dplyr 计数变量的一个特定值 [英] dplyr count number of one specific value of variable

问题描述

假设我有一个这样的数据集:

Say I have a dataset like this:

id <- c(1, 1, 2, 2, 3, 3)
code <- c("a", "b", "a", "a", "b", "b")
dat <- data.frame(id, code)

即，

    id  code
1   1   a
2   1   b 
3   2   a
4   2   a
5   3   b
6   3   b

使用 dplyr，我如何计算每个 id 有多少个 a

Using dplyr, how would I get a count of how many a's there are for each id

即

   id  countA
1   1   1
2   2   2
3   3   0

我正在尝试这样的东西，但不起作用，

I'm trying stuff like this which isn't working,

countA<- dat %>%
group_by(id) %>%
summarise(cip.completed= count(code == "a"))

上面给了我一个错误，错误:没有适用于group_by_"的方法应用于类逻辑"的对象"

The above gives me an error, "Error: no applicable method for 'group_by_' applied to an object of class "logical""

感谢您的帮助！

推荐答案

尝试以下方法:

library(dplyr)
dat %>% group_by(id) %>%
  summarise(cip.completed= sum(code == "a"))

Source: local data frame [3 x 2]
    id cip.completed
  (dbl)         (int)
1     1             1
2     2             2
3     3             0

这是可行的，因为逻辑条件 code == a 只是一系列零和一，而这一系列的总和就是出现的次数.

This works because the logical condition code == a is just a series of zeros and ones, and the sum of this series is the number of occurences.

请注意，无论如何，您不一定在 summarise 中使用 dplyr::count，因为它是 summarise 调用任一 的包装器>n() 或 sum() 本身.参见 ?dplyr::count.如果您真的想使用 count，我想您可以通过首先过滤数据集以仅保留 code==a 中的所有行，然后使用 count 然后会给你所有严格的正数(即非零)计数.例如，

Note that you would not necessarily use dplyr::count inside summarise anyway, as it is a wrapper for summarise calling either n() or sum() itself. See ?dplyr::count. If you really want to use count, I guess you could do that by first filtering the dataset to only retain all rows in which code==a, and using count would then give you all strictly positive (i.e. non-zero) counts. For instance,

dat %>% filter(code==a) %>% count(id)

Source: local data frame [2 x 2]

     id     n
  (dbl) (int)
1     1     1
2     2     2

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