Relation of Elements of matrix
3 views (last 30 days)
Show older comments
ANAND ASP
on 29 Jan 2021
Commented: Walter Roberson
on 29 Jan 2021
A = [1 2
2 6
2 3
2 7
3 4
5 4
6 3
4 7]
i have matrix A,
if i select B=2, i want output as 1 2 3 6 7, it shows relatiionship of 2
if i select B=3, i want output as 2 3 4 6, it shows relatiionship of 3
Accepted Answer
Walter Roberson
on 29 Jan 2021
Edited: Walter Roberson
on 29 Jan 2021
A = [1 2
2 6
2 3
2 7
3 4
5 4
6 3
4 7]
B = 2
unique([B; A(A(:,1) == B,2); A(A(:,2) == B,1)]).'
However, the basis for including the number itself is not clear. If B does not occur at all in A, then I would argue it would be more appropriate for the result to be empty than for it to include B itself.
6 Comments
Paul Hoffrichter
on 29 Jan 2021
Edited: Paul Hoffrichter
on 29 Jan 2021
After adding a note that removing the sort was faster, I then started thinking that the two line solution might actually be faster than the more one-line solution. (Usually, I think of one-line solutions as not only more elegant, but also, usually also faster.)
Yet, the two line solution was 4-5 times faster than the one line solution. Not that it matters much since the time for either one is negligible.
B = 12345;
n = 1e8;
A = randi(n/2000,n,2);
tic
z1 = unique([B; A(A(:,1) == B,2); A(A(:,2) == B,1)]).';
toc
tic
[r, ~] = ind2sub( size(A), find(A==B) );
z2 = unique( A( r ,: ) )';
toc
Elapsed time is 0.665452 seconds.
Elapsed time is 0.154779 seconds.
Walter Roberson
on 29 Jan 2021
B = 12345;
n = 5e7;
A = randi(n/2000,n,2);
tic
z1 = unique([B; A(A(:,1) == B,2); A(A(:,2) == B,1)]).';
toc
tic
[r, ~] = ind2sub( size(A), find(A==B) );
z2 = unique( A( r ,: ) )';
toc
tic
z3 = unique(A(any(A == B,2),:)).';
toc
More Answers (1)
Paul Hoffrichter
on 29 Jan 2021
B = 3;
[r, ~] = ind2sub( size(A), find(A==B) );
z = A(sort(r),:);
zu = unique(z(:))'
2 Comments
Paul Hoffrichter
on 29 Jan 2021
B = 2;
[r, ~] = ind2sub( size(A), find(A==B) );
z = unique( A( sort(r) ,: ) )'
Paul Hoffrichter
on 29 Jan 2021
Don't need the sort, so this is faster:
[r, ~] = ind2sub( size(A), find(A==B) );
z = unique( A( r ,: ) )'
See Also
Categories
Find more on Logical in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!