# "symmetrical" rows of matrix

4 views (last 30 days)
Michal on 10 Feb 2020
Commented: Michal on 11 Feb 2020
I have integer matrix A (nA x c) with even number of columns (e.g. mod(c,2) = 0) and unique rows.
How to effectivelly (by speed and memory optimized function "symmetricRows") find the "symmetric" rows of matrix A iA1 and iA2, where "symmetric" rows iA1 and iA2 are defined as:
all(A(iA1,1:end/2) == A(iA2,end/2+1:end) & A(iA1,end/2+1:end) == A(iA2,1:end/2),2) = true
Example:
A = [1 1 1 1;
2 2 2 2;
1 2 3 4;
4 3 2 1;
2 2 3 3;
3 4 1 2;
3 3 2 2]
[iA1, iA2] = symmetricRows(A)
iA1 =
1
2
3
5
iA2 =
1
2
6
7
Typical size of matrices A: nA ~ 1e4-1e6, c ~ 60 - 120
The problem is motivated by pre-processing of large dataset, where "symmetrical" rows are irrelevant from the point of user defined distance metric.

Michal on 11 Feb 2020
Edited: Michal on 11 Feb 2020
I present the best solution so far:
d = ~pdist2(A(:,1:end/2), A(:,end/2+1:end));
[iA1, iA2] = find(triu(d & d.'));
Michal on 11 Feb 2020
Yes, defintely, MATLAB Online is not proper way how to compute any memory or CPU intensive task at all ... :)

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