what is the functionality of "&: " operator in ruby? [duplicate]

Question

Possible Duplicate:
What does map(&:name) mean in Ruby?

I came across a code snippet which had the following

a.each_slice(2).map(&:reverse)

I do not know the functionality of &: operator. How does that work?

Awl

This is a duplicate of no less than 17 other questions that have already been asked and answered here on StackOverflow: Understanding [ClassOne, ClassTwo].each(&:my_method), What does map(&:name) mean in Ruby?, What exactly is &:capitalize in Ruby?, Ruby/Ruby on Rails ampersand colon shortcut, Ruby : &:symbol syntax, … – Jerz

… What is this &:last Ruby Construct Called?, What do you call the &: operator in Ruby?, What does map(&:name) do in this Ruby code?, What are :+ and &+ in ruby?, &:views_count in Post.published.collect(&:views_count), Ruby Proc Syntax, How does “(1..4).inject(&:+)” work in Ruby, … – Jerz

… What does following statement &:property ?, What does the & mean in the following ruby syntax?, Why would one use the unary operator on a property in ruby? i.e &:first, how does Array#map have parameter to do something like this?, and what does &: mean in ruby, is it a block mixed with a symbol?. – Jerz

Answer 1

There isn't a &: operator in Ruby. What you are seeing is the & operator applied to a :symbol.

In a method argument list, the & operator takes its operand, converts it to a Proc object if it isn't already (by calling to_proc on it) and passes it to the method as if a block had been used.

my_proc = Proc.new { puts "foo" }

my_method_call(&my_proc) # is identical to:
my_method_call { puts "foo" }

So the question now becomes "What does Symbol#to_proc do?", and that's easy to see in the Rails documentation:

Turns the symbol into a simple proc, which is especially useful for enumerations. Examples:

# The same as people.collect { |p| p.name }
people.collect(&:name)

# The same as people.select { |p| p.manager? }.collect { |p| p.salary }
people.select(&:manager?).collect(&:salary)

Twelfthtide

@kikito: ruby-doc.org/core-1.9.3/Symbol.html#method-i-to_proc – Meadowsweet

@kikito: actually it was first implemented in Rails, but everyone loved it so it made its way into the core. – Meadowsweet

Is there a way to pass and argument to the method being called? salary for example – Stayathome

Answer 2

By prepending & to a symbol you are creating a lambda function that will call method with a name of that symbol on the object you pass into this function. Taking that into account:

ar.map(&:reverse)

is roughly equivalent to:

ar.map { |element| element.reverse }

Tactile