How to convert 2D array to 1D array in Fortran code?

How do I convert r(i,j) to a 1D array so that I can sort the number easily?

program sort
  implicit none
  character CN*8,O*7
  integer j,iconf,nconf
  integer i,nbins,t
  integer n,nmax,ind,num,b
  parameter (n=216)
  double precision xbox,rq
  parameter (nmax=3091,nconf=1)
  double precision atom(nmax),id(nmax),ox(nmax),oy(nmax),oz(nmax)
  double precision xij,yij,zij,rij
  double precision r(n,n),A(n)
  open(unit=10,status='unknown',file='1000.gro')
   do iconf= 1,nconf
    write(*,*)iconf
     read(10,*)
     read(10,*)
   do i=1,n
     read(10,'(A8,A7,1i5,3f8.3)')CN,O,num,ox(i),oy(i),oz(i)
   enddo
     read(10,*)xbox        ! read the xbox for PBC

  open(unit=3,file='dist.txt')

   do i=1,n
    do j=1,n
   if(i .ne. j) then
   xij=ox(i)-ox(j)
   yij=oy(i)-oy(j)
   zij=oz(i)-oz(j)
   r(i,j)=dsqrt(xij**2 + yij**2 + zij**2)
      write(3,'(i3,2x,i3,4x,f17.15)') i,j, r(i,j)
    endif 
    enddo
    enddo
    enddo
    END

I have to calculate the distance and save this in array as r (i,j). I want to convert r(i,j) to a 1 dimensional array, so that I can sort the r(i,j)easily.

Solution

This seems tailor made for the reshape function: https://gcc.gnu.org/onlinedocs/gfortran/RESHAPE.html In this tiny example, reshape is used first to make a two-dimensional array A, then reshape is called again to make the one-dimension array C.

Program reshape_demo
    use, intrinsic :: iso_c_binding

    implicit none
    real(kind=c_float),allocatable :: A(:,:),C(:)
    integer(kind=c_int) :: krow
    allocate(A(3,3))
    A=reshape((/1,2,3,4,5,6,7,8,9/),(/3,3/))
    do krow=1,3
       write(*,fmt="(1p3e10.3)")A(krow,:)
    end do
    C=reshape(A,(/9/))
    write(*,fmt="(9(1x,f4.1))")C
End Program reshape_demo