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Re: Struggling with list element assignment in functions


Hi,

your function work fine, with out a error message with my Mathematica 4.2
and the shorter version

SetAttributes[halo, HoldFirst];

halo[x_?MatrixQ] /; Length[x] > 4 :=
  (x[[1]] = x[[(Length[x] - 1)]];
    x[[Length[x]]] = x[[2]])

run also without problems.
Can you be more specific about the errors?

Regards
  Jens



Michael Tobis wrote:
> 
> For context, I am trying to create a toroidal data structure by
> causing edges of an array to be copies of
> rows and columns on the first interior line of the opposite edge.
> 
> I'm not entirely clear on what Hold does, but it seemed to solve my
> problem when wrapping the rows of the array:
> 
> ===
> Attributes[haloarray] = HoldFirst;
> haloarray[array_]/;MatrixQ[array] := Module[{},
>                 If [Length[array] <4 || Length[array[[1]] ]< 4,
>                         (Print["haloarray: matrix too small"];Return[Null])];
>                Map[halo,array];
>                array[[1]] = array[[Length[array] - 1]];
>                array[[Length[array]]] = array[[2]];
>         ]
> 
> Attributes[halo] = {};
> halo[x_]/;MatchQ[x,z_List] := Module[{y},
>                 y = x;
>                 If [Length[x] <4,(Print["halo: list too short"];Return[Null])];
>                y[[1]] = x[[(Length[x] - 1)]];
>                y[[Length[x]]] = x[[2]];
>         ]
> ===
> so, by putting in the HoldFirst, I could assign directly to the called
> array in haloarray[] rather than having to copy it twice, to and from
> a dummy array. Imagine my disappointment, then, in trying to extend
> this idea to the halo[] routine. This
> ===
> Attributes[halo] = HoldFirst;
> halo[x_]/;MatchQ[x,z_List] := Module[{},
>                 If [Length[x] <4,(Print["halo: list too short"];Return[Null])];
>                x[[1]] = x[[(Length[x] - 1)]];
>                x[[Length[x]]] = x[[2]];
>         ]
> ===
> refuses to do the assignments and yields all sorts of error messages.
> Any ideas? It's not a showstopper for me but I'd like to know what's
> up.
> 
> thanks
> Michael Tobis
> http://geosci.uchicago.edu/~tobis


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