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Why avoid Ruby's for .. in?

Renan Lourençoni Nobile on September 19, 2018

So I'm starting to learn Ruby and found a great resource, Learn Ruby in Y minutes. It's great, very fast paced as I expected, since I'm an experien...

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Dan Fockler • Sep 19 '18

One thing to think about is that the for counter in 1..5 is really only used for looping syntax, whereas most other ruby Enumerable methods like #map and #reduce have similar syntax as #each, so consistency is likely a factor in why people prefer one over the other.

Renan Lourençoni Nobile • Sep 19 '18

Looking from that perspective it's a good option to use each.

Thank you

Andy Zhao (he/him) • Sep 19 '18

Given this:

banana = { name: "banana", flavor: "sweet", texture: "soft" }
apple = { flavor: "sweet", texture: "crispy" }
grapefruit = { name: "grapefruit", flavor: "sour", texture: "juicy" }

fruit_basket = [banana, apple, grapefruit]

for fruit in fruit_basket
  puts "#{fruit[:name]} is #{fruit[:flavor]}"
end

fruit_basket.each do |fruit|
  puts "#{fruit[:name]} is #{fruit[:flavor]}"
end

I would say, yeah there's not much of a difference in terms of readability.

BUT there is a good reason. Try running this code example:

Basically, for..in loop can redefine things that you didn't intend to, and that can cause problems.

Reference: Tutorials Point - Ruby Loops

Renan Lourençoni Nobile • Sep 20 '18

Hey Andy, I just ran the code and it really makes much more sense to use each method. I believe there are use cases for the for .. in syntax right?

Anyway, thank you for the information.

Renan Lourençoni Nobile • Sep 19 '18

I'm on mobile right now, when I get a chance I'll run it on my laptop, but knowing that both loops act differently and for in might cause inconstancy is a great point towards each.

Thank you

John Carroll • Sep 20 '18 • Edited

I believe there are use cases for the for .. in syntax right?

So I don't have anything against the for .. in syntax personally, but no, there are no special use cases for it. I mean, I'm sure if you tried you could construct a situation where for might be preferable to .each, but really, it's presence in the language at this point is just vestigial.

I'm not sure I've ever seen it used outside of a tutorial.

I was amused to see that Crystal-lang went so far as to remove it altogether.

Ben Halpern • Sep 19 '18

Performance differences seem negligible gist.github.com/jodosha/229951

As for better/cleaner, Rubyists definitely love .each, and for practical purposes, it's probably worth just drinking the kook-aid rather than stirring things up with Ruby dogma.

But I'm happy to hear the bikeshedding on this. 😄

Philip Hallstrom • Sep 22 '18

I've been writing Ruby for 12-13 years and have never once used or even come across for in real life. Definitely bikeshedding :)

More seriously though, you can't pass a lambda to a for loop like you can each that I'm aware of. Which is a huge part of Ruby's Enumerable ecosystem.