Re: Replace elements in a matrix

• To: mathgroup at smc.vnet.net
• Subject: [mg116454] Re: Replace elements in a matrix
• From: Peter Breitfeld <phbrf at t-online.de>
• Date: Wed, 16 Feb 2011 04:33:52 -0500 (EST)
• References: <ijdo8r\$f7a\$1@smc.vnet.net>

```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.
>

Try this:

apos=Position[a,1];
bvals=Extract[b,apos];