Intersection of two tables

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Vilém Frynta
Vilém Frynta am 24 Feb. 2023
Kommentiert: Vilém Frynta am 24 Feb. 2023
Hi,
I'm trying to intersect two tables. They share same column with strings, and I want strings that are in both of these tables (intersection).
Example tables:
A = table();
A.user = ["user1";"user2";"user3";"user4"];
A.value = [10;20;30;40]
A = 4×2 table
user value _______ _____ "user1" 10 "user2" 20 "user3" 30 "user4" 40
B = table();
B.user = ["user3";"user4";"user5"];
B.value = [300;400;500]
B = 3×2 table
user value _______ _____ "user3" 300 "user4" 400 "user5" 500
I would like to get a logical vector (index) which would tell me positions of users, that are in both tables, which are user3 and user4 in this example. Values of the users are not important here, i need only those indexes.
Although this seems simple in my head, I wasn't able to do it in Matlab. I tried using outerjoin(), ismember() and intersect(), but always some errors and empty logical vectors as results.

Akzeptierte Antwort

Les Beckham
Les Beckham am 24 Feb. 2023
Bearbeitet: Les Beckham am 24 Feb. 2023
A = table();
A.user = ["user1";"user2";"user3";"user4"];
A.value = [10;20;30;40]
A = 4×2 table
user value _______ _____ "user1" 10 "user2" 20 "user3" 30 "user4" 40
B = table();
B.user = ["user3";"user4";"user5"];
B.value = [300;400;500]
B = 3×2 table
user value _______ _____ "user3" 300 "user4" 400 "user5" 500
Since you say you only care about the indices but you didn't say which table you wanted them for, here are both.
idxBinA = ismember(B.user, A.user) % where members of A.user are found in B.user
idxBinA = 3×1 logical array
1 1 0
idxAinB = ismember(A.user, B.user) % where members of B.user are found in A.user
idxAinB = 4×1 logical array
0 0 1 1
Does that get you what you need?
  1 Kommentar
Vilém Frynta
Vilém Frynta am 24 Feb. 2023
Yes, it looks similar to what I've tried.
But I do fail when applying this solution into my code. My logical vectors are full of zeros. It appears that strings in my table are also inside a cell, which is the issue..

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Weitere Antworten (1)

Askic V
Askic V am 24 Feb. 2023
Bearbeitet: Askic V am 24 Feb. 2023
Maybe this code can help you:
A = table();
A.user = {'user1';'user2';'user3';'user4'};
A.value = [10;20;30;40]
A = 4×2 table
user value _________ _____ {'user1'} 10 {'user2'} 20 {'user3'} 30 {'user4'} 40
B = table();
B.user = {'user3';'user4';'user5'};
B.value = [300;400;500]
B = 3×2 table
user value _________ _____ {'user3'} 300 {'user4'} 400 {'user5'} 500
ind = ismember(B.user, A.user)
ind = 3×1 logical array
1 1 0
Or you can slightly modified what @Les Beckham suggested:
A = table();
A.user = {"user1";"user2";"user3";"user4"};
A.value = [10;20;30;40]
A = 4×2 table
user value ___________ _____ {["user1"]} 10 {["user2"]} 20 {["user3"]} 30 {["user4"]} 40
B = table();
B.user = {"user3";"user4";"user5"};
B.value = [300;400;500]
B = 3×2 table
user value ___________ _____ {["user3"]} 300 {["user4"]} 400 {["user5"]} 500
idxBinA = ismember([B.user{:}], [A.user{:}])
idxBinA = 1×3 logical array
1 1 0
  3 Kommentare
Vilém Frynta
Vilém Frynta am 24 Feb. 2023
I have even converted the table column into a string, to get rid of the cell.
Still not possible. I'm trying to intersect 21x1 string and 28x1 string and still it doesn't work. Not with ismember and and interesct. :(
Vilém Frynta
Vilém Frynta am 24 Feb. 2023
Oh I found out the problem, thanks for help!

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