Re: Mapping to a specific column or row in a matrix
- To: mathgroup at smc.vnet.net
- Subject: [mg103017] Re: Mapping to a specific column or row in a matrix
- From: Valeri Astanoff <astanoff at gmail.com>
- Date: Thu, 3 Sep 2009 19:57:44 -0400 (EDT)
- References: <h7o86n$nj6$1@smc.vnet.net>
On 3 sep, 13:10, 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. Good day, A double transpose seems to be slightly faster than map+mapat : In[1]:= mat=ConstantArray[1,{1000,500}]; In[2]:= res=Map[MapAt[#/10&,#,300]&,mat]//Timing//Short Out[2]//Short= {0.047,{<<1>>}} In[3]:= res=(tm=Transpose[mat];tm[[300]]/=10;tm//Transpose)//Timing// Short Out[3]//Short= {0.031,{<<1>>}} -- V.Astanoff