Adding a column in data.table with looking up a value in another data.table

Question

I have a question regarding adding a column to a data.table based on info in another data.table.

This is how my data looks:

dt.1: One column with 1.9 million Product ID's
0771044445
0827229534
0827229534
0738700797
etc.

dt.2: Two columns with 7 million Product ID's + Categories
0842328327  Book
0842328327  Book
1577943082  DVD
Etc.

Now I want to create a column in dt.1 which adds the category by looking up this category in dt.2. All ID's of dt.1 are in dt.2.

I tried:

 dt.1[,group := ID %in% dt.2, by = dt.2$category]

But I get the following error:

  Error in `[.data.frame`(dt.amazon.similar, , `:=`(group, asin %in% dt.amazon.reviews),  : unused argument (by = dt.amazon.reviews$group)

Try joining them first. – David Arenburg Mar 07 '17 at 12:24 — David Arenburg, Mar 07 '17 at 12:24

score 1 · Answer 1 · edited Mar 07 '17 at 22:39

1

You could also use fast join syntax for inplace editing of dt1

dt.1[dt.2, on="Product ID", category:=category]

edited Mar 07 '17 at 22:39

David Arenburg

91,361
17
137
196

answered Mar 07 '17 at 13:15

Billy34

1,777
11
11

Thanks, this works! – Max Mar 07 '17 at 14:03

Adding a column in data.table with looking up a value in another data.table

1 Answers1