#hello ruby

A delightful way to teach kids about computers | Linda Liukas

Computer code is the next universal language, and its syntax will be limited only by the imaginations of the next generation of programmers. Linda Liukas is helping to educate problem-solving kids, encouraging them to see computers not as mechanical, boring and complicated but as colorful, expressive machines meant to be tinkered…

Linda Liukas - Hello Ruby

Dostałem właśnie wersję w PDF i jaram się jak dziecko! :-)

Ciekawe jak wyjdzie edycja drukowana, a wysyłka ma zacząć się już niebawem.

I recently funded the “Hello Ruby" programming book on Kickstarter to help teach my kids programming. I was asked if I want a personal message in a custom sketch. This is the message I requested:

%w[i love you].each{|m|eval "def #{m}(n=nil);\"#{m} \#\{n ?n :'too'\}\";end"}

Try it out in irb, and then type:

i love you

I hope my kids will like it. ☺

Explanation of the code

This code has a number of things going on with it. Let’s take some time to break the code down. First of all, let’s expend the code out to multiple lines, expand out the ternary operator to an if statement and give the variables sane (read: not one-letter) names:

%w[i love you].each do |method_name| eval <<-eos def #{method_name}(parameter = nil) second_word = if parameter != nil parameter else 'too' end "#{method_name} \#\{second_word\}" end eos end

Hopefully, that is a little clearer, but let’s break it down.

First off, what the heck does %w[i love you] do? Well, this is a shortcut for creating an array. So:

%w[i love you] == ['i', 'love', 'you']

Next up is the each statement. It will take whatever is inside the follow block and executes it for each string in the array [‘i’, ‘love’, ‘you’], passing in the string to the variable “method_name.” That means for each iteration “method_name" will equal something different:

method_name = 'i' # iteration 1 (first) method_name = 'love' # iteration 2 method_name = 'you' # iteration 3 (last)

Next up is the eval statement. In Ruby (and many other languages), the eval command takes a string and turns it into a code. Eval is short for evaluate. So the following string will be turned into actual code, but with the “method_name" variable interpreted:

def #{method_name}(parameter = nil) second_word = if parameter != nil parameter else 'too' end "#{method_name} \#\{second_word\}" end

The trickiest bit here is the escaped “\#\{second_word\}”. After this is evaluated it expands to “#{second_name}.” We need this because second_name is a local variable in the method, and needs to be interpreted when it is called, not when it is created.

Note: the «-eos … eos in the expanded code is a way in Ruby to create a multi-line string.

So this code will be evaluated three times, making three methods:

# iteration 1 (first) def i(parameter = nil) second_word = if parameter != nil parameter else 'too' end "i #{second_word}" end # iteration 2 def love(parameter = nil) second_word = if parameter != nil parameter else 'too' end "love #{second_word}" end # iteration 3 (last) def you(parameter = nil) second_word = if parameter != nil parameter else 'too' end "you #{second_word}" end

Next, you see that the parameter defaults to nil, and second_name is then assigned to the result of the following if block:

if parameter != nil parameter else 'too' end

This is the same as the following ternary operation that you can see in the one-liner:

parameter ?parameter :'too'

It says “if you passed in a parameter, set the second word to that, otherwise set the second parameter to the string ‘too.’”

The last line of all the methods simply returns the name of the method plus the second word, or ‘too’ if you pass nothing into it.

That means the following method calls produce the following strings:

you() == "you too"

and of course:

love("you too") == "love you too"

therefore:

love(you()) == "love you too"

and:

i("love you too") == "i love you too"

thus:

i(love(you())) == "i love you too"

But in Ruby, parentheses are optional making:

i(love(you())) == (i love you)

And that is why typing i love you in irb will produce the string “i love you too”