R:在每列的每两行中找到最大值 [英] R: finding maximum value every two rows in each column

问题描述

我想每2行(例如)在每一列中找到最大值.如何在R中做到这一点?例如

I want to find maximum value in each column for every 2 rows (say). How to do that in R? For example

matrix(c(3,1,20,5,4,12,6,2,9,7,8,7), byrow=T, ncol=3) 

我想要这样的输出

matrix(c(5,4,20,7,8,9), byrow=T, ncol=3) 

推荐答案

这是一种实现方法.

1. 定义一个向量，该向量包含有关所需的groups的信息.在这种情况下，我使用rep重复数字序列.
2. 然后定义一个辅助函数来计算数组的列最大值-这是max的简单apply.
3. 最后，将sapply与匿名函数一起使用，该匿名函数会将colMax应用于每个分组的数组子集.

1. Define a vector that contains information about the groups you want. In this case, I use rep to repeat a sequence of numbers.
2. Then define a helper function to calculate the column maximum of an array — this is a simple apply of max.
3. finally, use sapply with an anonymous function that applies colMax to each of your grouped array subsets.

代码:

groups <- rep(1:2, each=2)
colMax <- function(x)apply(x, 2, max)
t(
    sapply(unique(groups), function(i)colMax(x[which(groups==i), ]))
)

结果:

     [,1] [,2] [,3]
[1,]    5    4   20
[2,]    7    8    9

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