r - 如何从 R 数据帧中获取条件结果

Question

这是我在这里的第一条消息。我正在尝试解决 edX R 类(class)中的 R 练习，但我陷入了困境。如果有人可以帮助我解决它，那就太好了。以下是给出的数据框和问题:

> students
   height shoesize gender population
1     181       44   male     kuopio
2     160       38 female     kuopio
3     174       42 female     kuopio
4     170       43   male     kuopio
5     172       43   male     kuopio
6     165       39 female     kuopio
7     161       38 female     kuopio
8     167       38 female    tampere
9     164       39 female    tampere
10    166       38 female    tampere
11    162       37 female    tampere
12    158       36 female    tampere
13    175       42   male    tampere
14    181       44   male    tampere
15    180       43   male    tampere
16    177       43   male    tampere
17    173       41   male    tampere

Given the dataframe above, create two subsets with students whose height is equal to or below the median height (call it students.short) and students whose height is strictly above the median height (call it students.tall). What is the mean shoesize for each of the above 2 subsets by population?

我已经能够创建两个子集 students.tall 和 students.short(均按 TRUE/FALSE 显示答案), 但我不知道如何通过人口获得平均值。数据应该像这样显示:

                    kuopio     tampere
students.short      xxxx       xxxx
students.tall       xxxx       xxxx

如果你能帮帮我，非常感谢!

Answer 1

我们可以根据中位数高度拆分

# // median height
medHeight <- median(students$height, na.rm = TRUE)

# // split the data into a list of data.frames using the 'medHeight'
lst1 <- with(students, split(students, height > medHeight))

然后遍历 list 使用来自 base R 的 aggregate

lapply(lst1, function(dat) aggregate(shoesize ~ population, 
        data = dat, FUN = mean, na.rm = TRUE))

---

但是，我们不需要创建两个单独的数据集或一个列表。可以通过将“人口”和使用 logical vector

创建的“grp”分组来完成

library(dplyr)
students %>%
     group_by(grp = height > medHeight, population) %>%
     summarise(shoesize = mean(shoesize))