使用data.table包的笛卡尔积 [英] Cartesian Product using data.table package

问题描述

使用R中的data.table包，我试图像在基R中那样使用merge方法创建两个data.tables的笛卡尔积。

Using the data.table package in R, I am trying to create a cartesian product of two data.tables using the merge method as one would do in base R.

根据以下内容进行操作：

In base the following works:

#assume this order data
orders <- data.frame(date = as.POSIXct(c('2012-08-28','2012-08-29','2012-09-01')),
                     first.name = as.character(c('John','George','Henry')),
                     last.name = as.character(c('Doe','Smith','Smith')),
                     qty = c(10,50,6))

#and these dates
dates <- data.frame(date = seq(from = as.POSIXct('2012-08-28'),
                               to = as.POSIXct('2012-09-07'), by = 'day'))

#get the unique customers
cust<-unique(orders[,c('first.name','last.name')])

#using merge from base R, get the cartesian product
merge(dates, cust, by = integer(0))

但是，相同的技术无法正常使用ng data.table并抛出此错误：

However, the same technique does not work using data.table and this error is thrown:

"Error in merge.data.table(dates.dt, cust.dt, by = integer(0)) : 
  A non-empty vector of column names for `by` is required."

#data.table approach
library(data.table)

orders.dt <- data.table(orders)

dates.dt <- data.table(dates)

cust.dt <- unique(orders.dt[, list(first.name, last.name)])

#try to use merge (data.table) in the same manner as base
merge(dates.dt, cust.dt, by = integer(0))

Error in merge.data.table(dates.dt, cust.dt, by = integer(0)) : 
  A non-empty vector of column names for `by` is required.

我希望结果能反映所有日期的所有客户名称，就像在base中一样，但是以以数据表为中心的方式进行。

I want the result to reflect all customer names for all dates, just like in base, but do it in a data.table-centric way. Is this possible?

推荐答案

如果从数据帧的第一个和最后一个构造全名，则可以使用 CJ （交叉联接）。您不能全部使用三个向量，因为将有99个项目。

If you construct full names from the first and last in the dataframes, you can use CJ (cross-join). You cannot use all three vectors since there would be 99 items.

> nrow(CJ(dates$date, cust$first.name, cust$last.name ) )
[1] 99

这将返回一个data.table对象：

This returns a data.table object:

> CJ(dates$date,paste(cust$first.name, cust$last.name) )
            V1           V2
 1: 2012-08-28 George Smith
 2: 2012-08-28  Henry Smith
 3: 2012-08-28     John Doe
 4: 2012-08-29 George Smith
 5: 2012-08-29  Henry Smith
 6: 2012-08-29     John Doe
 7: 2012-08-30 George Smith
 8: 2012-08-30  Henry Smith
 9: 2012-08-30     John Doe
10: 2012-08-31     John Doe
11: 2012-08-31 George Smith
12: 2012-08-31  Henry Smith
13: 2012-09-01     John Doe
14: 2012-09-01 George Smith
15: 2012-09-01  Henry Smith
16: 2012-09-02 George Smith
17: 2012-09-02  Henry Smith
18: 2012-09-02     John Doe
19: 2012-09-03  Henry Smith
20: 2012-09-03     John Doe
21: 2012-09-03 George Smith
22: 2012-09-04  Henry Smith
23: 2012-09-04     John Doe
24: 2012-09-04 George Smith
25: 2012-09-05 George Smith
26: 2012-09-05  Henry Smith
27: 2012-09-05     John Doe
28: 2012-09-06 George Smith
29: 2012-09-06  Henry Smith
30: 2012-09-06     John Doe
31: 2012-09-07 George Smith
32: 2012-09-07  Henry Smith
33: 2012-09-07     John Doe
            V1           V2

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