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?

Answer 1

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.