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MathGroup Archive 2006

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Re: Sorting Rows of a Matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg65138] Re: Sorting Rows of a Matrix
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Wed, 15 Mar 2006 06:29:58 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

On 3/14/06 at 6:00 AM, gregory.lypny at videotron.ca (Gregory Lypny)
wrote:

>How can I sort the rows of a matrix by using one or more of the
>columns as sort keys just like in a spreadsheet?  For example, if
>matrix M is 12 x 2 with numbers in the first column and unordered
>month numbers in the second, how would I order the rows by month?

There are a number of ways to achieve what you want. One way to do it would be based on the documentation for Sort, i.e.,

Sort[M, OrderedQ@{#1[[2]],#2[[2]]}&]

That is use the second argument for Sort to specify the sort order.

Equivalent results can be obtained with

M[[Ordering[M[[All,2]]]]]

to order by the second column

I find for me, this second method runs faster for large arrays.
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