R help - May 2006 - newbie question: ROW average

Dimitris Rizopoulos wrote:
> look at ?rowMeans; you can also use "apply(mat, 1, mean)" but 
> rowMeans() is better.
	By my reading of the question, this is not what
	Ezhil wants.  He said:

	``I have a 992 x 74 matrix. I would like to form a new matrix
	  by averaging each 4 rows from the original one.''

	I.e. he wants (I think) the first row of the new matrix
	to be the mean of the first 4 rows of the old one, the
	second row of the new matrix to be the mean of rows 5
	through 8 of the old one, and so on.

	One way this could be done is via

	> m.new <- t(apply(array(t(m.old),dim=c(74,4,992/4)),c(1,3),mean))

			cheers,

				Rolf Turner
				rolf at math.unb.ca

Peter Dalgaard wrote:
> How about 
> 
> dim(M) <- c(4,248,74)
> mn <- apply(M, c(2,3), mean)
	Hey!  That's sexy!  Much better than my kludgy
	suggestion!

			cheers,

				Rolf

yes you're right; it was my mistake.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://med.kuleuven.be/biostat/
     http://www.student.kuleuven.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Rolf Turner" <rolf at math.unb.ca>
To: <dimitris.rizopoulos at med.kuleuven.be>; <ezhil02 at yahoo.com>
Cc: <r-help at stat.math.ethz.ch>
Sent: Monday, May 29, 2006 1:55 PM
Subject: Re: [R] newbie question: ROW average

> Dimitris Rizopoulos wrote:
> 
>> look at ?rowMeans; you can also use "apply(mat, 1, mean)" but
>> rowMeans() is better.
> 
> By my reading of the question, this is not what
> Ezhil wants.  He said:
> 
> ``I have a 992 x 74 matrix. I would like to form a new matrix
>   by averaging each 4 rows from the original one.''
> 
> I.e. he wants (I think) the first row of the new matrix
> to be the mean of the first 4 rows of the old one, the
> second row of the new matrix to be the mean of rows 5
> through 8 of the old one, and so on.
> 
> One way this could be done is via
> 
> > m.new <- t(apply(array(t(m.old),dim=c(74,4,992/4)),c(1,3),mean))
> 
> cheers,
> 
> Rolf Turner
> rolf at math.unb.ca
>
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