Fortran FFTW example

fftw_ex.f90

program main
    use ISO_C_BINDING
    implicit none

    include 'fftw3.f03'

    real, allocatable :: test_in(:)
    integer(8) :: plan_c2r, plan_r2c
    integer :: n, i

    real, allocatable :: r(:), corr1(:), corr2(:)
    integer(8) :: plan_c2r_corr(2), plan_r2c_corr(2)
    integer :: n_corr, i_corr

    n = 5
    allocate(test_in(n+2))

    test_in = 0.

    call dfftw_plan_dft_r2c_1d(plan_r2c, n, test_in, test_in, FFTW_MEASURE)
    call dfftw_plan_dft_c2r_1d(plan_c2r, n, test_in, test_in, FFTW_MEASURE)

    test_in = [1., 2., 3., 4., 5.]

    print *, "IN0: "
    write(*, *) (test_in(i),i = 1,n)
    call dfftw_execute_dft_r2c(plan_r2c, test_in, test_in)

    print *, "OUT: "
    write(*, *) (test_in(i),i = 1,n)

    call dfftw_execute_dft_c2r(plan_c2r, test_in, test_in)

    do i = 1, n
        ! normalization
        test_in(i) = test_in(i) / n
    end do

    print *, "IN2: "
    write(*, *) (test_in(i),i = 1,n)

    call dfftw_destroy_plan(plan_r2c)
    call dfftw_destroy_plan(plan_c2r)

    deallocate(test_in)
    
    
    ! corr
    n_corr = 192
    allocate(r(n_corr))
    allocate(corr1(n_corr+2))
    allocate(corr2(n_corr+2))

    call dfftw_plan_dft_r2c_1d(plan_r2c_corr(1), n_corr, corr1, corr1, FFTW_MEASURE)
    call dfftw_plan_dft_c2r_1d(plan_c2r_corr(1), n_corr, corr1, corr1, FFTW_MEASURE)
    call dfftw_plan_dft_r2c_1d(plan_r2c_corr(2), n_corr, corr2, corr2, FFTW_MEASURE)
    call dfftw_plan_dft_c2r_1d(plan_c2r_corr(2), n_corr, corr2, corr2, FFTW_MEASURE)

    open(100, FILE='corr.plt', ACTION='READ')

    do i = 1, n_corr
        read(100, *) r(i),corr1(i), corr2(i)
        print *, r(i), corr1(i), corr2(i)
    end do
    close(100)
    call dfftw_execute_dft_r2c(plan_r2c_corr(1), corr1, corr1)
    call dfftw_execute_dft_r2c(plan_r2c_corr(2), corr2, corr2)

    open(100, FILE='E1D.plt', ACTION='WRITE')
    do i = 1, n_corr/2
        write(100, '(I4, 3ES20.9)') &
            i,  &
            2.0*corr1(2*(i-1)+1), &
            2.0*corr2(2*(i-1)+1)

    end do
    close(100)

    deallocate(r)
    deallocate(corr1)
    deallocate(corr2)
    do i = 1, 2
        call dfftw_destroy_plan(plan_r2c_corr(i))
        call dfftw_destroy_plan(plan_c2r_corr(i))
    end do


    stop
end program

Looks like a nice example, but where is fftw3.f03

Looks like a nice example, but where is fftw3.f03

You should provide with include path in Makefile or compile option. If you install FFTW3 you can find easily.

I have installed FFTW3, and how to provide with include path in makefile or compile option?