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Array#include? provides only a weaker information than what Array#index provides, i.e., when Array#index returns nil, the corresponding method call with Array#include? will return false, and when Array#index returns an integer, Array#include? returns true. Furthermore, comparing the two indicates that there is no significant difference in speed; rather Array#index often shows a better result than Array#include?:

a = %w[boo zoo foo bar]

t = Time.now
10000.times do
  a.include?("foo")
end
puts Time.now - t # => 0.005626235

t = Time.now
10000.times do
  a.index("foo")
end
puts Time.now - t # => 0.003683945

Then, what is the purpose of Array#include?? Can't all code using it be rewritten using Array#index?

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  • Related: stackoverflow.com/questions/25779451/… Commented Sep 11, 2014 at 6:15
  • Your linked question answers the performance issues, but one reason that Array#include? is present is because it is inherited from Enumerable. Commented Sep 11, 2014 at 6:18
  • @Sparhawk Right. But Array could have overwritten include? with its current index functionality. That would have made it simpler (except for the trickiness of ? used for non-boolean). Commented Sep 11, 2014 at 6:24
  • 1
    I guess if there's not a strong argument to change the behaviour, then it'd be best to keep the inherited behaviour for consistency with other enumerables. Not necessarily very convincing, I know, but perhaps the argument for consistency with ?s suggests a similar thought process. Commented Sep 11, 2014 at 6:32

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3

I know this isn't an official reason, but I can think of a few things:

  • Clarity: as a name, include? makes more sense at first sight, and also allows easy visual confirmation of code correctness by identifying itself as a boolean predicate. This follows the concept of making wrong code look wrong (see http://www.joelonsoftware.com/articles/Wrong.html)
  • Good typing: If all you want is a boolean value for a boolean check, making that a number could lead to bugs
  • Cleanliness: Isn't it nicer to see a printed output of "true" rather than going back to C and having no boolean to speak of?
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4 Comments

@sawa: In C, there is no boolean type. 0 is false, everything else is true. People started doing all sorts of clever tricks using integers as booleans, and vice versa, and multiplying them, and all that, and it just turns into a mess.
Technically C does have a boolean type. Unless by "C" you mean "whatever old version of C that MS supports".
@muistooshort: What is it? I've never seen it.
It is part of C99 but you have to #include <stdbool.h> to use it.

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