# Extrange behaviour using "if XXX ..." and "XXX if ..." **URL:** \ **Category:** Ruby\ **Created:** [March 30, 2008, 12:04am UTC](https://www.ruby-forum.com/t/extrange-behaviour-using-if-xxx-and-xxx-if/133313 "2008-03-30T00:04:02Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![ISSSSaki\_Baz\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/51bf81/32.png) [@ISSSSaki\_Baz\_C](https://www.ruby-forum.com/u/ISSSSaki_Baz_C)\ **Post date:** [March 30, 2008, 12:04am UTC](https://www.ruby-forum.com/t/extrange-behaviour-using-if-xxx-and-xxx-if/133313/1 "2008-03-30T00:04:02Z")
Hi, note the difference in those both cases: a) if a=“QWEQWE” : a end =\> “QWEQWE” b) a if a=“QWEQWE” NameError: undefined local variable or method `a’ for main:Object I know that the behaviour in a) is no recommended and maybe deprecated in a future, but why it works in a) and not in b) ? Thanks. ---
**Author:** ![ISSSSaki\_Baz\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/51bf81/32.png) [@ISSSSaki\_Baz\_C](https://www.ruby-forum.com/u/ISSSSaki_Baz_C)\ **Post date:** [March 30, 2008, 12:24am UTC](https://www.ruby-forum.com/t/extrange-behaviour-using-if-xxx-and-xxx-if/133313/2 "2008-03-30T00:24:06Z")
Iñaki Baz C. wrote: > I know that the behaviour in a) is no recommended and maybe deprecated in a > future, but why it works in a) and not in b) ? A method call without parentheses and a plain variable look the same, so Ruby tries to determine between the two by looking for an assignment as it parses your script. If it sees you assigning a value to “a”, then it knows that “a” is a variable, otherwise it assumes that “a” is a method call. Parsing occurs from the top down and from left to right. In case (a) the assignment to “a” occurs before any other reference, so Ruby knows it’s a variable. In case (b), there has not yet been an assignment before the reference to a, so Ruby assumes that “a” is a method call, yet there is no definition for a so you get the message. ---
**Author:** ![ISSSSaki\_Baz\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/51bf81/32.png) [@ISSSSaki\_Baz\_C](https://www.ruby-forum.com/u/ISSSSaki_Baz_C)\ **Post date:** [March 30, 2008, 12:45am UTC](https://www.ruby-forum.com/t/extrange-behaviour-using-if-xxx-and-xxx-if/133313/3 "2008-03-30T00:45:18Z")
El Domingo, 30 de Marzo de 2008, Tim H. escribió: > In case (b), there has not yet been an assignment before the reference > to a, so Ruby assumes that “a” is a method call, yet there is no > definition for a so you get the message. b) a if a=“QWEQWE” NameError: undefined local variable or method `a’ for main:Object But why the first “a” is interpreted before the “if” stament? Imagine this example: defined?a =\> nil a=“BLABLA” if 2 == 3 =\> nil a =\> nil After this code “a” will remain “nil” (or the previous value it had), so: a=“BLABLA” hasn’t been interpreted since the “if” stament failed. Also note this other example: defined?kk =\> nil kk if nil =\> nil Why in this last case the code doesn’t return: "undefined local variable or method kk’ " ? I assume the reason: “kk” is not interpreted if the condition fails. In b) the first “a” is then interpreted AFTER the condition success, so in that moment “a” has been already declared (into the condition), so it would exist and not give an error. Am I not right? ---
**Author:** ![ISSSSaki\_Baz\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/51bf81/32.png) [@ISSSSaki\_Baz\_C](https://www.ruby-forum.com/u/ISSSSaki_Baz_C)\ **Post date:** [March 30, 2008, 5:17am UTC](https://www.ruby-forum.com/t/extrange-behaviour-using-if-xxx-and-xxx-if/133313/4 "2008-03-30T05:17:45Z")
El Domingo, 30 de Marzo de 2008, Tim H. escribió: > to the left of the assignment to a. When Ruby first encounters a > reference to a it assumes that since it has not yet seen any assigment > to a, then a must be a method name. Ok, I understand. Thanks a lot. ---
**Author:** ![ISSSSaki\_Baz\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/51bf81/32.png) [@ISSSSaki\_Baz\_C](https://www.ruby-forum.com/u/ISSSSaki_Baz_C)\ **Post date:** [March 30, 2008, 1:07am UTC](https://www.ruby-forum.com/t/extrange-behaviour-using-if-xxx-and-xxx-if/133313/5 "2008-03-30T01:07:37Z")
Iñaki Baz C. wrote: > After this code “a” will remain “nil” (or the previous value it had), so: > Why in this last case the code doesn’t return: You must distinguish between parsing and execution. Ruby parses your program before Ruby executes your program. Ruby decides how to interpret the reference to a during parsing, before Ruby executes the if statements in your examples. Their success or failure has nothing to do with the decision. In this case a if a = “qweqwe” Remember parsing occurs from left to right. Ruby sees the reference to a to the left of the assignment to a. When Ruby first encounters a reference to a it assumes that since it has not yet seen any assigment to a, then a must be a method name. ---
**Author:** ![ISSSSaki\_Baz\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/51bf81/32.png) [@ISSSSaki\_Baz\_C](https://www.ruby-forum.com/u/ISSSSaki_Baz_C)\ **Post date:** [March 30, 2008, 6:50am UTC](https://www.ruby-forum.com/t/extrange-behaviour-using-if-xxx-and-xxx-if/133313/6 "2008-03-30T06:50:54Z")
Iñaki Baz C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > Remember parsing occurs from left to right. Ruby sees the reference to a > > to the left of the assignment to a. When Ruby first encounters a > > reference to a it assumes that since it has not yet seen any assigment > > to a, then a must be a method name. > > Ok, I understand. Thanks a lot. I asked the same question a few months ago, and got some great answers: \<[http://groups.google.com/group/comp.lang.ruby/browse\_thread/thread/f03e](http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/f03e) 291e32ea2570/49ca1f19297bece6?lnk=st&q=#49ca1f19297bece6\> Understanding this point has been very useful to me subsequently…! m. ---
**Author:** ![ISSSSaki\_Baz\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/51bf81/32.png) [@ISSSSaki\_Baz\_C](https://www.ruby-forum.com/u/ISSSSaki_Baz_C)\ **Post date:** [March 30, 2008, 11:28pm UTC](https://www.ruby-forum.com/t/extrange-behaviour-using-if-xxx-and-xxx-if/133313/7 "2008-03-30T23:28:10Z")
El Domingo, 30 de Marzo de 2008, matt neuburg escribió:\> I asked the same question a few months ago, and got some great answers: > \<[http://groups.google.com/group/comp.lang.ruby/browse\_thread/thread/f03e](http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/f03e) > 291e32ea2570/49ca1f19297bece6?lnk=st&q=#49ca1f19297bece6\> > > Understanding this point has been very useful to me subsequently…! m. Thanks for the link matt,it’s very useful to understand this “issue”. Regards.