Re: Struggling with list element assignment in functions
- To: mathgroup at smc.vnet.net
- Subject: [mg45580] Re: Struggling with list element assignment in functions
- From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
- Date: Fri, 16 Jan 2004 06:04:56 -0500 (EST)
- Organization: Universitaet Leipzig
- References: <bu2o9h$jeg$1@smc.vnet.net>
- Reply-to: kuska at informatik.uni-leipzig.de
- Sender: owner-wri-mathgroup at wolfram.com
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