Search code examples
fortranfftw

How is a 3D complex array laid out in memory

I would like to know how a 3d complex array is laid out in a computer's memory? An example in Fortran would be of some help.

I'm trying to do a 1-dimensional sine transformation of a 3d complex array and I'm using a routine from FFTW. http://www.fftw.org/fftw3_doc/Advanced-Complex-DFTs.html#Advanced-Complex-DFTs

So I need to figure out the values for the parameters(i.e.howmany, istride and idist) in the FFT routine. Thanks.

Solution

  • Complex Fortran scalar values are stored as pairs of values [real, imaginary]. All Fortran arrays are column-major order, that means that the first index is the most frequently changing one. And each element of a complex array is a (real, imaginary) pair.

    A Fortran array 

    complex(c_float_complex) :: A(nx, ny, nz)

    is like a C array

    float A[nz][ny][nx][2]

    (yes, I know there is an optional complex type in modern C as well).

    So the sequence goes like this:

    (re(1,1,1),im(1,1,1)), (re(2,1,1,),im(2,1,1)), (re(3,1,1,),im(3,1,1)) ... (re(nx,1,1,),im(nx,1,1)), (re(1,2,1),im(1,2,1)) ...

    Se the column-major order link for more details.