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MathGroup Archive 2000

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Re: making a column into a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg22990] Re: [mg22964] making a column into a list
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Sun, 9 Apr 2000 01:45:53 -0400 (EDT)
  • References: <8cnv2d$5bb@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Mark:
You report
>     To get a column from a matrix in list form:
> IN>
> matrix = {{1, 2}, {1, 2}, {1, 2}}
> column = Column[matrix, 1]
> OUT>
> {{1, 2}, {1, 2}, {1, 2}}
> {1, 1, 1}

This doesn't work for me  with Mathematica 4.0.

matrix = {{1, 2}, {1, 2}, {1, 2}}
column = Column[matrix, 1]

{{1, 2}, {1, 2}, {1, 2}}

Column[{{1, 2}, {1, 2}, {1, 2}}, 1]

Did you mean TakeColumns from the package?

<< LinearAlgebra`MatrixManipulation`

TakeColumns[matrix, {1}]

    {{1}, {1}, {1}}

-------------------------------------

But in Mathematica 4.0 we can use

matrix[[All, {1}]]

{{1}, {1}, {1}}

And this is much faster:

matrix = Table[Random[], {50000}, {10}];

TakeColumns[matrix, {1}]; // Timing

{1.37 Second, Null}

matrix[[All, {1}]]; // Timing

{0.11 Second, Null}


--
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565







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