Selecting the top X values of each individual column in R

Question

Say you have the following data set.

df1<-matrix(data = 1:10,
            nrow = 5,
            ncol = 5)
colnames(df1)=c("a","b","c","d","e")

How would you extract the top X values from each individual column as a new data frame?

The expected output would be something like this (for the top 3 values in each column

a	b	c	d	e
5	10	5	10	5
4	9	4	9	4
3	8	3	8	3

Answer 1

You can use apply to apply a function to each column (MARGIN = 2). Here, the function is \(x) head(sort(x, decreasing = T), 3), which sorts the column by decreasing order, and select the top three values (head(x, 3)).

apply(df1, 2, \(x) head(sort(x, decreasing = T), 3))

     a  b c  d e
[1,] 5 10 5 10 5
[2,] 4  9 4  9 4
[3,] 3  8 3  8 3

_{Note: \(x) is a shorthand for function(x) in lambda-like functions since 4.1.0.}

Answer 2

We can sort, then use head:

head(apply(df1, 2, sort, decreasing = TRUE), 3)
#      a  b c  d e
# [1,] 5 10 5 10 5
# [2,] 4  9 4  9 4
# [3,] 3  8 3  8 3