CODEWITHSUNDEEP
matlab
Pavan Yalamanchili
Pavan Yalamanchili

Reputation: 12099

Comparison of complex numbers in MATLAB

I am trying to figure out how MATLAB compares complex numbers using the following code. I am not sure if this is expected behavior or if I have uncovered a bug.

The documentation for max says the following:

When X is complex, the maximum is computed using the magnitude MAX(ABS(X)). In the case of equal magnitude elements, then the phase angle MAX(ANGLE(X)) is used.

The behavior of max matches the documentation as expected.

>> a = complex(rand(3,1), rand(3,1))

a =

   0.8147 + 0.9134i
   0.9058 + 0.6324i
   0.1270 + 0.0975i

>> b = complex(imag(a), real(a))

b =

   0.9134 + 0.8147i
   0.6324 + 0.9058i
   0.0975 + 0.1270i

>> max(a, b)

ans =

   0.8147 + 0.9134i
   0.6324 + 0.9058i
   0.0975 + 0.1270i

>> a > b

ans =

     0
     1
     1

>> angle(a) > angle(b)

ans =

     1
     0
     0

>> abs(a) == abs(b)

ans =

     1
     1
     1

However when I try to use greater than operator, ">", matlab seems to use just the real part for comparison. 

>> a = complex(rand(5,1), rand(5,1))

a =

   0.1576 + 0.1419i
   0.9706 + 0.4218i
   0.9572 + 0.9157i
   0.4854 + 0.7922i
   0.8003 + 0.9595i

>> b = complex(imag(a), real(a))

b =

   0.1419 + 0.1576i
   0.4218 + 0.9706i
   0.9157 + 0.9572i
   0.7922 + 0.4854i
   0.9595 + 0.8003i

>> max(a, b) == a

ans =

     0
     0
     0
     1
     1

>> a > b

ans =

     1
     1
     1
     0
     0

>> real(a) > real(b)

ans =

     1
     1
     1
     0
     0

Is there any particular reason the behavior changes in this manner from max to > ?

Upvotes: 5

Views: 3241

Answers (1)

bendervader
bendervader

Reputation: 2660

This is from

doc >

The test compares only the real part of numeric arrays

It so happens that the implementations of > looks only at the real part. The design decision from the Matlab team seems legit.

The overwhelming majority of operations that involve the comparison operator are intended to work with real numbers. Adding a special behavior for a basic operation like > to handle complex numbers will cause a big on hit for the 90% of code that does not require it. Especially, that there is not standard way to compare complex numbers. It depends on your application.

Upvotes: 3

Related Questions