如何根据最后两行的值对 data.frame 进行子集化？

Question

###the original data

df1 <- data.frame(a=c(2,2,5,5,7), b=c(1,5,4,7,6))
df2 <- data.frame(a=c(2,2,5,5,7,7), b=c(1,5,4,7,6,3))

when the a column value of the last two rows are not equal (here the 4th row is not equal to the 5th row, namely, 5!=7), I want to subset the last row only.

#input

 > df1
      a b
    1 2 1
    2 2 5
    3 5 4
    4 5 7
    5 7 6

#output

> df1
  a b
1 7 6

when the a column value of the last two rows are equal (here 5th row is equal to the 6th row, namely, 7=7, I want to subset the last two rows

#input

> df2
  a b
1 2 1
2 2 5
3 5 4
4 5 7
5 7 6
6 7 3

#output

> df2
  a b
1 7 6
2 7 3

Answer 1

You can write a function to check last two row values for a column :

return_rows <- function(data) {
  n <- nrow(data)
  if(data$a[n] == data$a[n - 1])
      tail(data, 2)
  else tail(data, 1)
}

return_rows(df1)
#  a b
#5 7 6

return_rows(df2)
#  a b
#5 7 6
#6 7 3

Answer 2

try it this way

library(tidyverse)
df %>% 
  filter(a == last(a))

  a b
5 7 6

  a b
5 7 6
6 7 3

Answer 3

我们可以使用base R subset

subset(df1, a == a[length(a)])