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Ruby Blocks

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Implementing Block Methods

3:30 with Jason Seifer

Blocks are easy to use in our classes. In this video, we'll practice writing a class that mimics arrays.

Code Samples

class MyArray
  attr_reader :array
  def initialize
    @array = []
  end

  def push(item)
    array.push(item)
  end

  def each(&block)
    i = 0
    while i < array.length
      block.call(array[i])
      i += 1
    end
    array
  end
end

my_array = MyArray.new
my_array.push(1)
my_array.push(2)
my_array.push(3)

my_array.each{ |item| puts "item: #{item}" }

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    Now that we know how blocks work. 0:00
    Let's practice a little bit more by re-implementing some methods that we've 0:02
    already seen. 0:06
    Okay. So the next thing that we're gonna do 0:08
    is take a look at a built in method by re-implementing it. 0:09
    And what we're gonna do 0:13
    is create a new class that functions kind of like an array. 0:15
    We're gonna call it my_array.rb. 0:19
    So this is going to need to be a class, MyArray. 0:22
    So we'll define the class, 0:27
    and we'll have the initialize method, and for 0:32
    right now we're going to use the built in array. 0:36
    Let's go ahead make that attr_reader, 0:40
    so that we don't have to use the at sign each time we wanna access it. 0:46
    And so we'll make a push method, so that we can append items to this array. 0:51
    We're not doing anything crazy with it. 0:55
    But what we really want to do is see how the each method works, 0:59
    so let's implement our own. 1:03
    And we can do that by writing the each method. 1:06
    We'll say the each method takes a block. 1:12
    So now, what do we do? 1:16
    Well, in an array, the each method iterates over each item in the array, 1:17
    and we pass a block to that method. 1:23
    So, we could say that the block is called on each item in the array. 1:27
    So, let's go ahead and iterate over each item, and we'll use a while loop for this. 1:32
    We'll say i is zero for our iterator. 1:38
    And while i is less than the array's length, 1:42
    we have a block there, and we return the array. 1:49
    So what goes inside this block? 1:53
    Well, we'll call the block that is passed in 1:55
    on the current item in the array. 2:02
    Now, we could run this right now, but this is currently an infinite loop. 2:08
    What we have to do is increment our iterator variable. 2:13
    So now, let's go ahead and create a new array. 2:20
    And we'll add some items to it. 2:27
    And now we can go ahead and write our block. 2:34
    My array.each|item|, 2:38
    we'll just say, puts "item". 2:42
    Now I'm going to save that and run this by typing ruby my_array.rb. 2:47
    Oh, oh. 2:54
    Undefined local variable or method i for main object. 2:56
    And that's on line 14 called from line 26. 3:01
    And the problem here is that on line 26, I tried to call i 3:08
    instead of item, item is what I'm passing in to the block. 3:13
    So that would be the issue. 3:20
    Let's clear the screen, and run this again. 3:21
    And we can see that this block is executed for each item on my_array. 3:23
    Great job, everybody. 3:29

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