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