replacing a variable using .each ruby

Question

I am a beginner at Ruby and have been having some trouble figuring out why I can't modify an initialized variable placed outside a block. For example, I want to do the following (but without a for loop because I heard using it in Ruby can cause some nasty bugs):

sentence = "short longest"

def longest_word(sentence)
  max_length = 0
  for word in sentence.split(" ")
    if word.length > max_length
      max_length = word.length
      max_word = word
    end
  end
  return max_word
end

What I tried:

def longest_word(sentence)
    max_length = 0
    sentence.split(" ").each do |word|
        if word.length > max_length
            max_length = word.length
            max_word = word
        end
    end
    return max_word
end

I understand that you can use something like this:

def longest_word(sentence)

     return sentence.split(" ").each.map {|word| [word.length, word]}.max[1]
end

as well but just wanted to figure out why I can't do the .each method in the same way I can do the for loop method. Any help would be appreciated!

"I can't modify an initialized variable placed outside a block" - Huh? You can and you successfully do. With the max_length variable, that is. You just forgot to initialize max_word outside as well. — Stefan Pochmann
– Stefan Pochmann, Commented Jan 24, 2018 at 12:49

Tom Lord · Accepted Answer · 2018-01-24 10:34:47Z

First of all, note that the easiest way to do this would be:

sentence.split(" ").max_by(&:length)

Your method, which can be slightly simplified to:

sentence.split(" ").map {|word| [word.length, word]}.max[1]

also works, but in a more confusing way.

Array comparison works in an “element-wise” manner. So for example, [2, "foo"] > [1, "bar"] because 2 > 1. This is why max works in this scenario: Because you're actually, indirectly, comparing the first element of each array.

Why can't I do the .each method in the same way I can do the for loop method?

Because a variable defined within a block is only accessible within that block.

This is a very common programming principle across virtually all languages, known as the scope of the variable.

def longest_word(sentence)
  max_length = 0
  sentence.split(" ").each do |word|
    if word.length > max_length
      max_length = word.length
      max_word = word  # <--- The variable is defined IN A BLOCK (scope) here
    end
  end
  return max_word      # <--- So the variable does not exist out here
end

A simple workaround (but as I mentioned above, this is not the actual solution I'd recommend!) is to initialise the variable outside the block:

def longest_word(sentence)
  max_length = 0
  max_word = nil   # <--- The variable is defined OUTSIDE THE BLOCK (scope) here
  sentence.split(" ").each do |word|
    if word.length > max_length
      max_length = word.length
      max_word = word
    end
  end
  return max_word  # <--- So the variable exists here
end

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