与最大值和因子汇总

Question

我有一个包含因子列的data.frame,我想在其上计算最大值(或最小值或分位数).我不能在因素上使用这些功能,但我想.

这是一些例子:

set.seed(3)
df1 <- data.frame(id = rep(1:5,each=2),height=sample(c("low","medium","high"),size = 10,replace=TRUE))
df1$height <- factor(df1$height,c("low","medium","high"))
df1$height_num <- as.numeric(df1$height)
# > df1
#    id height height_num
# 1   1    low          1
# 2   1   high          3
# 3   2 medium          2
# 4   2    low          1
# 5   3 medium          2
# 6   3 medium          2
# 7   4    low          1
# 8   4    low          1
# 9   5 medium          2
# 10  5 medium          2

我可以轻松地做到这一点:

aggregate(height_num ~ id,df1,max)
#   id height_num
# 1  1          3
# 2  2          2
# 3  3          2
# 4  4          1
# 5  5          2

但不是这个:

aggregate(height ~ id,df1,max)
# Error in Summary.factor(c(2L, 2L), na.rm = FALSE) : 
#   ‘max’ not meaningful for factors

我想采用最大的"高度",并在我的聚合表中保持与原始表中相同的级别.在我的真实数据中,我有很多列,我想保持我的因素排序,以保持我的情节清洁和一致.

我可以这样做,并在其他聚合函数中使用以下结构:

use_factors <- function(x,FUN){factor(levels(x)[FUN(as.numeric(x))],levels(x))}
aggregate(height ~ id,df1,use_factors,max)
#   id height
# 1  1   high
# 2  2 medium
# 3  3 medium
# 4  4    low
# 5  5 medium

或者我可以超载max min median和quantile我想的功能但是我觉得我肯定会重新发明轮子.

是否有捷径可寻 ？

Answer 1

实际上, 如果使用有序因子,则可以进行所需的聚合.

set.seed(3)
df1 <- data.frame(id = rep(1:5,each=2),height=sample(c("low","medium","high"),size = 10,replace=TRUE))
df1$height <- factor(df1$height,c("low","medium","high"), ordered = TRUE)
df1$height_num <- as.numeric(df1$height)

aggregate(height~id, df1, max) 
  id height
1  1   high
2  2 medium
3  3 medium
4  4    low
5  5 medium