title: Confusion about DO loop in a FORTRAN program (Beginner level)
body: <p>I'm just starting out learning to program and FORTRAN 95 is my first language. I'm trying to complete Question 4 i) at the bottom of page 45 of <a href="http://www.maths.usyd.edu.au/u/david/fplecs12.pdf" rel="nofollow">this</a> pdf file. For reference, here is my whole source code:</p>
  
  <pre><code>PROGRAM PARACHUTIST
  ! Tabulation of parachutist's descent z and and speed zdot
  ! as functions of time t
  
   !Assign the program's associated constants
  
  IMPLICIT NONE
    REAL z, zdot, g, U1, U2, z0, u0, t0, q0, t, x,c,s
  INTEGER I
  
  
  g=9.8
  U1=54
  U2=5
  !Break z0 down a little with q0
  q0=COSH(g*t0/U1)
  z0=U1**2/g*LOG(q0)
  u0=U1*TANH(g*t0/U1)
  
    !Prompt for and read in the free-fall time
  
    Print*, 'Input free-fall time in seconds:'
    Read*, t0
  
    !Print the table headings
    WRITE(*,1000)
  
  1000 FORMAT (6X, 'TIME', 6X, 'DISTANCE', 6X, 'VELOCITY', /6X, '(SEC)', 7X, '(M)', 10X, '(M/SEC)',&amp;
      /6X, '0.0', 10X, '0.0', 10X, '0.0' )
  
    !Loop covering the specified times
    t=0
  ! I know I'm meant to start some DO loop here, but unsure
  ! how to set it up. 
       ! Calculate the distance above ground
  200         IF(t&lt;=15) THEN
          x=g*t/U1
          z=U1**2/g*LOG(COSH(x))
          zdot=U1*TANH(x)
       Elseif(t&gt;15) THEN
          x=g*(t-t0)/U2  !store re-used expressions
          c=cosh(x)
          s=sinh(x)
          z= z0 + (U2**2/g)*LOG(c+ u0/U2*s)
          zdot=U2*(U2*s+u0*c)/(U2*c+u0*s) 
       Endif
  
       !Print a line of table using T formats
       WRITE(*,100) t, z, zdot
  100      Format(4X, F5.2, 6X, F7.2, 6X, F7.2)
  
       !Stop with message if landed
       If(z.GE.500) THEN
      STOP
  
       !If we haven't yet landed then increment t as in 
      !   problem specs
       If(t&lt;15) then
          t=t+1
  
       Elseif(t.GE.15) then
          t=t+10
  ENDIF
  GOTO 200
  300 STOP
  ENDIF
    !End of the t-loop
  
  END PROGRAM PARACHUTIST
  </code></pre>
  
  <p>Now, I think I've assigned variable types/values correctly and printed out the headings in a suitable format (though I'm never certain, if you see any errors please tell me), my main problem occurs from "!Loop covering the specified times". That whole block of code, to be honest, I'm totally confused about. I'm not really certain about how to construct the loop, I've only done them on much simpler problems and can't wrap my head around this one. Can someone please take a look and give me some suggestions? </p>