[R] group means

[Henrik Bengtsson]

When all groups have the same number of elements and the groups are
consecutive I normally transform the vector into a matrix where each
column contains data from one group. Then I perform whatever on each
group using apply():

x <- c(3.4, 6.0, 2.5, 7.5, 1.8, 4.2, 6.4, 5.7, 17.2, 13.5)
xm <- matrix(x, nrow=5)   # matrix() "fills by column" by default
print(xm)
#      [,1] [,2]
# [1,]  3.4  4.2
# [2,]  6.0  6.4
# [3,]  2.5  5.7
# [4,]  7.5 17.2
# [5,]  1.8 13.5
m <- apply(xm, MARGIN=2, FUN=mean, na.rm=TRUE) # MARGIN=2 means "along
columns" or "columnwise"
print(m) 
# [1] 4.24 9.40

Hope this helps

Henrik Bengtsson

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> -----Original Message-----
> From: r-help-admin at stat.math.ethz.ch 
> [mailto:r-help-admin at stat.math.ethz.ch] On Behalf Of Jeremy Z Butler
> Sent: den 21 februari 2003 09:44
> To: r-help at stat.math.ethz.ch
> Subject: [R] group means
> 
> 
> Hi,
> Any hints on how I would generate the means of each 5 number 
> group in a 
> column of numbers in data.frame form. i.e. get mean of first 
> five in column 
> and then mean of second five in column etc. etc.
> 
> 1   3.4
> 2   6.0
> 3   2.5
> 4   7.5
> 5   1.8
> 6   4.2
> 7   6.4
> 8   5.7
> 9   17.2
> 10  13.5
> 
> Grateful for any suggestions
> Jeremy
> 
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