Question

if I have the following dataframe:

value    factorA    factorB
1           a          e
2           a          f
3           a          g
1           b          k
2           b          l
3           b          m
1           c          e
2           c          g

how can I get for each factorA the highest value and the entry from factorB associated with it i.e.

value   factorA factorB

3         a        g
3         b        m
2         c        g

Is this possible without first using

blocks<-split(factorA, list(), drop=TRUE)

and then sorting each block$a as this will be performed many times and number of blocks will always change.

Answer 1

Here is one option, using base R functions:

maxRows <- by(df, df$factorA, function(X) X[which.max(X$value),])
do.call("rbind", maxRows)
#   value factorA factorB
# a     3       a       g
# b     3       b       m
# c     2       c       g