barf now!"

See?  We gave Ruby a list of things to do, but we didn't tell it how to do all of those things.  Make sense?  Here's what a more complete program might look like:

So now, if you told the robot to grab_backpack, grab_water_bottle, or go_into_garage, it knows what to do!

Oh my gosh, do you see what just happened?  Somehow you've ended up learning about a whole page of computer code!  If you showed this code to most grownups, they'd tell you it's too hard to figure out.  Just tell them all you're doing is defining some functions, duh!  We haven't even done the hard stuff yet!