计算R中每行中特定列的下3行的最大值

我想创建一个列"Y",它给出R中每行中特定列"X"的下三行的最大值.

例如-

X  Y
1  4 (4=max(2,3,4))
2  7 (7= max(3,4,7))
3  9 (9=max(4,7,9))
4
7
9
2
4

有人能帮忙吗？

鉴于:

x <- c(1, 2, 3, 4, 7, 9, 2, 4)

你可以看一下这个embed功能:

embed(x, 3)
#      [,1] [,2] [,3]
# [1,]    3    2    1
# [2,]    4    3    2
# [3,]    7    4    3
# [4,]    9    7    4
# [5,]    2    9    7
# [6,]    4    2    9

请注意,这不是正确的长度,我们对第一行不感兴趣,所以让我们做一个修改:

embed(c(x[-1], 0, 0, 0), 3)
#      [,1] [,2] [,3]
# [1,]    4    3    2
# [2,]    7    4    3
# [3,]    9    7    4
# [4,]    2    9    7
# [5,]    4    2    9
# [6,]    0    4    2
# [7,]    0    0    4
# [8,]    0    0    0

从那里,它应该是一个简单的:

apply(embed(c(x[-1], 0, 0, 0), 3), 1, max)
# [1] 4 7 9 9 9 4 4 0

为方便起见,作为一个功能:

this_by_n <- function(invec, n = 3, pad_val = NA, FUN = sum) {
  FUN <- match.fun(FUN)
  apply(embed(c(invec[-1], rep(pad_val, n)), n), 1, {
    function(x) if (all(is.na(x))) NA else FUN(x[!is.na(x)])
  })
}

试试看:

this_by_n(x, 3, NA, mean)
this_by_n(x, 2, NA, max)
this_by_n(x, 4, NA, min)