Re: Two questions about modification of a matrix.

*To*: mathgroup at smc.vnet.net*Subject*: [mg101061] Re: Two questions about modification of a matrix.*From*: Peter Breitfeld <phbrf at t-online.de>*Date*: Tue, 23 Jun 2009 07:04:05 -0400 (EDT)*References*: <h1nevg$8s9$1@smc.vnet.net>

Mayasky 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 elements > that satisfy a certain condition) > Try this: For your condition i*j>N: A = Array[Subscript[a, ##] &, {4, 4}]; MapIndexed[If[Greater[Times @@ #2, 4], 0, #1] &, A, {2}] // MatrixForm to replace negatives with 0: (B = RandomInteger[{-5, 5}, {4, 4}]) // MatrixForm Map[If[# < 0, 0, #] &, B, {2}] // MatrixForm -- _________________________________________________________________ Peter Breitfeld, Bad Saulgau, Germany -- http://www.pBreitfeld.de