Ruby code what does "&:" do here

Question

def reverse_words(s)
    s.split.map(&:reverse).join(' ')
end

This code reverses each word in a sentence. But I do not understand "&:" in the code. Can someone explain that to me?

Check out this question http://stackoverflow.com/questions/1217088/what-does-mapname-mean-in-ruby — sdc, Apr 15 '17 at 01:39
SymbolHound is your friend for hard to Google things like this: http://symbolhound.com/?q=ruby+%26%3A — mu is too short, Apr 15 '17 at 02:08

score 1 · Answer 1 · answered Apr 15 '17 at 01:44

map expects a code block that takes one argument. What you would normally do is to call reverse on that argument:

map {|elt| elt.reverse }

With the & syntax you can shorten this to

map(&:reverse)

The colon is there to make a symbol out of the name reverse.

sdc · Accepted Answer · 2017-04-15T02:50:19.840

0

The & means that reverse is referencing a function, not a block.

This method assumes that the caller will pass it a String object.

First the method #splits the string on whitespaces into an array (if the string has no whitespaces it creates an array with the string as the only element)
Calls the Array#map method on the new array and passes it a reference to the String#reverse method.

The & tells the map method that the input is a reference to a method and not a standard block

edited Apr 15 '17 at 02:50

answered Apr 15 '17 at 01:42

sdc

This has nothing to do with method references. A method reference would be something like `each(&method(:puts))`. Also, this has nothing to do with `map`. `map` doesn't know whether you pass a block directly or via the unary prefix ampersand operator. – Jörg W Mittag Apr 15 '17 at 07:06

2 Answers2