我想在一列中重新排序因子的级别，但在分组列定义的组内。
简单示例数据集：
df <- structure(list(a_factor = structure(1:6, .Label = c("a", "b",
"c", "d", "e", "f"), class = "factor"), group = structure(c(1L,
1L, 1L, 2L, 2L, 2L), .Label = c("group1", "group2"), class = "factor"),
value = 1:6), class = "data.frame", row.names = c(NA, -6L
))
> df
a_factor group value
1 a group1 1
2 b group1 2
3 c group1 3
4 d group2 4
5 e group2 5
6 f group2 6
更准确地说，我如何重新排序因子级别，例如按valuewhere降序df$group == "group1"，但按 …