There is more than one thing amiss in this piece of code. Probably the problem you have in mind is caused by your 'if' statement:
Both x and y are (presumably) two-element vectors, so the logical expression
also has two logical elements. One of these is "x(1)==0|y(1)==0" and the other is "x(2)==0|y(2)==0". The 'if' will execute if both of these are true, and that is clearly not what you intended. For example the 'if' will execute in case x(1)==0 and y(2)==0, even though neither the x vector nor the y vector is a zero vector. What you should have written is:
if (x(1)==0 and x(2)==0) | (y(1)==0 and y(2)==0)
or more compactly
You could even do:
if norm(x)==0 | norm(y) == 0
My next objection is requiring exact equality in
You could well have essentially parallel vectors but due to round-off errors in the computation of c, there would not be exact equality to 1 or -1. You need to allow a tolerance for a small deviation from exact equality here. The same applies to the test "elseif c == 0" for orthogonality.
One final complaint. You take the test for zero vector too late to avoid getting a NaN for an answer in computing c. You should have made this test beforehand to avoid a possible NaN.
(Note: As you undoubtedly realize, you are computing the cosine of the angle rather than the angle between those two vectors.)
