r - 使用 dplyr 获取以变量为条件的 R 数据帧的第 n、第一或最后一行

Question

由于 nth(y, -1) 返回的是值而不是数据帧，并且到目前为止我没有成功使用 row_number() 或 slice() 我想知道我是否遗漏了任何明显的东西或者应该重新考虑该方法:

我创建了一个函数:

ranking <- function(df, state, num = "last"){
    ## Setting num for "last" condition to n() gives error
    num <- ifelse(num=="first", 1, ifelse(num=="last", -1, num)) 
    df %>%
        filter(State == state) %>%
        arrange(y, State) %>%
        slice(num)  ## Does not work for -1
}

所以对于下面的df:

df <- data.frame(State=c("TX","TX","TX","MD"),y=c(5,2,3,4))
   State y
1     TX 5
2     TX 2
3     TX 3
4     MD 4

我想为适当的“最后一个”和第 n 个函数调用返回以下内容:

ranking(df, "TX", "last")
   State y
1     TX 5
ranking(df, "TX", 2)
   State y
1     TX 3

Answer 1

library(dplyr)
ranking <- function(df, state, num = "last"){
  ## Setting num for "last" condition to n() gives error
  df %>%
    filter(State == state) %>%
    arrange(y) %>%
    slice(ifelse(num=="first", 1, ifelse(num=="last", n(), num)) )
}


df <- data.frame(State=c("TX","TX","TX","MD", "MD"),y=c(5,2,3,4,9))
ranking(df, "TX", "last")
ranking(df, "TX", 2)
ranking(df, "MD", "first")

# State     y
#    TX     5
# State     y
#    TX     3
# State     y
#    MD     4