赋值与data.table联接操作，多次匹配 [英] Assignment with data.table join operation, multiple matches

问题描述

我有两个data.tables：

I have two data.tables:

dt1 = data.table(a=c('a','b'))
dt2 = data.table(a=c('a','b','b'))

合并 dt1 [dt2，on ='a'] 提供

   a
1: a
2: b
3: b

所以当我执行操作 dt1 [dt2，on ='a'，c：= 1] b
$ b

so when I perform the operation dt1[dt2, on='a', c:= 1] I expect

   a c
1: a 1
2: b 1
3: b 1

但我得到

   a c
1: a 1
2: b 1

？

推荐答案

我们需要使用

dt2[dt1, c := 1, on = "a"]
dt2
#   a c
#1: a 1
#2: b 1
#3: b 1

如果我们不想更改初始数据集dt1

If we are not interested in changing the initial dataset 'dt1', then

dt1[dt2, c(.SD, c= 1), on = 'a']
#   a c
#1: a 1
#2: b 1
#3: b 1

---

OP的方法的问题是在连接之后，赋值（：= ）在第一个数据集（'dt1'）中，它只有2行，因此，分配的值也将在这2行而不是3行。一个选项是将其分配给第二个数据集（如第一个方法中所示），或者通过连接新列c创建一个新数据集。

---

The problem in the OP's approach is that after the join, the assignment (:=) happens in the first dataset ('dt1') and it has only 2 rows, so, the assigned values will be also be in those 2 rows instead of 3 rows. One option is to assign it to the second dataset (as showed in the first method) or we create a new dataset by concatenating a new column 'c'.

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