使用 group_by(多个变量)时的 dplyr 问题 [英] dplyr issues when using group_by(multiple variables)

问题描述

我想开始使用 dplyr 代替 ddply 但我不知道它是如何工作的(我已经阅读了文档).

I want to start using dplyr in place of ddply but I can't get a handle on how it works (I've read the documentation).

例如，为什么当我尝试 mutate() 时，group_by"函数无法正常工作?

For example, why when I try to mutate() something does the "group_by" function not work as it's supposed to?

看 mtcars:

图书馆(汽车)

假设我制作了一个 data.frame，它是 mtcars 的摘要，按cyl"和gear"分组:

Say I make a data.frame which is a summary of mtcars, grouped by "cyl" and "gear":

df1 <- mtcars %.%
            group_by(cyl, gear) %.%
            summarise(
                newvar = sum(wt)
            )

然后说我想进一步总结这个数据框.使用 ddply，它会很简单，但是当我尝试使用 dplyr 时，它实际上并不是分组依据":

Then say I want to further summarise this dataframe. With ddply, it'd be straightforward, but when I try to do with with dplyr, it's not actually "grouping by":

df2 <- df1 %.%
            group_by(cyl) %.%
            mutate(
                newvar2 = newvar + 5
            )

仍然产生未分组的输出:

Still yields an ungrouped output:

  cyl gear newvar newvar2
1   6    3  6.675  11.675
2   4    4 19.025  24.025
3   6    4 12.375  17.375
4   6    5  2.770   7.770
5   4    3  2.465   7.465
6   8    3 49.249  54.249
7   4    5  3.653   8.653
8   8    5  6.740  11.740

我的语法有问题吗?

如果我用 plyr 和 ddply 来做这件事:

If I were to do this with plyr and ddply:

df1 <- ddply(mtcars, .(cyl, gear), summarise, newvar = sum(wt))

然后得到第二个df:

df2 <- ddply(df1, .(cyl), summarise, newvar2 = sum(newvar) + 5)

但是同样的方法，在 summarise() 函数中使用 sum(newvar) + 5 不适用于 dplyr ...

But that same approach, with sum(newvar) + 5 in the summarise() function doesn't work with dplyr...

推荐答案

让 Dickoa 的答案更进一步——正如 Hadley 所说的总结剥离了单层分组".它从您应用它的相反顺序剥离分组，以便您可以使用

Taking Dickoa's answer one step further -- as Hadley says "summarise peels off a single layer of grouping". It peels off grouping from the reverse order in which you applied it so you can just use

mtcars %>%
 group_by(cyl, gear) %>%
 summarise(newvar = sum(wt)) %>%
 summarise(newvar2 = sum(newvar) + 5)

请注意，如果您在第二行中使用 group_by(gear, cyl)，这将给出不同的答案.

Note that this will give a different answer if you use group_by(gear, cyl) in the second line.

为了让您的第一次尝试成功:

And to get your first attempt working:

df1 <- mtcars %>%
 group_by(cyl, gear) %>%
 summarise(newvar = sum(wt))

df2 <- df1 %>%
 group_by(cyl) %>%
 summarise(newvar2 = sum(newvar)+5)

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