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Relation of Elements of matrix

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ANAND ASP
ANAND ASP 2021-1-29
评论： Walter Roberson 2021-1-29
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

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Walter Roberson
Walter Roberson 2021-1-29
编辑：Walter Roberson 2021-1-29
Ran in:
A = [1 2
2 6
2 3
2 7
3 4
5 4
6 3
4 7]
A = 8×2
1 2 2 6 2 3 2 7 3 4 5 4 6 3 4 7
B = 2
B = 2
unique([B; A(A(:,1) == B,2); A(A(:,2) == B,1)]).'
ans = 1×5
1 2 3 6 7
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 个评论
显示 4更早的评论
Paul Hoffrichter
Paul Hoffrichter 2021-1-29
编辑：Paul Hoffrichter 2021-1-29
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
Walter Roberson 2021-1-29
Ran in:
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
Elapsed time is 0.759719 seconds.
tic
[r, ~] = ind2sub( size(A), find(A==B) );
z2 = unique( A( r ,: ) )';
toc
Elapsed time is 0.251981 seconds.
tic
z3 = unique(A(any(A == B,2),:)).';
toc
Elapsed time is 0.306312 seconds.

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更多回答（1 个）

Paul Hoffrichter
Paul Hoffrichter 2021-1-29
B = 3;
[r, ~] = ind2sub( size(A), find(A==B) );
z = A(sort(r),:);
zu = unique(z(:))'
  2 个评论
Paul Hoffrichter
Paul Hoffrichter 2021-1-29
B = 2;
[r, ~] = ind2sub( size(A), find(A==B) );
z = unique( A( sort(r) ,: ) )'
Paul Hoffrichter
Paul Hoffrichter 2021-1-29
Don't need the sort, so this is faster:
[r, ~] = ind2sub( size(A), find(A==B) );
z = unique( A( r ,: ) )'

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