How do I initialize a complex array to zeros in MATLAB?

MATLAB documentation recommends that pre-allocation of variables speeds up performance. I would like to pre-allocate a complex array in my code to zeros. However, when I type the following commands:

x = zeros(100,1) + j*zeros(100,1);

MATLAB creates a 100-by-1 real double array. I would like to create a complex array and initialize both the real and imaginary parts to zeros.

Answer 1

[MathWorks Support Team]

A complex number can be created in MATLAB using the COMPLEX function. To initialize a complex number with zero as the real part and non-zero imaginary part, enter the following at the MATLAB command prompt

a = zeros(1,100);

x = complex(a,0);

To verify that the above command actually creates a 100-by-1 complex array, enter the following at the MATLAB command prompt

whos x

If you were to create the complex arrary with the above code and then assign a pure real number to one part of x or perform an arithmetic operation with a real number, MATLAB will de-allocate the complex part of the variable x . Type the following commands to observe this behavior:

x = x + 1.0;

whos x

When a real number, 1.0 was added to x, MATLAB de-allocates the complex part of the variable x and creates a 100-by-1 real double array. To keep the complex part, wrap COMPLEX around the expression.

[Mike Beck]

Not sure what you mean by "wrap COMPLEX around the expression".

a=zeros(3,5)

z=complex(a)

z(3,3) = z(3,3)+1

z=complex(a)

z(3,3) = complex(z(3,3)+1)

The last line returns z as a real array

[James Tursa]

You did an operation that resulted in the imaginary part of z being identically 0. MATLAB detected this result and deallocated the imaginary part. So you would have to follow this up with:

z = complex(z)

I think TMW comments were meant to be interpreted as a whole array assignment. E.g.,

x = complex(x + 1.0);

[Qu Cao]

Thank you for clarifying the answer.

Answer 2

[Royi Avital]

In later versions one could do something like vB = zeros(3, 1, 'like', 1j);.