What does string * '' mean in Ruby?

Question

I was looking through some Rails source code and came across

# File vendor/rails/activesupport/lib/active_support/vendor/builder-2.1.2/builder/css.rb, line 129
129:     def target!
130:       @target * ''
131:     end

What does the * '' do? Is that multiplication by an empty string...? And why would you do that.

Answer 1

This is a bizarre syntax. These are equivalent:

>> [1, 2, 3] * 'joiner'
=> "1joiner2joiner3"

>> [1, 2, 3].join 'joiner'
=> "1joiner2joiner3"

so in this case it joins all the entries of @target into one string, with nothing between the entries.

Note: if you do something like [1, 2, 3] * 3 (using an int instead of a str), you'll get three concatenated copies of the array instead.

Answer 2

It does the same thing as:

["foo","bar","baz"].join

http://ruby-doc.org/core/classes/Array.html#M002210

Per Z.E.D.'s suggestion, you would use it if you want to confuse people and make your code more error prone.

Answer 3

Really cryptic code indeed.

After checking the source code, I realized that @target is actually an Array instance, I know you can do stuff like this

[5] * 5 # => [5,5,5,5,5]

I don't know where Array#* is defined (maybe in ActiveSupport), but what I can tell you is that, this is the behaviour when it gets multiplied by a String

[1,2,3] * 'comma' # => "1comma2comma3"
[1,2,3] * '' # => '123'

So I can infer it is concatanating all the elements of the array without any separators.

Answer 4

Two comments:

1. Having a ! end a method name implies that it's a mutating operation, which this example doesn't seem to be.

2. As others have stated it's indeed cryptic. I would go for @target.to_s or @target.join

Answer 5

Array#* with a String argument is equivalent to Array#join.