Ruby 103: Iterators & Blocks

Iterators

Iteration is the process of programatically interacting with a collection of values, one at a time. We call it “looping” sometimes, and we do a lot of it. Here is a simple loop; fire up irb and give it a try:

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10.times { |n| puts n } # So... what happened

Question: What is times?

We can iterate all kinds of things, but probably the most common and most fun is iterating an Array. They’re especially suited for iteration because they, by default, have a objective order to their elements. Here’s an example of iterating an Array using the each method:

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[10, 20, 30, 40, 50].each do |number|
  puts number * 10
end

Cool, right? each iterated the Array, starting with the first value, multiplied the value by 10, and spit it out. It knew what to do on each step of the iteration because we provided a block of instructions.

Let’s talk about Blocks…

Blocks are not a unique features of Ruby, but they’re one that many people talk about (and more people use without realizing it!) What is a block?

A section of code which is grouped together.

Zzzzz….

A block is a piece of code that is not executed immediately, but is stored in a variable or passed to a method, and is potentially executed at a later moment in time.

Sound complicated? Let’s look at some Ruby.

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[1,2,3,4].each do |number|
  puts number * 100 # <= this is a block. :)
end

[1,2,3,4].each { |number|
  puts number * 100 # <= this is a block too! :D
}

number * 100 is a chunk of code, similar to what would be in a method, but in this case we don’t need to go through all the trouble of creating a method to iterate over some numbers.

Block can be identified by either do ... end or { }.

Block arguments are defined using a pair of | (pipe) characters. They look like a slide, so I like to say that we slide arguments into the block. For example:

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[1,2,3].each do |number|
  puts number # wheeeee! Right down the slide!
end

number is the defined argument, this means that objects given to the block are going to be assigned to the number variable within the scope of the block.

Exercise: Walk Through a Loop:

It’s very important to understand what’s happening during iteration. The best way to get comfortable with loops is to make a chart showing how values change during iteration. Here’s an example of a loop and it’s corresponding value chart:

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[11,5,8,100].each do |n|
  puts n * 4
end

With a pair, figure out what’s happening on each iteration of the following loop, on paper, using the handy table template:

You’ll need more rows, but you get the idea.

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y = 0
[9,8,7,6,5,4,3,2,1].each do |x|
  if x % 2 == 0
    y = y + x
    puts y
  else
    puts x
  end
end