Re: Mapping to a specific column or row in a matrix
- To: mathgroup at smc.vnet.net
- Subject: [mg103001] Re: [mg102985] Mapping to a specific column or row in a matrix
- From: Adriano Pascoletti <adriano.pascoletti at dimi.uniud.it>
- Date: Thu, 3 Sep 2009 19:54:46 -0400 (EDT)
- References: <200909031110.HAA24167@smc.vnet.net>
2009/9/3 pfalloon <pfalloon at gmail.com>
> 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}]
>
>
mat.DiagonalMatrix[{1, 1, 1/10, 1, 1}]
MapAt[#/10 &, mat, {#, 3} & /@ Range@Length@mat]
Transpose[MapAt[#/10 &, Transpose@mat, {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}]
>
>
> MapAt[#/10 &, mat, {3}]
Adriano Pascoletti
Any thoughts/suggestions?
>
> Thanks,
> Peter.
>
>
- References:
- Mapping to a specific column or row in a matrix
- From: pfalloon <pfalloon@gmail.com>
- Mapping to a specific column or row in a matrix