# [R] group means

Spencer Graves spencer.graves at pdf.com
Fri Feb 21 01:12:03 CET 2003

```How about:

> group.means <- function(x, k=5){
+  n.gps <- floor(length(x)/k)
+  rep(1, k) %*% array(x[1:(k*n.gps)], dim=c(k, n.gps))
+ }
> group.means(c(3.4, 6.0, 2.5, 7.5, 1.8, 4.2, 6.4, 5.7, 17.2, 13.5))
[,1] [,2]
[1,] 21.2   47

Best Wishes,
Spencer Graves

Henrik Bengtsson wrote:
> 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
>
> Home: 201/445 Royal Parade, 3052 Parkville
> Office: Bioinformatics, WEHI, Parkville
> +61 (0)412 269 734 (cell), +61 (0)3 9345 2324 (lab),
> +1 (508) 464 6644 (global fax)
> hb at wehi.edu.au, http://www.maths.lth.se/~hb/
> Time zone: +11h UTC (Sweden +1h UTC, Calif. -8h UTC)
>
>
>>-----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
>>
>>______________________________________________
>>R-help at stat.math.ethz.ch mailing list
>>http://www.stat.math.ethz.ch/mailman/listinfo/> r-help
>>
>>
>
>
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```