For each row return the column name of the largest value

by

One option using your data (for future reference, use set.seed() to make examples using sample reproducible):

DF <- data.frame(V1=c(2,8,1),V2=c(7,3,5),V3=c(9,6,4))

colnames(DF)[apply(DF,1,which.max)]
[1] "V3" "V1" "V2"

A faster solution than using apply might be max.col:

colnames(DF)[max.col(DF,ties.method="first")]
#[1] "V3" "V1" "V2"

…where ties.method can be any of "random" "first" or "last"

This of course causes issues if you happen to have two columns which are equal to the maximum. I’m not sure what you want to do in that instance as you will have more than one result for some rows. E.g.:

DF <- data.frame(V1=c(2,8,1),V2=c(7,3,5),V3=c(7,6,4))
apply(DF,1,function(x) which(x==max(x)))

[[1]]
V2 V3 
 2  3 

[[2]]
V1 
 1 

[[3]]
V2 
 2

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