Aggregate multiple rows of the same data.frame in R based on common values in given columns

Question

I have a data.frame that looks like this:

# set example data
df <- read.table(textConnection(\"item\\tsize\         


        

Answer 1

The data.table solution...

require(data.table)
DT <- data.table(df)

DT[ , lapply(.SD , mean ) , by = item ]
   item size weight value
1:    A    2      3     5
2:    B    1      2     3
3:    C    3      2     1