Debugging DO loop / IF blocks in a Fortran 90 program (Beginner level)

Question

I would be grateful if someone could help me out here. I'm just starting out learning how to program, so there is a great chance I'm missing something very obvious. I'm trying to write a program in Fortran 90 that solves question 4 i) on page 45 of this pdf. I have finally managed to get my code to compile to something, but now that something is somewhat rubbish, the data it produces is crazy (as time increases, I get a decrease in distance after whatever I input at t0). Can someone spot my mistake? I realize this is quite a lot of code to look through, I'm sorry for asking so much of you. Thanks in advance for looking through!

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   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)',&
        /6X, '0.0', 10X, '0.0', 10X, '0.0' )

      !Loop covering the specified times
      t=0 

    DO I=0,20

  ! Calculate the distance above ground
200         IF(t<=t0) THEN
            x=g*t/U1
            z=U1**2/g*LOG(COSH(x))
            zdot=U1*TANH(x)

    Elseif(t>t0) 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
        PRINT*, 'LANDED'
        STOP
         !If we haven't yet landed then increment t as in 
        !   problem specs
         Elseif(t<15) then
            t=t+1
         Elseif(t.GE.15) then
            t=t+10

        ENDIF

     !End of the t-loop
      END DO

  END PROGRAM PARACHUTIST

Answer 1

I wrote this as two comments, but it was really too lengthy. Go ahead and delete it all if you had planned to do so. I just browsed through a document comparing Fortran77 and "modern" Fortran90. (I coded in Fortran77 when I was just starting school, awhile ago...). Here are some suggestions:

Be careful with your use of "ELSEIF". It is usually okay for ELSE and IF to have the space omitted, but that is not true otherwise with free-format code (I think the only other instances of space-optional are DOUBLE PRECISION, ELSE IF, GO TO, END DO, and END IF).

An advantage of using Fortran90 is that you shouldn't even need ELSE IF's, (nor computed GOTO's!) as there is SELECT CASE.

You shouldn't need FORMAT either, as it can be incorporated directly with a format string in the READ or WRITE statement itself.

Yes, you can use either the old Fortran 77 operators .GE..GT..EQ..NE..LE..LT. or the new ones >= > == /= <= <. However, I'm not sure if you should mix them, which I noticed in your code.

EDIT: The second link above, about Control Structures, describes how you can use DO loops instead of IF's in Fortran90, sections 3.2 - 3.5. You can use named DO's, indefinite DO loops, DO WHILE's, all sorts of things! There are examples too. (The name of the entire document is Fortran90 for Fortran77 Programmers.)