find highest value within factor levels

r

Solution

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

Problem

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.

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