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Re: Why Return[] does not work?

  • To: mathgroup at
  • Subject: [mg109454] Re: Why Return[] does not work?
  • From: Albert Retey <awnl at>
  • Date: Tue, 27 Apr 2010 08:28:42 -0400 (EDT)
  • Organization: CNNTP
  • References: <hr6ihp$2f8$>

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[
   Print["Outer step ", xi];
    Print[xi, yi];
    If[xi == yi, Print["Returning True"]; Throw[True]],
    {yi, y}
    ], {xi, x}

depending on what you try to achieve there might be better mechanisms
than do loops which stop at certain conditions...



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