How to get FORTRAN 77 to read input?

Question

I am using some old fortran code for a biology project I am doing. I am posting the relevant snippets here. Here is a subroutine called "READCN". Earllier in the program MAXN was set to 108.

    OPEN ( UNIT = CNUNIT, FILE = CNFILE,
 :         STATUS = 'OLD', FORM = 'UNFORMATTED' )
    READ ( CNUNIT ) N, BOX
    IF ( N .GT. MAXN ) STOP ' N TOO LARGE '
    READ ( CNUNIT ) ( RX(I), I = 1, N ), ( RY(I), I = 1, N )

    CLOSE ( UNIT = CNUNIT )

    RETURN
    END

I am inputting a file called "data.dat" to the program. Here is the file:

10, 4
0.8147,    0.1576
0.9058,    0.9706
0.1270,    0.9572
0.9134,    0.4854
0.6324,    0.8003
0.0975,    0.1419
0.2785,    0.4218
0.5469,    0.9157
0.9575,    0.7922
0.9649,    0.9595

Nevertheless, I always get the message "N TOO LARGE". Any advice? Thanks!

Answer 1

Don't open as unformatted, it will read your file as if it were binary data. Open as formatted instead, and use "*" format. Also, don't read in one line, as you would not read your data in the expected order.

  program bob
  implicit none
  integer cnunit, n, maxn, box, i
  parameter(maxn=108, cnunit=10)
  real rx(maxn), ry(maxn)
  open(unit=cnunit, file='bob.txt', status='old', form='formatted')
  read(cnunit, *) n, box
  print *, 'n=', n, 'box=', box
  if(n .gt. maxn) stop 'n too large'

  do i=1, n
     read(cnunit, *) rx(i), ry(i)
     print *, rx(i), ry(i)
  end do

  close(unit=cnunit)
  end

Alternately, if you can't change the code, then change your input file to fit the needs of your program. The input file you give simply won't work: you need binary data, in the format expected by your compiler (there is the usual, non portable "record size"), and data must be given column-wise, not row-wise.