calculating trace of matrix without calculating the matrix itself

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Amir
Amir 2017-2-7
编辑： Roger Stafford 2017-2-7
Let's say I have this matrix
tmp = magic(10);
and then I have
vecx= [1,2]
vecy=[3,4]
If I write tmp(vecx,vecy) it generates the matrix of
A= [tmp(1,3), tmp(1,4); tmp(2,3), tmp(2,4)]
but what I want to get is
B=[tmp(1,3), tmp(2,4)].
I understand B= trace(A), but the problem is in my actually code, I can't calculate A, because it is too large and I go out of memory.
How can I calculate B without explicitly calculating A?
Thanks,
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James Tursa
James Tursa 2017-2-7
We can't answer unless we know what A is and how it is currently calculated. Until we know that, how can we possibly tell you how to calculate only the trace elements of it?

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回答（1 个）

Roger Stafford
Roger Stafford 2017-2-7
编辑：Roger Stafford 2017-2-7
Let tmp be an n by n matrix.
B = tmp(vecx+n*(vecy-1)).';
(Note: The ‘trace’ is the sum of the diagonal elements of A, not their vector.)
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Walter Roberson
Walter Roberson 2017-2-7
The above is pretty much the internal formula used by sub2ind, but calling sub2ind() specifically can be easier to read and understand (but direct calculation can sometimes be much faster.)

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