Error with find(X) statement

I'm trying to run a simple calculation to find the non-trivial columns of a unitary matrix. All the unitaries in this case are of the form:

[ 1 0 0 0,
   0 a b 0,
   0 c d 0,
 0 0 0 1]

where a b c d form the 'non-trivial columns', in whatever columns.

Which is to say, if you sum the matrix either column wise or row wise, the sum must always equal 1, or be the non-trivial columns I'm trying to find. So the way I'm doing it is:

states=find(sum(UNITARY_MATRIX)~=1.0)

Which generally outputs something like:

Unitary number: 5
   1.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i
   0.0000 + 0.0000i   0.8165 + 0.0000i   0.0000 + 0.0000i  -0.0000 - 0.5774i
   0.0000 + 0.0000i   0.0000 + 0.0000i   1.0000 + 0.0000i   0.0000 + 0.0000i
   0.0000 + 0.0000i  -0.0000 + 0.5774i   0.0000 + 0.0000i  -0.8165 + 0.0000i

sum of column values
   1.0000 + 0.0000i   0.8165 + 0.5774i   1.0000 + 0.0000i  -0.8165 - 0.5774i

Non trivial columns:
     2     4

This is working very well for all the unitaries I'm checking, except for the following: (this is the output in the terminal)

Unitary number: 6
   1.0000 + 0.0000i   0.0000 + 0.0000i  -0.0000 - 0.0000i  -0.0000 - 0.0000i
   0.0000 + 0.0000i   1.0000 - 0.0000i   0.0000 + 0.0000i  -0.0000 - 0.0000i
  -0.0000 + 0.0000i   0.0000 - 0.0000i   0.7071 - 0.0000i  -0.7071 - 0.0000i
  -0.0000 - 0.0000i  -0.0000 - 0.0000i   0.0000 - 0.7071i   0.0000 - 0.7071i

sum of column values
   1.0000 + 0.0000i   1.0000 + 0.0000i   0.7071 - 0.7071i  -0.7071 - 0.7071i

Non trivial columns:
     2     3     4
  

Any idea why this is happening? This is the only matrix this is happening with. Since clearly the non-trivial columns should be 3 and 4. Is this to do with how find(X) reads complex numbers?

Answer 1

[Star Strider]

Taking a wild guess, since the test is:

states=find(sum(UNITARY_MATRIX)~=1.0)

it is likely encountering floating-point approximation error, since the sum may not be exactly equal to 1.0, the criterion for the test.

Perhaps something like:

states=find(abs(1.0 - sum(UNITARY_MATRIX)) <= 1E-8)

or something similar (incorporating a range of some sort for the test) would work.

If this does not address what you want to do, I will delete my Answer.

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[Anu Raghunathan]

Great that works perfectly! Only thing is I'm looking for elements where the sum is not equal to 1.0, so I just amended your code to:

states=find(abs(1.0 - sum(UNITARY_MATRIX)) > 1E-8);

And it works perfectly!! Thanks!

[Star Strider]

As always, my pleasure!

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