使用 dplyr 按组计算行数 [英] Count number of rows by group using dplyr

问题描述

我正在使用 mtcars 数据集.我想找到特定数据组合的记录数.与 SQL 中的 count(*) group by 子句非常相似.来自 plyr 的 ddply() 对我有用

I am using the mtcars dataset. I want to find the number of records for a particular combination of data. Something very similar to the count(*) group by clause in SQL. ddply() from plyr is working for me

library(plyr)
ddply(mtcars, .(cyl,gear),nrow)

有输出

  cyl gear V1
1   4    3  1
2   4    4  8
3   4    5  2
4   6    3  2
5   6    4  4
6   6    5  1
7   8    3 12
8   8    5  2

使用此代码

library(dplyr)
g <- group_by(mtcars, cyl, gear)
summarise(g, length(gear))

有输出

  length(cyl)
1          32

我发现各种函数可以传递给 summarise() 但似乎没有一个对我有用.我发现的一个函数是 sum(G)，它返回

I found various functions to pass in to summarise() but none seem to work for me. One function I found is sum(G), which returned

Error in eval(expr, envir, enclos) : object 'G' not found

尝试使用 n()，返回

Error in n() : This function should not be called directly

我做错了什么?我怎样才能让 group_by()/summarise() 为我工作?

What am I doing wrong? How can I get group_by() / summarise() to work for me?

推荐答案

dplyr 中有一个特殊的函数 n() 来计算行数(可能在组内):

There's a special function n() in dplyr to count rows (potentially within groups):

library(dplyr)
mtcars %>% 
  group_by(cyl, gear) %>% 
  summarise(n = n())
#Source: local data frame [8 x 3]
#Groups: cyl [?]
#
#    cyl  gear     n
#  (dbl) (dbl) (int)
#1     4     3     1
#2     4     4     8
#3     4     5     2
#4     6     3     2
#5     6     4     4
#6     6     5     1
#7     8     3    12
#8     8     5     2

但是 dplyr 还提供了一个方便的 count 函数，它的功能完全相同，但输入次数更少:

But dplyr also offers a handy count function which does exactly the same with less typing:

count(mtcars, cyl, gear)          # or mtcars %>% count(cyl, gear)
#Source: local data frame [8 x 3]
#Groups: cyl [?]
#
#    cyl  gear     n
#  (dbl) (dbl) (int)
#1     4     3     1
#2     4     4     8
#3     4     5     2
#4     6     3     2
#5     6     4     4
#6     6     5     1
#7     8     3    12
#8     8     5     2

这篇关于使用 dplyr 按组计算行数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！