Ruby or Rails array iteration syntax

Question

I came across a way in which a program extracted the name property of an object and found the syntax a little peculiar. This is dealing with the results of JSON response.

Our JSON response would be the following =

[{"id"=>9, "name"=>"Baked Potato w/ Cheese", "instructions"=>nil}, 
{"id"=>12, "name"=>"Baked Brussel Sprouts", "instructions"=>nil}]

results = JSON.parse(response.body)

def extract_name
  ->(object) { object["name"] }
end

results.map(&extract_name)

So I understand that results.map(&extract_name) returns the name of the JSON objects, I just don't understand how.

I'm unfamiliar with the ->(object) { object["name"] } syntax. Are there are other shorthand ways of doing this that may help me get a better idea of this type of syntax?

@sschmeck's answer is good, but it's worth noting that this isn't great code. This code initializes a new lambda every time extract_name is called. It would be better to assign the lambda to a variable or constant, e.g. EXTRACT_NAME = ->(object) { object["name"] }; results.map(&EXTRACT_NAME). — Jordan Running
– Jordan Running, Commented Feb 5, 2016 at 22:40
@Jordan: Are you sure about that? I'm pretty sure that e.map(&m) will be parsed as e.map(&(m())) so m will only be called once and then & will be applied to its return value only once. — mu is too short
– mu is too short, Commented Feb 5, 2016 at 22:47
Yes, you're correct, but what I wrote is "This code initializes a new lambda every time extract_name is called." It won't be called for every iteration, but it will be called every time results.map(&extract_name) is called. — Jordan Running
– Jordan Running, Commented Feb 5, 2016 at 22:49
@Jordan So if I were to write two test's each one running the results.map(&extract_name), I will be initializing two lambdas as opposed to just setting it to a constant where it is only initialized once? To my understanding, this is more efficient yes? — Nappstir
– Nappstir, Commented Feb 6, 2016 at 2:00

Community · Accepted Answer · 2017-05-23 12:15:38Z

2

The arrow -> is a short syntax to create lambas. See "What do you call the -> operator in Ruby?".

An alternative way could be the following snippet:

results.map { |object| object["name"] }

edited May 23, 2017 at 12:15

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answered Feb 5, 2016 at 21:51

sschmeck

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1 Comment

Jordan Running Over a year ago

object[:name] should be object["name"] for equivalence with OP's code.

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