# Ruby gotcha? \[\].all? always returns true **URL:** \ **Category:** Ruby\ **Created:** [April 27, 2007, 12:55am UTC](https://www.ruby-forum.com/t/ruby-gotcha-all-always-returns-true/93054 "2007-04-27T00:55:47Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ad7895/32.png) [@Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/u/Dan_Stevens_SIAmAI)\ **Post date:** [April 27, 2007, 12:55am UTC](https://www.ruby-forum.com/t/ruby-gotcha-all-always-returns-true/93054/1 "2007-04-27T00:55:47Z")
Something in caught me out today. I wanted to test that the contents of the array where all instances of particular class, and if not, do stuff. The example isn’t exactly what my code did, but it illustrates my point: if not my\_arr.all? { |i| i.is\_a(Integer) } # Do stuff end I expecting it to ‘do stuff’, however I found it was not. I discovered that ‘my\_arr’ was empty, which was not a surprise, but what did surprise me was that all? called on an empty array appears to always return true. I don’t know about you, but these seems counter-intuitive to me. Does anyone agree or disagree? I’ve now resorted to any? instead, like as follows: if not my\_arr.any? { |i| not i.is\_a(Integer) } # Do stuff end As far as it seems [].any? always returns false. The code should also be slightly more efficient as it doesn’t always have to check every item of the array, not that I expect it to get very big. ---
**Author:** ![Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ad7895/32.png) [@Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/u/Dan_Stevens_SIAmAI)\ **Post date:** [April 27, 2007, 2:17am UTC](https://www.ruby-forum.com/t/ruby-gotcha-all-always-returns-true/93054/2 "2007-04-27T02:17:52Z")
On 4/26/07, Dan Stevens (IAmAI) [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: ---
**Author:** ![Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ad7895/32.png) [@Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/u/Dan_Stevens_SIAmAI)\ **Post date:** [April 27, 2007, 2:32am UTC](https://www.ruby-forum.com/t/ruby-gotcha-all-always-returns-true/93054/3 "2007-04-27T02:32:53Z")
On Apr 27, 2007, at 12:55 AM, Dan Stevens (IAmAI) wrote: > that ‘my\_arr’ was empty, which was not a surprise, but what did > surprise me was that all? called on an empty array appears to always > return true. > > I don’t know about you, but these seems counter-intuitive to me. Does > anyone agree or disagree? Yeah, it is suprising at first sight but that’s standard semantics in math. A for all statement like that holds always for the empty set because otherwise you’d need to show an element of the empty set (impossible) for which the test is false. The reason is not for all x in S P(x) is logically equivalent to there exists x in S such that not P(x) This is a corner case, note that it does not depend on P. – fxn ---
**Author:** ![Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ad7895/32.png) [@Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/u/Dan_Stevens_SIAmAI)\ **Post date:** [April 27, 2007, 2:21pm UTC](https://www.ruby-forum.com/t/ruby-gotcha-all-always-returns-true/93054/4 "2007-04-27T14:21:33Z")
“Dan Stevens (IAmAI)” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: > anyone agree or disagree? > It seems perfectly intuitive to me. There’s a local law that states that all dogs must wear leashes when not confined behind a fence. We are in full compliance with that law, even though our front yard has no fence and we own no leashes - this is because we have no dogs. > I’ve now resorted to any? instead, like as follows: > > if not my\_arr.any? { |i| not i.is\_a(Integer) } > > # Do stuff > > end > > As far as it seems [].any? always returns false. The code should also > be slightly more efficient as it doesn’t always have to check every > item of the array, not that I expect it to get very big. Actually, all? has the same efficiency characteristics. That is, the instant it finds something false, it returns false. ---
**Author:** ![Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ad7895/32.png) [@Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/u/Dan_Stevens_SIAmAI)\ **Post date:** [April 27, 2007, 8:39pm UTC](https://www.ruby-forum.com/t/ruby-gotcha-all-always-returns-true/93054/5 "2007-04-27T20:39:49Z")
On Apr 27, 2007, at 2:21 PM, Daniel M. wrote: > > return true. > > > > I don’t know about you, but these seems counter-intuitive to me. Does > > anyone agree or disagree? > > It seems perfectly intuitive to me. > > There’s a local law that states that all dogs must wear leashes when > not confined behind a fence. We are in full compliance with that law, > even though our front yard has no fence and we own no leashes - this > is because we have no dogs. A good explanation. Let me elaborate it a bit more. The parallelism there goes: "you are within the law if all your dogs satisfy ". Can I say that "all my dogs satisfy " is true if I have no dog? Why? In what sense can I say something holds in an empty collection? To see why the response is yes, I think we need to go a little step further: The point is that a policeman should prove one of your dogs does not satisfy to say you’ve break the law. It’s because that’s impossible that you are within the law as far as that law concerns. Therefore, since you are either within or outside the law, "all your dogs satisfy " necessarily holds. That’s the formal equivalence I mentioned earlier (what makes logicians define for all and there exists that way is our intuitive reasoning about dogs and laws, formal logic as such does not justify per se that’s OK that way.) – fxn