什么是<在data.table中代表on = [英] What does < stand for in data.table joins with on=

问题描述

联接数据表:

X <- data.table(A = 1:4, B = c(1,1,1,1)) 
#    A B
# 1: 1 1
# 2: 2 1
# 3: 3 1
# 4: 4 1

Y <- data.table(A = 4)
#    A
# 1: 4

通过

X[Y, on = .(A == A)]
#    A B
# 1: 4 1

返回预期结果.但是，我希望这行代码:

returns the expected result. However, I would expect the line:

X[Y, on = .(A < A)]
#    A B
# 1: 4 1
# 2: 4 1
# 3: 4 1

返回

   A B
1: 1 1
2: 2 1
3: 3 1

因为关键字on:

指示应将x中的哪些列与i中的哪些列进行连接，以及要与之连接的二进制运算符的类型

Indicate which columns in x should be joined with which columns in i along with the type of binary operator to join with

根据?data.table.没有明确提到加入的方式，当然也没有像我所猜测的那样. <如何精确地将x中的列与i中的列连接起来?

according to ?data.table. The way the joining is done is not explicitly mentioned, and certainly it is not as I have guessed. How exactly < joins columns in x with columns in i?

推荐答案

进行非等额联接时，如X[Y, on = .(A < A)] data.table 返回Y的A列(i -data.table).

When doing a non-equi join like X[Y, on = .(A < A)] data.table returns the A-column from Y (the i-data.table).

要获得理想的结果，您可以执行以下操作:

To get the desired result, you could do:

X[Y, on = .(A < A), .(A = x.A, B)]

给出:

   A B
1: 1 1
2: 2 1
3: 3 1

在下一个版本中，data.table 将同时返回两个A列. 请参阅此处进行讨论.

In the next release, data.table will return both A columns. See here for the discussion.

这篇关于什么是<在data.table中代表on =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！