write in array format in fortran

Question

I try to write an output file.dat with an nxn matrix format .

I write the code but the output is a column of value f.

Now the problem is: how can i change the output-format of the file to write?

from: 1 2 4 5 ...

to: 1,2,3,4 // 5,6,8,.. //

program eccen
    implicit none
    integer, parameter:: grid=800
    integer::i,j,k,n,m
    real*8,allocatable::f(:,:)
    real*8::xx(grid),yy(grid),mval,Mxval
    real*8,allocatable::x(:),y(:)

    open(10,file='3d_disk.txt')
    n=0
        DO
                READ(10,*,END=100)
                n=n+1
        END DO

 100     continue
        rewind(10)

    allocate(x(n),y(n))

    do i=1, n
        read(10,*) x(i),y(i)
    end do


    mval=-20.
    Mxval=20.
    do i=1, grid
        xx(i) = mval + ((Mxval - mval)*(i-1))/(grid-1)
        yy(i) = mval + ((Mxval - mval)*(i-1))/(grid-1)
    end do

    open(20,file='3d_map.dat')

    allocate(f(n,n))
    f=0
    do i=1,grid
        do j=1,grid
            m=0.
            do k=1, n
                if (x(k) > xx(i) .and. x(k) < xx(i+1) .and. &
                 & y(k) > yy(j) .and. y(k) < yy(j+1)) then  
                    m=m+1 ! CONTA IL NUMERO DI PARTICELLE
                end if
            end do
            f(i,j)=float(m+1)

I thing that the modification must be here from this:

                write(20,*) f(i,j)
            end do
            write(20,*)
     print *,i
        end do    
end program eccen

to:

    do i=1,grid
      do j=1,grid
        write(20,*) f(i,j)
      end do
    end do

end do
            write(20,*)
     print *,i
        end do

 end program eccen

Answer 1

This statement

write(20,*) f(i,j)

will write the value of f(i,j) then move to the next line, so the file you're getting is exactly what your code is specifying. If you want the file to contain n rows each with n values try

write(20,*) f(i,:)

which ought to make a good stab at writing the whole of row i of the array to a single line in the output file. Of course, this makes your loop over j redundant so you can remove it.