# [R] Odp: means and variances of several groups in the matrix

Petr PIKAL petr.pikal at precheza.cz
Mon Apr 21 12:40:15 CEST 2008

```Hi

r-help-bounces at r-project.org napsal dne 21.04.2008 09:03:30:

>
> Dear R users,
>
> I have 32 observations in data x.  After sorting this, I want to compute
> means and variances of 3 groups divided by "nr".
>
> Actually, the number of groups is flexible.  Any suggestion will be
greatly
> appreciated.
>
> Kathryn Lord
>
>
---------------------------------------------------------------------------
> x=rnorm(32)
> y=sort(x)
>
> nr=matrix(c(12,11,10,10,10,11),2,3)
> > nr
>      [,1] [,2] [,3]
> [1,]   12   10   10    -> sum=32
> [2,]   11   10   11    -> sum=32
>
> For the 1st row in "nr", index of y = (1,..,12,   13,...,23,   24,...32)
>
> I want to compute means and variances for 3 groups
>
> (1st group is 1 through 12; 2nd group is 13-23; 3rd group is 24-32)
>
>
> For the 2nd row in "nr", index of y = (1,..,11,   12,...,22,   23,...32)
>
> also, I want to compute means and variances for 3 groups
>
> (1st group is 1 through 11; 2nd group is 12-22; 3rd group is 23-32)
>

If you know that your vector is sorted you can do

sapply(split(y,rep(1:3, times=nr[1,])), mean)

Regards
Petr

>
>
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