Is there a fast mechanism to interleave the real and imaginary parts of a large array of complex numbers?

Which is the fastest way to convert a very large array of complex numbers in the following format :

[ real1 + img1;

  real2 + img2;

  ....

]

 to the following format:

[real1 img1 real2 img2 .....]

Input Data:

>> N = 20971520;

>> a = randn(1,N)+1i*randn(1,N);

>> b = zeros(1, 2*numel(a));

 

Answer 1

[MathWorks Support Team]

The following are a few of the possible and fast solutions:

1)  b(1:2:end) = real(a);

     b(2:2:end) = imag(a);

This is the slowest of the 3 approaches as it involves indexing.

2) c = ([real(a);imag(a)]); c = c(:);

This approach uses linear indexing and is faster than 1) but slightly slower than 3)

3) c = reshape([real(a);imag(a)],[],1);

This approach appears to be the fastest of the 3 because it involving only data shuffling and no indexing.

There are other solutions involving logical indexing and using 'kron' to interleave data but they are slower than the 3 suggested solutions.

[Bruno Luong]

@Ian Atkinson you must misstype semincolon ";" with comma ","; and to use 2) and 3) your a vector must be row vector not column vector.

NOTE that the expression

[ real1 + img1;
    real2 + img2;
    ....
    ]

written in the question is wrong it also mix up between comma and semicolon, not counting there must be a missing i or j somewhere.