hw2-1.f

       ! use f90 or f95 for this
       program hw2
       implicit none
       
       ! vars
       double precision :: h, p
       integer :: loop
       double precision, dimension(11) :: trueVals

       h = 1.0
       loop = 0
       trueVals(1)  = -1.2625
       trueVals(2)  = -0.916
       trueVals(3)  = -0.912535
       trueVals(4)  = -0.91250035
       trueVals(5)  = -0.9125000035
       trueVals(6)  = -0.912500000035
       trueVals(7)  = -0.91250000000035
       trueVals(8)  = -0.9125000000000035
       trueVals(9)  = -0.912500000000000035
       trueVals(10) = -0.91250000000000000035
       trueVals(11) = -0.9125000000000000000035

       ! It looks like fortran mangles the true values.
       ! Compare the above values to the outputted values.

       write(*, *)
1      if (loop /= 11) then
           p = prime(0.5, h)
           write(*, *) "h =", h, ": f'(x) =", p
           write(*, *) "true value =", trueVals(loop + 1)
           write(*, *) "true error =", abs(trueVals(loop + 1) - p)
           write(*, *)
           write(7, *) h, abs(trueVals(loop + 1) - p)
           h = h * 0.1
           loop = loop + 1
           goto 1
       end if


       contains
       function f(x)
           double precision f, x
           f = -0.1 * x**4 - 0.15 * x**3 - 0.5 * x**2
           f = f - 0.25 * x + 1.2
           return
       end function f

       function prime(x, h)
           double precision :: prime
           real :: x
           double precision :: h
           prime = (f(x + h) - f(x - h)) / (2.0 * h)
           return
       end function prime

       end program hw2