连接两个数据表,只使用R中第二个dt的一个列 [英] join two data tables and use only one column from second dt in R
问题描述
编辑,使其更清洁。
假设我有两个数据表(dt1和dt2),我想使用数据表获得dt3。 A,B,C,E,F,G,H是列名。 dt1键是列A,dt2键是列E.数据表具有不同的行数。我想保留DT1的所有列,并从DT2只添加一个列(H)到连接的数据表。最终,我将它存储为DT1(虽然我把它显示为dt3下面)。
我如何实现它与数据表?我有一个包含合并+数据框的丑陋解决方案。
dt1
ABC
1 4 7
2 5 8
3 6 9
2 20 21
dt2
EFGH
1 10 13 16
3 12 15 18
2 11 14 17
dt3
ABCH
1 4 7 16
2 5 8 17
3 6 9 18
2 20 21 17
a df1
并从 df2添加
H
运算符(:=
)将二进制连接与
setkey(setDT(dt1),A)
dt1 [dt2,H:= iH]
(v> = 1.9.5),我们可以通过在 setDT
中指定键
@Arun)
setDT(dt1,key =A)[dt2,H:= iH] / code>
编辑24/7/2015 p>
现在,您可以使用上的新参数运行二进制连接,而无需设置键
setDT(dt1)[dt2,H:= iH,on = c(A =E)]
/ pre>
Edited to make it cleaner.
Let's say I have two data tables (dt1 and dt2), and I want to get dt3 using data tables. A,B,C,E,F,G,H are column names. dt1 key is column A, and dt2 key is column E. Data tables have different number of rows. I want to keep all the columns from DT1, and add only one column (H) from DT2 to the joined data table. Eventually, I will store this as DT1 (though I showed it as dt3 below).
How can I achieve it with data tables? I have an ugly solution with merge + data frames.
dt1
A B C
1 4 7
2 5 8
3 6 9
2 20 21
dt2
E F G H
1 10 13 16
3 12 15 18
2 11 14 17
dt3
A B C H
1 4 7 16
2 5 8 17
3 6 9 18
2 20 21 17
解决方案 In order to perform a left join to df1
and add H
column from df2
, you can combine binary join with the update by reference operator (:=
)
setkey(setDT(dt1), A)
dt1[dt2, H := i.H]
See here and here for detailed explanation on how it works
With the devel version (v >= 1.9.5) we could make it even shorter by specifying the key
within setDT
(as pointed by @Arun)
setDT(dt1, key = "A")[dt2, H := i.H]
Edit 24/7/2015
You can now run a binary join using the new on
parameter without setting keys
setDT(dt1)[dt2, H := i.H, on = c(A = "E")]
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