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Re: does not work?

  • To: mathgroup at
  • Subject: [mg109463] Re: does not work?
  • From: Bill Rowe <readnews at>
  • Date: Wed, 28 Apr 2010 01:59:36 -0400 (EDT)

On 4/27/10 at 7:41 AM, dims12 at (Dims) wrote:

>I wrote a function to check if two lists contain equal members:

>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:

It sounds like you intended each of the Do loops to access each
list element by element. If I have this correct the following
modifications to your code should work as you intended

   y_List] := (Do[(Print["Outer step ", xi];
     Do[(Print[x[[xi]], y[[yi]]];
       If[x[[xi]] == y[[yi]], (Print["Returning True"];
Return[True])]), {yi,
       Length@y}]), {xi, Length@x}]; False)

But while this corrects the code to do what I believe you are
asking for this is not how I would approach this problem.

It appears you intend for the output to be True whenever both
lists contain at least one element in common. If so, the
following should do what you want.

In[1]:= intersects[x_List, y_List] :=
  If[Intersection[Flatten@x, Flatten@y] == {}, False, True]

In[2]:= intersects[{a, b}, {c, a}]

Out[2]= True

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