Question

I have a dataframe like this:

block   plot    date    data
1   1   aug 11.95171507
1   1   aug 18.41451063
1   2   aug 9.506155236
1   2   aug 13.26259947
1   3   aug 17.53616835
1   3   sep 15.40950767
2   1   sep 23.03616678
2   1   sep 17.07067258
2   2   sep 11.58278798
2   2   sep 13.15443304

I would like to calculate the means of data across plot based on block and date. Eventually, i would like to have 5 means.
Thanks for your help.

Answer 1

Probably the path of least resistance is to use plyr:

library(plyr)
ddply(yourData, c("block", "date"), summarize, outVal = mean(data))

You can do similar things with data.table, aggregate, by and probably a whole host of other functions. Take a few minutes to peruse the R tag here on SO.