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Re: Rather simple function returns curious result.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg131393] Re: Rather simple function returns curious result.
  • From: Bob Hanlon <hanlonr357 at gmail.com>
  • Date: Wed, 3 Jul 2013 04:57:26 -0400 (EDT)
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  • References: <20130702044842.DB5BC6A5F@smc.vnet.net>

Clear[f];


f[run : {___List}] := Module[
   {n},
   n = run // Length;
   If[n != 2, n, {}]];


f /@ Table[{}, {n, 0, 4}, {n}]


{0, 1, {}, 3, 4}



Bob Hanlon




On Tue, Jul 2, 2013 at 12:48 AM, James Stein <mathgroup at stein.org> wrote:

> For reasons that escape me, the simple function below fails to return
> an empty List when n==2; Is this a bug? If not, what is the
> explanation? If the return value for n==2 is changed to an integer or
> a string, the function behaves as expected.(Of course, this bizarre
> function is the result of simplifying a more reasonable one.)
>
> Clear[ f ];
> f [run : { List___ } ] := Module [ { n },
>    n = run // Length;
>    If [ n != 2, Return [ n ] ];
>    Module [ { } ,
>     { } (* Return Empty List if n==2 *)
>     ]
>    ] ;
> f [ { } ]
> f [ { { } } ]
> f [ { { }, { } } ]
> f [ { { }, { }, { } } ]
> f [ { { }, { }, { }, { } } ]
>
> When I run the above, I get these four outputs:
> 0
> 1
> Sequence[{}, {}][]
> 3
> 4
>
>



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