question about mean

Store your 'matrix' as a data frame.  Call it 'SchoolVals' say.  Then

SchoolMeans <- with(SchoolVals, tapply(value, school, mean))

should do it.  If you have missing values you want to ignore:

SchoolMeans <- with(SchoolVals, tapply(value, school, mean, na.rm = TRUE))




Bill Venables
http://www.cmis.csiro.au/bill.venables/ 


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of cindy Guo
Sent: Monday, 9 February 2009 1:32 PM
To: r-help at r-project.org
Subject: [R] question about mean

Hello, everyone,

I have a matrix like following:

school  value
A          .1
A          .2
A          .15
A          .2
B          .3
B          .5
C          .3
C          .3
C          .4
C          .5
C          .6
C          .9
C          1

I want to get the mean 'value' for each 'school', but each school has
different length. Is there any way to do this fast? Because my data has
hundreds of thousands of rows.

Thank you,

Cindy


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Store your 'matrix' as a data frame.

Surely it's a data frame already, since ``school'' is character or  
factor,
	and ``value'' is (must be?) numeric.

	People have this unfortunate predilection to refer to data frames as
	matrices, and this messes them, and everybody else, up.

		cheers,

			Rolf


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On 9/02/2009, at 4:40 PM, Bill.Venables at csiro.au wrote:

Store your 'matrix' as a data frame.

    Surely it's a data frame already, since ``school'' is character or
factor,
    and ``value'' is (must be?) numeric.

    People have this unfortunate predilection to refer to data frames as
    matrices, and this messes them, and everybody else, up.

still, a naive user's messing up data frames with matrices seems no
worse than an experrienced (?) r programmer's messing up lists with vectors.

vQ