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

aggregate(value ~ item + size + weight, FUN = mean, data=df)

  item size weight value
1    B    1      2     3
2    C    3      2     1
3    A    2      3     5