pre-allocation of a compex matrix
in [Matlab]
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From: Marios Karaoulis on 8 Jun 2010 13:25 How can i pre-allocate a complex matrix with zeros everywhere? If i use this command x=zeros(5,6)+j*zeros(5,6), I end up with a real matrix if I use this x=zeros(5,6)+j*ones(5,6); then I do not have zeros in the imaginary part. Any ideas?
From: dpb on 8 Jun 2010 13:43 Marios Karaoulis wrote: > How can i pre-allocate a complex matrix with zeros everywhere? > If i use this command > x=zeros(5,6)+j*zeros(5,6), I end up with a real matrix > > if I use this > x=zeros(5,6)+j*ones(5,6); > then I do not have zeros in the imaginary part. > Any ideas? Just use x=zeros(5,6) When you reference an element w/ a nonzero imaginary component, ML will automagically handle it... Example... >> z=zeros(3,1); >> z(2) = i z = 0 0 + 1.0000i 0 >> --
From: Wayne King on 8 Jun 2010 13:53 Marios Karaoulis <marios.karaoulis(a)gmail.com> wrote in message <0e877227-c134-4d60-ba37-cb631943f345(a)a16g2000prg.googlegroups.com>... > How can i pre-allocate a complex matrix with zeros everywhere? > If i use this command > x=zeros(5,6)+j*zeros(5,6), I end up with a real matrix > > if I use this > x=zeros(5,6)+j*ones(5,6); > then I do not have zeros in the imaginary part. > Any ideas? Hi Marios, If you want the Attributes field to show up as complex, you can do x = complex(zeros(5,6), zeros(5,6)); but I'm not sure what difference it makes. How does it matter that the entries of the matrix don't display as 0+j0? You can still do things like: x=zeros(5,6); y = randn(5,6)+1j*randn(5,6); x+y % the sum is defined Wayne
From: Marios Karaoulis on 8 Jun 2010 14:15 I think, that matlab automatically checks if there is reason for a matrix to be complex or not (so reduce the memory size). This way it is slower if you preallocate a complex matrix as real, and then during execution you change an element so it can be compelx. Then matlab re-preallocate another matrix so it can handle complex numbers.
From: Bruno Luong on 8 Jun 2010 14:21
Marios Karaoulis <marios.karaoulis(a)gmail.com> wrote in message <0e877227-c134-4d60-ba37-cb631943f345(a)a16g2000prg.googlegroups.com>... > How can i pre-allocate a complex matrix with zeros everywhere? > If i use this command > x=zeros(5,6)+j*zeros(5,6), I end up with a real matrix > > if I use this > x=zeros(5,6)+j*ones(5,6); > then I do not have zeros in the imaginary part. > Any ideas? There is no need to preallocate the imaginary part: as soon as the first true complex element is assigned to your array, the whole memory block (same size as the real part) reserved for imaginary part will be allocated. All you need is allocated with zeros(), don't bother with the rest. Bruno |