Re: Re: Mapping to a specific column or row in a matrix
- To: mathgroup at smc.vnet.net
- Subject: [mg103020] Re: [mg103011] Re: Mapping to a specific column or row in a matrix
- From: "Ned Lieb" <okty.gy.ned at gmail.com>
- Date: Fri, 4 Sep 2009 03:15:09 -0400 (EDT)
- Reply-to: okty.gy.ned at gmail.com
Yep, there is. Double square brackets indicate coordinates.
To define a function as row x in a table defined as Table, for example, you'd write something like f=Table[[x]][All]]. I assume you can drop the same syntax into a mapping function.
------Original Message------
From: Vince
To: Mathgroup
Sent: Sep 3, 2009 7:56 PM
Subject: [mg103020] [mg103011] Re: Mapping to a specific column or row in a matrix
On Sep 3, 7:10 am, pfalloon <pfall... at gmail.com> wrote:
> Hi,
> I'm wondering if there is a more simple/elegant way to Map a function
> to a specific row or column of a matrix. The best I can come up
> requires both Map and MapAt, but it feels like there should be
> something more succinct (I suspect I may be overlooking something
> obvious).
>
> Example: suppose I want to divide entries in the 3rd column by 10.
>
> (* my clunky solution *)
> mat = ConstantArray[1, {10,5}];
> res = Map[MapAt[#/10 &, #, 3] &, mat]
>
> Note that the desired output is the entire matrix, so that rules out
> something like:
>
> mat[[All,3]]/10
>
> One thing that would seem natural would be to allow "All" as an
> element specification in the MapAt function. Thus, what I'm trying to
> do would be expressed as:
>
> (* doesn't work, but would be nice *)
> MapAt[#/10 &, mat, {All,3}]
>
> and if I wanted to do it to a specific row (say, the 3rd) it would be:
>
> (* doesn't work, but would be nice *)
> MapAt[#/10 &, mat, {3, All}]
>
> Any thoughts/suggestions?
>
> Thanks,
> Peter.
Consider in-place modification, if you're working with a variable/
symbol:
mat = ConstantArray[1, {10,5}];
mat[[All,3]] /= 10;
or more generally:
mat[[All,3]] = op @ mat[[All, 3]];
where 'op' can of course be a function of more than its target.
Vince Virgilio