使用R中的data.table包的笛卡尔积

Question

使用R中的data.table包,我试图使用merge方法创建两个data.tables的笛卡尔积,就像在基数R中一样.

在基础上,以下工作:

#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,使用data.table相同的技术不起作用,并抛出此错误:

"merge.data.table中的错误(dates.dt,cust.dt,by = integer(0)):需要一个非空的列名向量__CODE__."

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

我希望结果反映所有日期的所有客户名称,就像在base中一样,但是以data.table为中心的方式.这可能吗？

Answer 1

如果从数据框中的第一个和最后一个构造全名,则可以使用CJ(交叉连接).你不能使用所有三个向量,因为有99个项目.

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

这将返回一个data.table对象:

> 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

Answer 2

merge.data.table(x, y)是一个包含调用的便捷函数x[y],因此合并需要基于两个列中的列data.table.(这就是错误消息试图告诉你的).

一种解决方法是向两个data.tables添加一个虚拟列,其唯一目的是使合并成为可能:

## Add a column "k", and append it to each data.table's vector of keyed columns.
setkeyv(cust.dt[,k:=1], c(key(cust.dt), "k"))
setkeyv(dates.dt[,k:=1], c(key(dates.dt), "k"))

## Merge and then remove the dummy column
res <- merge(dates.dt, cust.dt, by="k")
head(res[,k:=NULL])
#          date first.name last.name
# 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

## Maybe also clean up cust.dt and dates.dt    
# cust.dt[,k:=NULL]
# dates.dt[,k=NULL]