# How to find a vector multiple times in an array?

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Tom Engels on 5 Aug 2021
Commented: Tom Engels on 5 Aug 2021
Good day to all,
I am facing the problem that I need to quickly find the positions of duplicates of a vector in an array.
Currently I am doing this with a for-statement. For this I run the whole array and compare the rows with my vector. The row has the same length as the vector. This leads to very high run times.
With "ismember" I can check if there are duplicates and also determine the position of the first duplicate. However, the vector can occur several times in the array.
Example:
A = [1 3 4] %x,y,z-coordinates - vector
B = [2 3 4; 1 3 4; 5 5 6; 3 4 6; 1 3 4]; %array
The goal here would be to get the indexes 2 and 5.
Can anyone help me with this?
Many greetings
Tom

Chunru on 5 Aug 2021
A = [1 3 4] %x,y,z-coordinates - vector
A = 1×3
1 3 4
B = [2 3 4; 1 3 4; 5 5 6; 3 4 6; 1 3 4]; %array
find(all(A==B, 2)) % 2 is for row dim
ans = 2×1
2 5

Bruno Luong on 5 Aug 2021
Edited: Bruno Luong on 5 Aug 2021
A = [1 3 4]; %x,y,z-coordinates - vector
B = [2 3 4; 1 3 4; 5 5 6; 3 4 6; 1 3 4]; %array
% which row match
find(ismember(B,A,'rows'))
ans = 2×1
2 5
Tom Engels on 5 Aug 2021
Thank you all very much. It works great!

R2020b

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