使用 data.table 的简单频率表 [英] Simple frequency tables using data.table

问题描述

我正在寻找一种通过 data.table 进行简单聚合/计数的方法.

I'm looking for a way to do simple aggregates / counts via data.table.

考虑 iris 数据，每个物种有 50 个观测值.要计算每个物种的观察结果，我必须对物种以外的列进行汇总，例如Sepal.Length".

Consider the iris data, which has 50 observations per species. To count the observations per species I have to summaries over a column other than species, for example "Sepal.Length".

library(data.table)
dt = as.data.table(iris)
dt[,length(Sepal.Length), Species]

我觉得这很令人困惑，因为乍一看我好像在对 Sepal.Length 做一些事情，而实际上只有物种才是重要的.

I find this confusing because it looks like I'm doing something on Sepal.Length at first glance, when really it's only Species that matters.

这是我想说的，但我没有得到有效的输出:

This is what I would prefer to say, but I don't get valid output:

dt[,length(Species), Species]

正确的输入和输出，但代码笨拙:

> dt[,length(Sepal.Length), Species]
Species V1
1:     setosa 50
2: versicolor 50
3:  virginica 50

输入输出不正确，但代码更好:

> dt[,length(Species), Species]
Species V1
1:     setosa  1
2: versicolor  1
3:  virginica  1

有没有优雅的方法解决这个问题?

推荐答案

data.table 有几个可以在 j 表达式中使用的符号.值得注意的是

data.table has a couple of symbols that can be used within the j expression. Notably

• .N 将为您提供每组中的行数.

• .N will give you the number of number of rows in each group.

查看 ?data.table 下的 by

高级:按by或i进行分组时，j表达式中可以使用符号.SD、.BY和.N，定义如下.

Advanced: When grouping by by or by i, symbols .SD, .BY and .N may be used in the j expression, defined as follows.

....

.N 是一个整数，长度为 1，包含组中的行数.

.N is an integer, length 1, containing the number of rows in the group.

例如:

dt[, .N ,by = Species]

     Species  N
1:     setosa 50
2: versicolor 50
3:  virginica 50

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