Re: Why Return[] does not work?

*To*: mathgroup at smc.vnet.net*Subject*: [mg109509] Re: Why Return[] does not work?*From*: David Bailey <dave at removedbailey.co.uk>*Date*: Sun, 2 May 2010 05:36:18 -0400 (EDT)*References*: <hr6ihp$2f8$1@smc.vnet.net> <hr6la2$5im$1@smc.vnet.net> <201004280558.BAA05169@smc.vnet.net> <hre94j$8s$1@smc.vnet.net>

Andrzej Kozlowski wrote: > I am not sure if anyone pointed out that if enough Returns are used the original code will work as expected: > > Intersects[x_List, y_List] := (Do[(Print["Outer step ", xi]; > Do[(Print[xi, yi]; > If[xi == yi, (Print["Returning True"]; > Return[Return[Return[True]]])]), {yi, y}]), {xi, x}]; False) > > In[2]:= Intersects[{a,b},{c,a}] > During evaluation of In[2]:= Outer step a > During evaluation of In[2]:= ac > During evaluation of In[2]:= aa > During evaluation of In[2]:= Returning True > Out[2]= True > > On 28 Apr 2010, at 14:58, Dims wrote: > >> On 27 =D0=B0=D0=BF=D1=80, 16:28, Albert Retey <a... at gmx-topmail.de> wrote: >>> Am 27.04.2010 13:41, schrieb Dims: >>> Intersects[x_List, y_List] := Catch[ >> Yes, thank you, I also came to conclusion, that catch works. But I >> think it is a bug with return, since manual says, that we CAN use >> Return[] inside Do[]. >> > > With Mathematica you always live and learn - I had never realised that would work - thanks! I do use Return sometimes, but if I need a loop, I normally use a While loop - which don't seem to be noticed by Return. Unlike some others, I think Return has its place, although I do wish its semantics were a little clearer. If you want a function to perform a few tests on its arguments to decide if its arguments are appropriate, I think it is much clearer to use If[xxx,Return[]]; rather than embed the rest of the function body inside the If. I also think Return is useful when making temporary alterations to some code - say adding a Return[] somewhere just to disable some code. David Bailey http://www.dbaileyconsultancy.co.uk