我有这样一个数据框:
df <- structure(list(a = c(NA, NA, 1L, 2L, 3L, 4L, 5L, 6L, 7L, 8L), b = c(NA, NA, NA, 1L, 2L, 3L, 4L, 5L, 6L, 7L), d = c(NA, NA, NA, NA, 1L, 2L, 3L, 4L, 5L, 6L)), .Names = c("a", "b", "d"), row.names = c(NA, -10L), class = "data.frame")
> df
a b d
1 NA NA NA
2 NA NA NA
3 1 NA NA
4 2 1 NA
5 3 2 1
6 4 3 2 …