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Re: Two questions about modification of a matrix.

• To: mathgroup at smc.vnet.net
• Subject: [mg101079] Re: [mg101041] Two questions about modification of a matrix.
• From: Chenguang Zhang <alix.zhang at gmail.com>
• Date: Tue, 23 Jun 2009 07:07:33 -0400 (EDT)
• References: <200906220822.EAA09140@smc.vnet.net>

Thanks, you excellent people! I really did not expect there to be so great
ways for that.

On Mon, Jun 22, 2009 at 11:18 PM, Scot Martin <smartin at seas.harvard.edu>wrote:

> >> I know a "for loop" can be used, I just wonder if there is any more
> concise and beautiful ways.
>
> You're right about Mathematica. Stop thinking about loops. Think about Map
> and Pattern. Here are some example solutions to the problems you posed.
>
> 1. n = 3; MapIndexed[(If[Times @@ #2 > n, 0, #1]) &, mat, {2}]
>
> 2. mat /. {_?Negative->0} (*This "?" is known as a PatternTest[]*)
>
>
>
> -----Original Message-----
> From: Mayasky [mailto:alix.zhang at gmail.com]
> Sent: Monday, June 22, 2009 04:22
> To: mathgroup at smc.vnet.net
> Subject: [mg101041] Two questions about modification of a matrix.
>
> 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)
>
>
>

• References:
  • Two questions about modification of a matrix.
    • From: Mayasky <alix.zhang@gmail.com>