Extract the maximum value within each group in a dataframe [duplicate]

Question

I have a data frame with a grouping variable ("Gene") and a value variable ("Value"):

Gene   Value A      12 A      10 B      3 B      5 B      6 C      1 D      3 D      4 

For each level of my grouping variable, I wish to extract the maximum value. The result should thus be a data frame with one row per level of the grouping variable:

Gene   Value A      12 B      6 C      1 D      4 

Could aggregate do the trick?

Answer 1

There are many possibilities to do this in R. Here are some of them:

df <- read.table(header = TRUE, text = 'Gene   Value A      12 A      10 B      3 B      5 B      6 C      1 D      3 D      4')  # aggregate aggregate(df$Value, by = list(df$Gene), max) aggregate(Value ~ Gene, data = df, max)  # tapply tapply(df$Value, df$Gene, max)  # split + lapply lapply(split(df, df$Gene), function(y) max(y$Value))  # plyr require(plyr) ddply(df, .(Gene), summarise, Value = max(Value))  # dplyr require(dplyr) df %>% group_by(Gene) %>% summarise(Value = max(Value))  # data.table require(data.table) dt <- data.table(df) dt[ , max(Value), by = Gene]  # doBy require(doBy) summaryBy(Value~Gene, data = df, FUN = max)  # sqldf require(sqldf) sqldf("select Gene, max(Value) as Value from df group by Gene", drv = 'SQLite')  # ave df[as.logical(ave(df$Value, df$Gene, FUN = function(x) x == max(x))),]