Re: Two questions about modification of a matrix.

*To*: mathgroup at smc.vnet.net*Subject*: [mg101072] Re: Two questions about modification of a matrix.*From*: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>*Date*: Tue, 23 Jun 2009 07:06:16 -0400 (EDT)*References*: <h1nevg$8s9$1@smc.vnet.net>

There must be 100s of ways to do this. Here's my version: n = 5; m = RandomReal[{-5, 5}, {n, n}] 1: MapIndexed[If[Times @@ #2 > n, 0, #1] &, m, {2}] 2: Map[If[# < 0, 0, #] &, m, {2}] A slightly more beginner-friendly formatted version of 1 would be: MapIndexed[ Function[{entry, indices}, If[Apply[Times, indices] > n, 0, entry] ], m, {2}] Cheers -- Sjoerd On Jun 22, 10:22 am, Mayasky <alix.zh... at gmail.com> wrote: > Suppose there is a N*N matrix. > > 1. How can I change all elements that satisfy i*j>N to 0 ? (Here i, j > is position of this element, as in mat[[i,j]]) > > 2. How can I change all negative elements in that matrix to 0 ? > > I know a "for loop" can be used, I just wonder if there is any more > concise and beautiful ways. (I doubt Mathematica doesn't have a 'find' > function as in another system which conveniently returns indice of elemen= ts > that satisfy a certain condition)