[R] How to average subgroups in a dataframe? (not sure how to apply aggregate(..))

[Tony Breyal]

Dear all,

Lets say I have the following data frame:

> set.seed(1)
> col1 <- c(rep('happy',9), rep('sad', 9))
> col2 <- rep(c(rep('alpha', 3), rep('beta', 3), rep('gamma', 3)),2)
> dates <- as.Date(rep(c('2009-10-13', '2009-10-14', '2009-10-15'),6))
> score=rnorm(18, 10, 3)
> df1<-data.frame(col1=col1, col2=col2, Date=dates, score=score)

    col1  col2       Date     score
1  happy alpha 2009-10-13  8.120639
2  happy alpha 2009-10-14 10.550930
3  happy alpha 2009-10-15  7.493114
4  happy  beta 2009-10-13 14.785842
5  happy  beta 2009-10-14 10.988523
6  happy  beta 2009-10-15  7.538595
7  happy gamma 2009-10-13 11.462287
8  happy gamma 2009-10-14 12.214974
9  happy gamma 2009-10-15 11.727344
10   sad alpha 2009-10-13  9.083835
11   sad alpha 2009-10-14 14.535344
12   sad alpha 2009-10-15 11.169530
13   sad  beta 2009-10-13  8.136278
14   sad  beta 2009-10-14  3.355900
15   sad  beta 2009-10-15 13.374793
16   sad gamma 2009-10-13  9.865199
17   sad gamma 2009-10-14  9.951429
18   sad gamma 2009-10-15 12.831509


Is it possible to get the following, whereby I am averaging the values
within each group of values in col2:

    col1  col2       Date     score   Average
1  happy alpha 13/10/2009  8.120639  8.721561
2  happy alpha 14/10/2009 10.550930  8.721561
3  happy alpha 15/10/2009  7.493114  8.721561
4  happy  beta 13/10/2009 14.785842 11.104320
5  happy  beta 14/10/2009 10.988523 11.104320
6  happy  beta 15/10/2009  7.538595 11.104320
7  happy gamma 13/10/2009 11.462287 11.801535
8  happy gamma 14/10/2009 12.214974 11.801535
9  happy gamma 15/10/2009 11.727344 11.801535
10   sad alpha 13/10/2009  9.083835 11.596236
11   sad alpha 14/10/2009 14.535344 11.596236
12   sad alpha 15/10/2009 11.169530 11.596236
13   sad  beta 13/10/2009  8.136278  8.288990
14   sad  beta 14/10/2009  3.355900  8.288990
15   sad  beta 15/10/2009 13.374793  8.288990
16   sad gamma 13/10/2009  9.865199 10.882712
17   sad gamma 14/10/2009  9.951429 10.882712
18   sad gamma 15/10/2009 12.831509 10.882712


My feeling is that I should be using the ?aggregate is some fashion
but can't see how to do it. Or possibly there's another function i
should be using?

Thanks in advance,
Tony

O/S: Windows Vista Ultimate
> sessionInfo()
R version 2.9.2 (2009-08-24)
i386-pc-mingw32

locale:
LC_COLLATE=English_United Kingdom.1252;LC_CTYPE=English_United Kingdom.
1252;LC_MONETARY=English_United Kingdom.
1252;LC_NUMERIC=C;LC_TIME=English_United Kingdom.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods
base