From help-octave-request at bevo dot che dot wisc dot edu Tue Dec  2 22:49:43 2003
Subject: Re: random sort
From: Heber Farnsworth <farnsworth at olin dot wustl dot edu>
To: Mike Miller <mbmiller at taxa dot epi dot umn dot edu>
Cc: Help-Octave List <help-octave at bevo dot che dot wisc dot edu>
Date: Tue, 2 Dec 2003 22:49:20 -0600

I think your problem is that you are trying to sort something the same 
size as X when you really only want to sort a vector with as many 
elements as X has columns.

If X is NxM then do

[S I] = sort(rand(N,1));
Y = X(:,I);

and Y has the same columns as X but in shuffled order.

You didn't mention whether you wanted to sample with replacement or 
not.  This just re-orders the columns.  It's a different trick if you 
want to admit the possibility that some columns of X may not show up in 
Y while others are repeated.  That's what you would do if you were 
boostrapping data for instance.

Heber

On Tuesday, December 2, 2003, at 09:02 PM, Mike Miller wrote:

> I don't know why I can't figure this out!  I'd swear I've done it 
> before.
> What's the trick to sorting columns in random order?  I have a matrix 
> with
> many columns.  Suppose the matrix is called 'X'.  It seems like I 
> should
> be able to use something like this:
>
> [S, I] = sort(rand(size(X)));
>
> Then I can use 'I' to sort the columns of X, right?  But how?  I just 
> know
> there's an easy trick.
>
> Thanks in advance for breaking my mental block.
>
> Mike
>
>
>
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