Re: Trying to use ReplacePart

*To*: mathgroup at smc.vnet.net*Subject*: [mg120022] Re: Trying to use ReplacePart*From*: Ray Koopman <koopman at sfu.ca>*Date*: Wed, 6 Jul 2011 05:39:44 -0400 (EDT)*References*: <201107010040.UAA24254@smc.vnet.net> <iup8eo$kt0$1@smc.vnet.net>

On Jul 3, 1:15 am, Iv=E1n Lazaro <gamins... at gmail.com> wrote: > Thanks for the help; both solutions are much simpler than mine! > > However, is there some way to tell ReplacePart to "remember" each > change it makes? ReplacePart makes the changes one at a time. replaceElements makes them simultaneously and will be much faster for large matrices with many elements to be replaced. replaceElements[matrix_, positionlist_, valuelist_] := Block[{cols = Length@matrix[[1]], vec = Flatten@matrix}, vec[[ positionlist.{cols,1} - cols ]] = valuelist; Partition[vec,cols] ] {rows, cols, n} = {4, 6, 9} matrix = ConstantArray[0, {rows, cols}]; positions = Transpose@{ RandomInteger[{1,rows}, n], RandomInteger[{1,cols}, n]} values = RandomInteger[{10,99}, n] {4,6,9} {{1,5},{2,3},{1,6},{1,2},{2,6},{2,5},{3,2},{2,3},{1,6}} {42,37,82,42,37,14,20,10,63} matrix1 = ReplacePart[matrix, Thread[positions -> values]] matrix2 = replaceElements[matrix, positions, values] matrix2 === matrix1 {{0,42,0,0,42,63},{0,0,10,0,14,37},{0,20,0,0,0,0},{0,0,0,0,0,0}} {{0,42,0,0,42,63},{0,0,10,0,14,37},{0,20,0,0,0,0},{0,0,0,0,0,0}} True