Ruby - test for array

Question

What is the right way to:

is_array("something") # => false         (or 1)

is_array(["something", "else"]) # => true  (or > 1)

or to get the count of items in it?

Answer 1

You probably want to use kind_of().

>> s = "something"
=> "something"
>> s.kind_of?(Array)
=> false
>> s = ["something", "else"]
=> ["something", "else"]
>> s.kind_of?(Array)
=> true

Answer 2

Are you sure it needs to be an array? You may be able to use respond_to?(method) so your code would work for similar things that aren't necessarily arrays (maybe some other enumberable thing). If you do actually need an array, then the post describing the Array#kind\_of? method is best.

['hello'].respond_to?('each')

Answer 3

Instead of testing for an Array, just convert whatever you get into a one-level Array, so your code only needs to handle the one case.

t = [*something]     # or...
t = Array(something) # or...
def f *x
    ...
end

Ruby has various ways to harmonize an API which can take an object or an Array of objects, so, taking a guess at why you want to know if something is an Array, I have a suggestion.

The splat operator contains lots of magic you can look up, or you can just call Array(something) which will add an Array wrapper if needed. It's similar to [*something] in this one case.

def f x
  p Array(x).inspect
  p [*x].inspect
end
f 1         # => "[1]"
f [1]       # => "[1]"
f [1,2]     # => "[1, 2]"

Or, you could use the splat in the parameter declaration and then .flatten, giving you a different sort of collector. (For that matter, you could call .flatten above, too.)

def f *x
  p x.flatten.inspect
end         # => nil
f 1         # => "[1]"
f 1,2       # => "[1, 2]"
f [1]       # => "[1]"
f [1,2]     # => "[1, 2]"
f [1,2],3,4 # => "[1, 2, 3, 4]"

And, thanks gregschlom, it's sometimes faster to just use Array(x) because when it's already an Array it doesn't need to create a new object.

Answer 4

[1,2,3].is_a? Array evaluates to true.

Answer 5

It sounds like you're after something that has some concept of items. I'd thus recommend seeing if it is Enumerable. That also guarantees the existence of #count.

For example,

[1,2,3].is_a? Enumerable
[1,2,3].count

note that, while size, length and count all work for arrays, count is the right meaning here - (for example, 'abc'.length and 'abc'.size both work, but 'abc'.count doesn't work like that).

Caution: a string is_a? Enumerable, so perhaps this isn't what you want... depends on your concept of an array like object.

Answer 6

Try:

def is_array(a)
    a.class == Array
end

EDIT: The other answer is much better than mine.

Answer 7

Also consider using Array(). From the Ruby Community Style Guide:

Use Array() instead of explicit Array check or [*var], when dealing with a variable you want to treat as an Array, but you're not certain it's an array.

# bad
paths = [paths] unless paths.is_a? Array
paths.each { |path| do_something(path) }

# bad (always creates a new Array instance)
[*paths].each { |path| do_something(path) }

# good (and a bit more readable)
Array(paths).each { |path| do_something(path) }