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Re: Given a matrix, find position of first non-zero element in each

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99510] Re: Given a matrix, find position of first non-zero element in each
  • From: Albert Retey <awnl at gmx-topmail.de>
  • Date: Thu, 7 May 2009 06:32:44 -0400 (EDT)
  • References: <gtrl9k$242$1@smc.vnet.net>

Hi,

> This is how I solved this problem and after a bit of  struggle. I wanted to 
> see if I could avoid using a Table, and solve it just using Patterns and 
> Position and Select, but could not so far.
> 
> 
> Table[Flatten[{i, Flatten[Position[A[[i,All]], _?(#1 != 0 & ), 1, 1]]}], {i, 
> 1, 6}]
> 
> Out[174]= {{1, 3}, {2, 1}, {3, 2}, {4, 1}, {5, 2}, {6, 2}}
> 
> I am not happy with the above solution. I am sure there is a better one (the 
> above also do not work well when one row has all zeros).
> 
> Do you see a better and more elegant way to do this?

I absolutley don't think that it is a value per se to avoid Table, but
here is one solution without Table:

MapIndexed[Flatten[{#2, Position[#1, _?(# != 0 &), 1, 1]}] &,A]

hth,

albert


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