program loops

      implicit none

*     Program to demonstrate use of loops in fortran
*    (Lines that start with * or C are comments; all code starts in
*     at least column 7 and can't go past 72 in general)
*
*    To use on athena: Download code from website to athena
*    home directory and then.
*    % ssh athena.dialup.mit.edu
*    % add  -f gcc-3.4
*    % g77 loops.f -o loops
*    % loops
*    Program to demonstrate loops
*    Standard do loop
*       I   I**2 
*       1      1
*       2      4
*       3      9
*       4     16
*       5     25
*       6     36
*       7     49
*       8     64
*       9     81
*      10    100
*    Output from do while loop
*       I   I**2 
*       1      1
*      2      4
*       3      9
*       4     16
*       5     25
*       6     36
*       7     49
*       8     64
*       9     81
*      10    100

***** START OF PROGRAM *****
* Non-executable statements
*     Declare variables first
      integer*4  i, j    ! This is a comment, i and j loop variables
      logical done       ! Logical variable type

* Start of excutable statements

****  Output what this program does
      write(*,100) 
 100  format('Program to demonstrate loops')

****  Standard loop
      write(*,200)
 200  format('Standard do loop',/,
     .       '   I   I**2 ')         ! This is a continuation line
*     This loop loops from 1 to 10 increments of 1.  We could use
*     integer variables instead of 1 and 10 e.g.
*     n = 1
*     m = 10
*     do i = n, m   ! is the equivalent code
*     do i = m, n, -1  ! count down instead of up.
      do i = 1, 10
         write(*,210) i,i**2
 210     format(1x,i3,1x,i6)
      end do

****  Do while loop
*     The other loop is a do while loop which is useful if the
*     bounds are not known.  (See inout.f)
      write(*,220)
 220  format('Output from do while loop',/,
     .       '   I   I**2 ')        
      done = .false.
      i = 0
      do while ( .not. done )
         i = i + 1
         if( i.ge.10 ) done = .true.
         write(*,210) i,i**2     ! Notice here we use the earlier format
      end do

      end