How to limit the number of characters to be printed per line in Fortran?

Question

This is the program I am compiling in Fortran. It calculates two different formulas for even and odd numbers and it stops when you get 1 as result. If I choose N = 50 for example, I get all the values displayed in the same line. I would like maximum 40 characters (including numbers and commas) printed per line. I also need to store the last numbers (4,2,1), but I don't know how.

That's what I am getting when I compile the program.

50, 25, 76, 38, 19, 58, 29, 88, 44, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1,

That's what I want (40 characters per line):

50, 25, 76, 38, 19, 58, 29, 88, 44, 22,

11, 34, 17, 52, 26, 13, 40, 20, 10, 5,

16, 8, 4, 2, 1,

program numbers
implicit none

integer(kind=4) :: i, n

print*,'Enter the N value'

read*, n

          do while (n.ne.1)
              if (mod(n,2) .eq.0) then
                    n = n/2
                    write(6,'(i0,a)',advance='no') n,','
               else if (mod(n,2) .ne.0) then
                    n = (3*n)+1
               write(6,'(i0,a)',advance='no') n,','
               else
                    cycle
               end if
          end do

end program numbers

Answer 1

You could tell the compiler break lines after a given number of characters, but that would also break them midnumber. If you want to avoid that, you could just count the number of characters. Maybe the easiest to do that would be to write to string and to use len_trim to figure out its length. For keeping the last three numbers around you could use an array of length three and put a number into index of current iteration modulo 3.

Something like this maybe:

program numbers
  implicit none

  integer :: i, n
  integer :: rowlen
  integer :: last(0:2)
  character(len=40) :: numstring = ''

  print*,'Enter the N value'

  read*, n

  last(0) = n
  write(numstring, '(i0)') n
  rowlen = len_trim(numstring)
  write(*,'(a)', advance='no') trim(numstring)

  do i=1,3*n
    if (mod(n,2) == 0) then
      n = n/2
    else
      n = (3*n)+1
    end if

    write(numstring, '(a,i0)') ', ', n
    rowlen = rowlen+len_trim(numstring)
    if (rowlen > 39) then
       write(*,'(a)') ','
       write(numstring, '(i0)') n
       rowlen = len_trim(numstring)
    end if
    write(*,'(a)', advance='no') trim(numstring)

    last(mod(i,3)) = n

    if (n == 1) EXIT
  end do

  write(*,*) ''
  if (i > 2) then
    write(*,*) 'Last three numbers:', last(mod(i-2,3)), &
      &                               last(mod(i-1,3)), &
      &                               last(mod(i,3))
  end if

end program numbers