Why Return[] does not work?
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From: Dims on 27 Apr 2010 07:41 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? 2) is there better way to implement intersection check? Thanks
From: Albert Retey on 27 Apr 2010 08:28 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 situations: 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
From: DrMajorBob on 28 Apr 2010 01:58 IMHO, Return doesn't work because it's a bad idea. Don't use it. Bobby On Tue, 27 Apr 2010 06:41:50 -0500, Dims <dims12(a)hotmail.com> wrote: > 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? > 2) is there better way to implement intersection check? > > Thanks > -- DrMajorBob(a)yahoo.com
From: dh on 28 Apr 2010 01:58 On 27.04.2010 13:41, Dims wrote: > 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? > 2) is there better way to implement intersection check? > > Thanks > Hi, Return does only return a single level . In your case only the inner Do loop is left. Use Throw and Catch to control how many levels to jump up. In general, try to avoid Return. Daniel -- Daniel Huber Metrohm Ltd. Oberdorfstr. 68 CH-9100 Herisau Tel. +41 71 353 8585, Fax +41 71 353 8907 E-Mail:<mailto:dh(a)metrohm.com> Internet:<http://www.metrohm.com>
From: Raffy on 28 Apr 2010 01:59
On Apr 27, 5:28 am, Albert Retey <a...(a)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; |