From: Gavin Simpson <gavin.simpson_at_ucl.ac.uk>

Date: Tue, 22 May 2007 18:21:24 +0100

Date: Tue, 22 May 2007 18:21:24 +0100

On Tue, 2007-05-22 at 19:01 +0200, Benoit Chemineau wrote:

> Hello,

*> I have a basic problem but i can't figure it out with the
**> table underneath. I would like to compute monthly averages.
**> I would like to have the average measure for month #5 for the first
**> three rows (the same number in the first three lines) and the average
**> measure for month #6 for the last four rows ((the same number in the first
**> three lines) in a separate vesctor (let's call it 'result')
**> I tried to use a "while" statement inside a "for" loop but it doesn't
**> seem to work.
**> Can someone please help me with this ?
**>
**> Measure Month
**> 2.28 5
**> 14.04 5
**> 0.60 5
**> 0.21 6
**> 0.96 6
**> 0.75 6
**> 1.28 6
*

If dat is a data frame containing your data:

*> dat
*

Measure Month

1 2.28 5 2 14.04 5 3 0.60 5 4 0.21 6 5 0.96 6 6 0.75 6 7 1.28 6

> aggregate(dat$Measure, by = list(Month = dat$Month), mean)

Month x

1 5 5.64 2 6 0.80

> tapply(dat$Measure, dat$Month, mean)

5 6

5.64 0.80

see ?aggregate and ?tapply for two solutions. The tapply one seems cleaner and easier to get the vector you need, the aggregate version needs an extra step:

aggregate(dat$Measure, by = list(Month = dat$Month), mean)$x

^^Note the $x at the end to subset the object returned by aggregate

**HTH
**
G

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