Re: Why Return[] does not work?

*To*: mathgroup at smc.vnet.net*Subject*: [mg109466] Re: Why Return[] does not work?*From*: Raffy <adraffy at gmail.com>*Date*: Wed, 28 Apr 2010 02:00:08 -0400 (EDT)*References*: <hr6ihp$2f8$1@smc.vnet.net> <hr6la2$5im$1@smc.vnet.net>

On Apr 27, 5:28 am, Albert Retey <a... at gmx-topmail.de> wrote: > Am 27.04.2010 13:41, schrieb Dims: > > > > > > > I wrote a function to check if two lists contain equal members: > > > Intersects[x_List, > > y_List] := (Do[(Print["Outer step ", xi]; > > Do[(Print[xi, yi]; > > If[xi == yi, (Print["Returning True"]; Return[True])]),= {yi, > > y}]), {xi, x}]; False) > > > But it returns False for interecting lists: > > > In[47]:= Intersects[{a, b}, {c, a}] > > During evaluation of In[47]:= Outer step a > > During evaluation of In[47]:= ac > > During evaluation of In[47]:= aa > > During evaluation of In[47]:= Returning True > > During evaluation of In[47]:= Outer step b > > During evaluation of In[47]:= bc > > During evaluation of In[47]:= ba > > Out[47]= False > > > Few questions: > > > 1) why does not Return[] interrupt the loops? > > Return behaves not as people expect in many circumstances. I found that > I almost never really need it in my code because for every possible use > case there are other mechanisms that work more reliable. > > > 2) is there better way to implement intersection check? > > There is Break which would be the natural choice to break loops, > but I think usually Throw and Catch are the best choice in these situatio= ns: > > Intersects[x_List, y_List] := Catch[ > Do[ > Print["Outer step ", xi]; > Do[ > Print[xi, yi]; > If[xi == yi, Print["Returning True"]; Throw[True]], > {yi, y} > ], {xi, x} > ]; > False > ] > > depending on what you try to achieve there might be better mechanisms > than do loops which stop at certain conditions... > > hth, > > albert Catch/Throw however cause overhead. If you're return is just a boolean, and your control structure depth is just 1, you could exploit the fact that: Do[Return[x],{1}] === x and Do[Null,{1}] === Null. intersects[x_List, y_List] := Do[If[xi == yi, Return[True]], {yi, y}, {xi, x}] === True;