考虑一个问题,你想将f(x,y)的频谱表示转换为坐标空间。因此,exp(i*x) + exp(-i*x) ----> f(x,y),其中f(x,y) = 一些系数 * cos(x)。
在列主排列中(我在下面用Fortran 2003/8编写了示例),频谱数组将被初始化为:
在变换后,您应该得到一个由实数(双精度)组成的tgt数组,其中所有列均相同,并且每列的行为都类似于cos(x)函数。
在列主排列中(我在下面用Fortran 2003/8编写了示例),频谱数组将被初始化为:
src=(0.,0.)
src(2,1)=(1.,0.) .
在变换后,您应该得到一个由实数(双精度)组成的tgt数组,其中所有列均相同,并且每列的行为都类似于cos(x)函数。
然而,使用下面给出的代码,我得到一个tgt数组,其中只有奇数列的行为正确,而偶数列则全为零。
我错在哪里了?
我的fftw3版本是3.4.4。
program c2r
! COMPLEX --> REAL 2D MPI TRANSFORM
! compiled using: mpif90 -O0 -g -I/usr/include c2rnot.F90 -o c2rnot -L /usr/lib/x86_64-linux-gnu -lfftw3_mpi -lfftw3
use, intrinsic :: iso_c_binding
use MPI
implicit none
include 'fftw3-mpi.f03'
integer :: ierr,id
! logical dimensions of the transform
integer(C_INTPTR_T), parameter :: N1 = 32
integer(C_INTPTR_T), parameter :: N2 = 32
! helper variables: storage size, extents along the slow dim, offsets
integer(C_INTPTR_T) :: alloc_local, slicec,slicer,offc,offr
! pointers to planner, real (image) and complex (original) arrays
type(C_PTR) :: plan, ctgt, csrc
! fortran array representation of csrc ...
complex(8), pointer :: src(:,:)
! ..., ctgt and a total array to hold the mpi-gathered result
real(8), pointer :: tgt(:,:),total(:,:)
integer(C_INTPTR_T) :: i,y0
! init mpi and fftw ...
call mpi_init(ierr)
call mpi_comm_rank(MPI_COMM_WORLD,id,ierr)
call fftw_mpi_init()
! done with init
! calculate local complex array storage requirements and layout
alloc_local = fftw_mpi_local_size_2d(N2,N1/2+1, &
& MPI_COMM_WORLD,slicec,offc)
! allocate local complex array
csrc = fftw_alloc_complex(alloc_local)
! fortran representation
call c_f_pointer(csrc, src, [N1/2+1,slicec])
! allocate local real array
ctgt = fftw_alloc_real(2*alloc_local)
! for non-transposed arrays slices are the same
slicer=slicec
offr=offc
! fortran representation
call c_f_pointer(ctgt, tgt, [2*(N1/2+1),slicer])
! allocate global container (holds gathered tgt arrays)
if (id==0) allocate(total(N1,N2))
! N1 - fast, N2 - slow dimension, in: src, out: tgt
plan = fftw_mpi_plan_dft_c2r_2d(N2,N1,src,tgt, MPI_COMM_WORLD, FFTW_MEASURE)
src=(0.D0,0.D0)
! ~ exp(i*x) + exp(-i*x) which should turn into ~ cos(x)
src(2,1)=(1.D0,0.D0)
tgt=0.D0
! do transform
call fftw_mpi_execute_dft_c2r(plan,src,tgt)
! collect tgt-s into total in process 0
call mpi_gather(tgt(1:n1,1:slicer),slicer*n1,mpi_double,total,slicer*n1,mpi_double,0,mpi_comm_world,ierr)
! The scalar field total has x: 1..N1, y:1..N2 layout. Print
! behaviour along the x-axis for a given, constant y. The result
! should be ~ cos(x) function for any y0 in [1..N2]. However, I get
! such result only for odd y0. For even, total(i,y0) is 0.
y0=3
if (id == 0) then
do i=1,n1
print *, i, total(i,y0)
enddo
endif
call mpi_finalize(ierr)
end program c2r