sum multiple columns by group with tapply

Question

I wanted to sum individual columns by group and my first thought was to use tapply. However, I cannot get tapply to work. Can tapply

Answer 1

tapply works on a vector, for a data.frame you can use by (which is a wrapper for tapply, take a look at the code):

> by(df.1[,c(3:5)], df.1$state, FUN=colSums)
df.1$state: AA
  apples cherries    plums 
     111      222      333 
------------------------------------------------------------------------------------- 
df.1$state: BB
  apples cherries    plums 
    -111     -222     -333