查找每个组的最大值并返回另一列 [英] Find max per group and return another column

问题描述

给出以下测试矩阵:

testMatrix <- matrix( c(1,1,2,10,20,30,300,100,200,"A","B","C"), 3, 4)

colnames(testMatrix) <- c("GroupID", "ElementID", "Value", "Name")

在这里，我想找到每个组的最大值，然后返回该列的名称. 例如.我期望1，A和2，C.如果与max并列，则第一场比赛就可以了. 之后，我必须使用新的"GroupName"列将其附加到矩阵上

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"

我该怎么做?

我已经有了组，最大值"组合:

I already have the Group, Max Value combination:

groupMax <- aggregate (as.numeric(testMatrix[,3]), by=list( testMatrix[,1] ), max )

我用来向矩阵中添加列的方式是这样的(假设已经有一个具有GroupID，Name组合的矩阵groupNames):

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] ) 

推荐答案

基本解决方案，不如Dan M的简单:

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)

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