使用data.table计数和汇总/汇总列 [英] Use data.table to count and aggregate / summarize a column

问题描述

我想对 data.table 中的一列进行计数和汇总，所以找不到最有效的方法。这似乎与我想要的 R用data.table汇总多列。

I want to count and aggregate(sum) a column in a data.table, and couldn't find the most efficient way to do this. This seems to be close to what I want R summarizing multiple columns with data.table.

我的数据

set.seed(321)
dat <- data.table(MNTH = c(rep(201501,4), rep(201502,3), rep(201503,5), rep(201504,4)), 
                  VAR = sample(c(0,1), 16, replace=T))

> dat
     MNTH VAR
 1: 201501   1
 2: 201501   1
 3: 201501   0
 4: 201501   0
 5: 201502   0
 6: 201502   0
 7: 201502   0
 8: 201503   0
 9: 201503   0
10: 201503   1
11: 201503   1
12: 201503   0
13: 201504   1
14: 201504   0
15: 201504   1
16: 201504   0

我想通过 MNTH 对 VAR 进行计数和求和使用data.table。预期结果：

I want to both count and sum VAR by MNTH using data.table. The desired result:

    MNTH COUNT VAR
1 201501     4   2
2 201502     3   0
3 201503     5   2
4 201504     4   2

推荐答案

<您所指的帖子提供了一种关于如何将一种聚合方法应用于多个列的方法。如果要将不同的汇总方法应用于不同的列，则可以执行以下操作：

The post you are referring to gives a method on how to apply one aggregation method to several columns. If you want to apply different aggregation methods to different columns, you can do:

dat[, .(count = .N, var = sum(VAR)), by = MNTH]

这将导致：

     MNTH count var
1: 201501     4   2
2: 201502     3   0
3: 201503     5   2
4: 201504     4   2

您还可以通过以下方式通过更新数据集将这些值添加到现有数据集中：

You can also add these values to your existing dataset by updating your dataset by reference:

dat[, `:=` (count = .N, var = sum(VAR)), by = MNTH]

：

> dat
      MNTH VAR count var
 1: 201501   1     4   2
 2: 201501   1     4   2
 3: 201501   0     4   2
 4: 201501   0     4   2
 5: 201502   0     3   0
 6: 201502   0     3   0
 7: 201502   0     3   0
 8: 201503   0     5   2
 9: 201503   0     5   2
10: 201503   1     5   2
11: 201503   1     5   2
12: 201503   0     5   2
13: 201504   1     4   2
14: 201504   0     4   2
15: 201504   1     4   2
16: 201504   0     4   2

有关如何使用 data.table 语法，请参见入门指南 在GitHub Wiki上。

For further reading about how to use data.table syntax, see the Getting started guides on the GitHub wiki.

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