在R中提取数据帧的子集

Question

Suppose I read a csv file into a data frame called "d". I wish to print the last 2 rows of this data frame. I tried the below but it is printing all the content starting from n-1. Can someone help me understand this behavior please?

> n<-nrow(d)
> n
[1] 153
> subset(d[n:n-1,])

Answer 1

You can just use tail

  tail(d, 2)

Will give the last two rows.

Answer 2

@mnel is correct that using tail() would probably be the easiest, however I think that your confusion has to do with how subset() and indexing work in general. In your example be mindful of how you index matrices and data.frames since

d[(n:n - 1), ]

is not the same as

d[n:(n-1), ]

so check the difference carefully since the order of operations are important to understand. The subset() function also indexes based on a logical indicator and has the form

 subset(object, subset = logicalvector)

where the logical vector gives the rows that you want to extract. See ?subset for more detail.

Answer 3

This is working for me ...

d <- matrix(1:10,nrow=5)
d
     [,1] [,2]
[1,]    1    6
[2,]    2    7
[3,]    3    8
[4,]    4    9
[5,]    5   10

d <- as.data.frame(d)
d
  V1 V2
1  1  6
2  2  7
3  3  8
4  4  9
5  5 10

n <- nrow(d)
> n
[1] 5
d[n:(n-1),] ## Specifying the number of the row inside the brackets.
 V1 V2
5  5 10
4  4  9

d[n:n-1,] ## without brackets it will do 5:5 -1 = 4, so printing only the fourth row
  V1 V2
4  4  9

Answer 4

我会用： tail(d, 2)或d[(n-1):n, ]希望它有所帮助