Find max per group and return another column

greatest-n-per-group, matrix, r

Solution

Base solution, not as simple as Dan M's:

testMatrix <- data.frame(GroupID = c(1,1,2), ElementID = c(10,20,30), 
    Value=c(300,100,200), Name=c("A","B","C"))

A <- lapply(split(testMatrix, testMatrix$GroupID), function(x) {
        x[which.max(x$Value), c(1, 4)]
    }
)
do.call(rbind, A)

Problem

Given the following test matrix: ``` testMatrix <- matrix( c(1,1,2,10,20,30,300,100,200,"A","B","C"), 3, 4) colnames(testMatrix) <- c("GroupID", "ElementID", "Value", "Name") ``` Here I want to find the max per group and then return the name of that column. E.g. I would expect 1, A and 2, C. If there is a tie with max, the first match would be fine. After that I would have to attach this to the matrix with a new Column "GroupName" How can I do this? I already have the Group, Max Value combination: ``` groupMax <- aggregate (as.numeric(testMatrix[,3]), by=list( testMatrix[,1] ), max ) ``` The way I used to add columns to my matrix works like this (let's assume there is also already a matrix groupNames with GroupID, Name combinations): ``` testMatrix <- cbind ( testMatrix, groupNames[match( testMatrix[,1], groupNames[,1] ), 2] ) ```

Original source