R: loop through data frame extracting subset of data depending on date

Question

I have a large data frame that consists of data that looks something like this:

        date    w    x    y    z    region
1    2012 01    21   43   12    3   NO         


        

Answer 1

After sub-setting your dataset by date, imagine that the function you would like to apply to each subset is to find the mean of the column x. You could do it this way: (df is your dataframe)

 library(plyr)
 ddply(df, .(date), summarize, mean = mean(x))