How do I pass an argument to array.map short cut?

Question

Given the following array a:

a = [1, 2, 3, 4, 5]

How do I do:

a.map { |num| num + 1 }

using the short notation:

a.map(&:+ 1)

or:

a.map(&:+ 2)

where 1 and 2 are the arguments?

Cheating: `[1, 2, 3, 4, 5].map(&:next) # => [2, 3, 4, 5, 6]` — Marc-André Lafortune, Mar 29 '12 at 20:59
You might be interested in Ruby's [Enumerators](http://www.ruby-doc.org/core-2.1.2/Enumerator.html). — alxndr, Jun 10 '14 at 04:54

score 43 · Answer 1 · edited May 23 '17 at 12:02

43

In this case you can do

a.map(&1.method(:+))

But only because 1 + x is usually the same as x + 1.

edited May 23 '17 at 12:02

Community

answered Mar 29 '12 at 21:44

Andrew Grimm

score 36 · Accepted Answer · edited Aug 03 '15 at 18:23

36

You can't do it like this. The & operator is for turning symbols into procs.

a = [1, 2, 3, 4, 5]  
puts a.map(&:to_s) # prints array of strings
puts a.map(&:to_s2) # error, no such method `to_s2`.

& is a shorthand for to_proc:

def to_proc
  proc { |obj, *args| obj.send(self, *args) }
end

It creates and returns new proc. As you see, you can't pass any parameters to this method. You can only call the generated proc.

edited Aug 03 '15 at 18:23

the Tin Man

answered Mar 29 '12 at 20:08

Sergio Tulentsev

you are right: since array.map(&:some_method) is short hand for: class Symbol def to_proc proc { |obj, *args| obj.send(self, *args) } end end. how do you pass in the *args? – not_nil Mar 29 '12 at 20:15
@not_nil: you don't. Use normal block/proc. – Sergio Tulentsev Mar 29 '12 at 20:18
2

@not_nil: Ruby doesn't support point-free style for anything that goes beyond `Symbol#to_proc`, because it lacks function composition and currying. Are you coming from a functional background? – Niklas B. Mar 29 '12 at 20:22
@sergio, thanks for the quick reply. This was more of a academic question than a real life problem. – not_nil Mar 29 '12 at 20:22
1

This is not right. & is not just for turning Symbols into Procs. It works, as far as I know, with anything that implements a `to_proc` method. You can define `to_proc` on `Array` to do what you want. See Boris Stitnicky's answer [here](http://stackoverflow.com/questions/1217088/what-does-mapname-mean-in-ruby) – Sean Mackesey Mar 12 '13 at 19:01
7

@SeanMackesey To be more accurate, `&` does not turn a symbol or anything with `to_proc` to a proc. It turns a proc to a block. Applying `to_proc` is due to implicit class casting. – sawa Mar 13 '13 at 03:58

tokland · Answer 3 · 2014-11-18T20:14:41.657

27

You cannot do it with map. But look at Facets' Enumerable#map_send:

require 'facets'
[1, 2, 3].map_send(:+, 1)
#=> [2, 3, 4]

Writing your own implementation is pretty straightforward:

module Enumerable
  def map_send(*args)
    map { |obj| obj.send(*args) }
  end
end

edited Nov 18 '14 at 20:14

answered Mar 29 '12 at 20:33

tokland

score 0 · Answer 4 · answered May 13 '13 at 13:27

0

If you really need that you can use Ampex library, but I don't know if it is still maintained.

answered May 13 '13 at 13:27

Hauleth

4 Answers4