# Silent out-of-index problems--how much of a problem are they **URL:** \ **Category:** Ruby\ **Created:** [October 18, 2006, 10:39am UTC](https://www.ruby-forum.com/t/silent-out-of-index-problems-how-much-of-a-problem-are-they/72655 "2006-10-18T10:39:50Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\ **Post date:** [October 18, 2006, 10:39am UTC](https://www.ruby-forum.com/t/silent-out-of-index-problems-how-much-of-a-problem-are-they/72655/1 "2006-10-18T10:39:50Z")
As a Python user starting to mess around with Ruby, one of my biggest concerns is the fact that so many operations on lists ‘fail silently’ (return nil) when given list indices that are out of bounds. In particular, I worry that this might produce hard-to-track down errors, as the problem might not show up until one tries to do something with the returned value(s); and by that may be somewhere far away in the code. Certainly the guilty list access won’t show up in the call stack. On the other hand, it’s often thought that dynamic languages will result in lots of difficult-to-find problems due to the lack of strong typing, but both Python and Ruby users know that’s not the case. So my question is, _in practice_, how difficult is it to track down index-out-of-bounds-errors in Ruby? It can certainly be a godawful problem in C, and I’m pretty sure it would be in Python, too, if exceptions weren’t thrown. And in Ruby, how does one track done where such errors actually occur? In Python, I can simply look at the stack trace. This isn’t a criticism of Ruby. (Well, one aspect maybe). I’m looking at Ruby because there are a lot of things (documentation tools, consistency, gems, others) that it does better than Python. But I do worry about silent index errors. Thanks, Ken ---
**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\ **Post date:** [October 18, 2006, 10:39am UTC](https://www.ruby-forum.com/t/silent-out-of-index-problems-how-much-of-a-problem-are-they/72655/2 "2006-10-18T10:39:57Z")
On 2006.10.13 13:34, Kenneth McDonald wrote: > but both Python and Ruby users know that’s not the case. > worry about silent index errors. > I can honestly say that I have never had this issue. I can see > that it might be worrying but it is just like any other language > feature–when you know it, you plan for it, you write your code > with it in mind. And this is not to say it is a terrible inconvenience to have to do this. The code seems to be more natural than with error handling. Oh–Arrays, not lists 🙂 ---
**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\ **Post date:** [October 18, 2006, 10:39am UTC](https://www.ruby-forum.com/t/silent-out-of-index-problems-how-much-of-a-problem-are-they/72655/3 "2006-10-18T10:39:58Z")
Kenneth McDonald wrote: > As a Python user starting to mess around with Ruby, one of my biggest > concerns is the fact that so many operations on lists ‘fail silently’ Is it _failing_, or simply returning a value that does not map to your current expectations? > … > This isn’t a criticism of Ruby. (Well, one aspect maybe). I’m looking at > Ruby because there are a lot of things (documentation tools, > consistency, gems, others) that it does better than Python. But I do > worry about silent index errors. Similar (rhetorical) question: What makes this an index _error_? I think that the degree of difficulty people have in tracking down undesired behavior is a factor of their beliefs about what the language “should” do. This is along the same lines of people having issues with zero being True, or with zero-based indexing, or significant indentation. There’s no objective reality for a programming language, no absolute right or wrong for how it behaves. The best one can hope for is a consistent internal universe. – James B. “People want simple stories.” ---
**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\ **Post date:** [October 18, 2006, 10:39am UTC](https://www.ruby-forum.com/t/silent-out-of-index-problems-how-much-of-a-problem-are-they/72655/4 "2006-10-18T10:39:58Z")
Kenneth McDonald wrote: > So my question is, _in practice_, how difficult is it to track down > index-out-of-bounds-errors in Ruby? It can certainly be a godawful > problem in C, and I’m pretty sure it would be in Python, too, if > exceptions weren’t thrown. And in Ruby, how does one track done where > such errors actually occur? In Python, I can simply look at the stack trace. I personally haven’t had trouble with this. I thought I’d point out that you can make Ruby yell if you like. In a simple example (using only one form of the [] access call): irb(main):001:0\> a = [0,1,2,3] =\> [0, 1, 2, 3] irb(main):002:0\> a[5] =\> nil irb(main):003:0\> class Array irb(main):004:1\> alias\_method :\_lookup, :[] irb(main):005:1\> def irb(main):006:2\> raise “OutOfBounds” unless idx\>=0 && idx\ \_lookup( idx ) irb(main):008:2\> end irb(main):009:1\> end =\> nil irb(main):010:0\> a[5] RuntimeError: OutOfBounds from (irb):6:in `[]’ from (irb):10 from :0 irb(main):011:0\> a[3] =\> 3 ---
**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\ **Post date:** [October 18, 2006, 10:40am UTC](https://www.ruby-forum.com/t/silent-out-of-index-problems-how-much-of-a-problem-are-they/72655/5 "2006-10-18T10:40:00Z")
Kenneth McDonald wrote: > So my question is, _in practice_, how difficult is it to track down > index-out-of-bounds-errors in Ruby? Truthfully, I’m not sure I’ve ever had one. The only real error I ever get from Ruby is when I attempt to call ‘foo’ on object nil. 😉 It’s sort of an “edge case” thing. Like in Java, the “pointerless” language, where the most common runtime error is “null pointer exception.” Of course, probably some percentage of those nils were from out-of-bound array accesses. Hal ---
**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\ **Post date:** [October 18, 2006, 10:39am UTC](https://www.ruby-forum.com/t/silent-out-of-index-problems-how-much-of-a-problem-are-they/72655/6 "2006-10-18T10:39:59Z")
Kenneth McDonald wrote: > So my question is, _in practice_, how difficult is it to track down > index-out-of-bounds-errors in Ruby? It can certainly be a godawful > problem in C, and I’m pretty sure it would be in Python, too, if > exceptions weren’t thrown. And in Ruby, how does one track done where > such errors actually occur? In Python, I can simply look at the stack > trace. In general, ruby encourages you to use iterators more than indexing (though of course not always), which avoids the issue. In many cases, you are expecting that the array may have some nil entries, so the bounds question isn’t really different from the question of whether there was a real object at that index. You have to deal with that case anyway (I presume even in python). Maybe you explicitly raise an exception when [] returns nil: elt = a[i] || (raise IndexError, “missing entry at #{i}”) (of course, this will also raise an error on a false value, as well as nil). ---
**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\ **Post date:** [October 18, 2006, 10:40am UTC](https://www.ruby-forum.com/t/silent-out-of-index-problems-how-much-of-a-problem-are-they/72655/7 "2006-10-18T10:40:55Z")
On Oct 12, 2006, at 11:34 PM, Kenneth McDonald wrote: > As a Python user starting to mess around with Ruby, one of my > biggest concerns is the fact that so many operations on lists ‘fail > silently’ (return nil) when given list indices that are out of bounds. As has been shown, Array does include a fetch() method that throws IndexError. I would say the fact that Ruby has an entire error type just for this case shows that it is not being ignored. In practice, this just isn’t much of an issue in Ruby. Like off-by- one errors, this pretty much just goes away with the pervasive use of iterators. > So my question is, _in practice_, how difficult is it to track down > index-out-of-bounds-errors in Ruby? Well, Rubyists are big fans of Test Driven Development (TDD). Two of the many advantages of TDD are that it causes you to write very small methods and isolate the issues of those methods with tightly focused tests. When doing this, it should be fairly rare that you are scanning hundreds of line of code for a trivial out of bounds error like this. That’s why we like it. 😉 James Edward G. II ---
**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\ **Post date:** [October 18, 2006, 10:43am UTC](https://www.ruby-forum.com/t/silent-out-of-index-problems-how-much-of-a-problem-are-they/72655/8 "2006-10-18T10:43:58Z")
-----BEGIN PGP SIGNED MESSAGE----- In article [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid), Kenneth McDonald [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > So my question is, _in practice_, how difficult is it to track down > index-out-of-bounds-errors in Ruby? It can certainly be a godawful > problem in C, and I’m pretty sure it would be in Python, too, if > exceptions weren’t thrown. And in Ruby, how does one track done where > such errors actually occur? In Python, I can simply look at the stack trace. If you use the Ruby error handling methodology, it’s generally pretty simple since nil has few methods. Most times if you’re accessing an Array element you’re going to do something with it, so the error crops up fairly immediately. Unfortunately, it does require this complicated setup %-). begin ` rescue => error print error.inspect end` ` that and some introspective code in method_missing will go a long ways toward catching most runtime errors. I’m pretty sure you can get a full stack trace if you need it, but I’ve never needed it. _ Booker C. Bense -----BEGIN PGP SIGNATURE----- Version: 2.6.2 iQCVAwUBRS+kxGTWTAjn5N/lAQGe4gQAgLETOnvA3IukC0PYARa3lLZUacvOGGlE uZ5vFv5xF+fho2cwm55i3sQlEJlRr8YmdgAdA3MfMLbFi/rELIMywSM6aJeQkq61 PkuqG+Swj8Xzq8pWEEbiTT6Q7zTnOyC0Qm84TZTOzNixX4ulQ2h2Dp87aIkkHV1M mWhp6Fzg3Zg= =YuKL -----END PGP SIGNATURE-----`