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

df$value <- ave(df$value,df$item,FUN=mean)
df[!duplicated(df$item),]

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