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Re: Replace elements in a matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg116447] Re: Replace elements in a matrix
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Wed, 16 Feb 2011 04:32:36 -0500 (EST)

kiz_0987 wrote:
> I'm new to Mathematica and have a question regarding matrices. I was
> sure that this problem must have been answered in the past, but could
> not find a solution.
> 
> I want to replace some elements in a matrix (based on value) with
> elements from another matrix at the same position. Eg:
> a = {{0, 1}, {1, 2}};
> b = {{1, 3}, {2, 5}};
> 
> I want to replace the elements in "a" which are equal to 1 (say) with
> the same position elements in b to give:
> c = {{0, 3}, {2, 2}};
> 
> How can I do this? I started with using Position and ReplacePart but
> could not figure out how to get it to work in this case (other cases
> are documented, such as replacing with a single value).
> 
> Any help appreciated. Thanks.
> 

Among other methods:

val = 1;
stencil = Unitize[a-val];

stencil*a + (1-stencil)*b

Out[10]= {{0, 3}, {2, 2}}

Daniel Lichtblau
Wolfram Research


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