What does %w(array) mean?

%w(foo bar) is a shortcut for ["foo", "bar"]. Meaning it's a notation to write an array of strings separated by spaces instead of commas and without quotes around them. You can find a list of ways of writing literals in zenspider's quickref.

---

I think of %w() as a "word array" - the elements are delimited by spaces and it returns an array of strings.

Here are all % literals:

%w() array of strings

%r() regular expression.

%q() string

%x() a shell command (returning the output string)

%i() array of symbols (Ruby >= 2.0.0)

%s() symbol

%() (without letter) shortcut for %Q()

The delimiters ( and ) can be replaced with a lot of variations, like [ and ], |, !, etc.

When using a capital letter %W() you can use string interpolation #{variable}, similar to the " and ' string delimiters. This rule works for all the other % literals as well.

abc = 'a b c'
%w[1 2#{abc} d] #=> ["1", "2\#{abc}", "d"]
%W[1 2#{abc} d] #=> ["1", "2a b c", "d"]

---

There is also %s that allows you to create any symbols, for example:

%s|some words|          #Same as :'some words'
%s[other words]         #Same as :'other words'
%s_last example_        #Same as :'last example'

Since Ruby 2.0.0 you also have:

%i( a b c )   # => [ :a, :b, :c ]
%i[ a b c ]   # => [ :a, :b, :c ]
%i_ a b c _   # => [ :a, :b, :c ]
# etc...

---

%W and %w allow you to create an Array of strings without using quotes and commas.

---

Though it's an old post, the question keep coming up and the answers don't always seem clear to me, so, here's my thoughts:

%w and %W are examples of General Delimited Input types, that relate to Arrays. There are other types that include %q, %Q, %r, %x and %i.

The difference between the upper and lower case version is that it gives us access to the features of single and double quotes. With single quotes and (lowercase) %w, we have no code interpolation (#{someCode}) and a limited range of escape characters that work (\\, \n). With double quotes and (uppercase) %W we do have access to these features.

The delimiter used can be any character, not just the open parenthesis. Play with the examples above to see that in effect.

For a full write up with examples of %w and the full list, escape characters and delimiters, have a look at "Ruby - %w vs %W – secrets revealed!"

---

Instead of %w() we should use %w[]

According to Ruby style guide:

Prefer %w to the literal array syntax when you need an array of words (non-empty strings without spaces and special characters in them). Apply this rule only to arrays with two or more elements.

# bad
STATES = ['draft', 'open', 'closed']

# good
STATES = %w[draft open closed]

Use the braces that are the most appropriate for the various kinds of percent literals.

[] for array literals(%w, %i, %W, %I) as it is aligned with the standard array literals.

# bad
%w(one two three)
%i(one two three)

# good
%w[one two three]
%i[one two three]

For more read here.

---

Excerpted from the documentation for Percent Strings at http://ruby-doc.org/core/doc/syntax/literals_rdoc.html#label-Percent+Strings:

Besides %(...) which creates a String, the % may create other types of object. As with strings, an uppercase letter allows interpolation and escaped characters while a lowercase letter disables them. These are the types of percent strings in ruby: ... %w: Array of Strings

---

I was given a bunch of columns from a CSV spreadsheet of full names of users and I needed to keep the formatting, with spaces. The easiest way I found to get them in while using ruby was to do:

names = %( Porter Smith
Jimmy Jones
Ronald Jackson).split('\n')

This highlights that %() creates a string like "Porter Smith\nJimmyJones\nRonald Jackson" and to get the array you split the string on the "\n" ["Porter Smith", "Jimmy Jones", "Ronald Jackson"]

So to answer the OP's original question too, they could have wrote %(cgi\ spaeinfilename.rb;complex.rb;date.rb).split(';') if there happened to be space when you want the space to exist in the final array output.

---