# Fortran program for oscillator using Euler method

• Fortran
koushan
I am trying to run a program with fortran. The program is about solving the Oscillator using Euler Method. I am trying to run this code and applying array arguments (as I want to extend it to 3 dimensions afterwards).
When I try to compile, it comes up with an error "Unclassifiable statement at (1)". I don't know how to resolved this error.
This is my code:

program Oscilator
implicit none

real :: h !(Step size)
integer, parameter :: n=100
real, dimension(1:n) :: x,v,t
external dx,dv
integer :: i

x(1) = 0.017
t(1) = 0.0
v(1) = 0.0

write(*,*) 't x v'

do while(t(i) <= n)
h = t(i+1) - t(i)
call Euler(h,x,v)
write(6,'(4(e12.5,3x))') t(i), x(i), v(i)

t(i)=t(i+1)
x(i)=x(i+1)
v(i)=v(i+1)

end do

end program Oscilator

function dx(t)
implicit none
real dx, t
dx = exp(-0.9*t)*(-2.5*exp(0.557*t)-2.8*exp(-0.557*t))
end function dx

function dv(t)
implicit none
real dv, t
dv=-0.2941*exp(-0.343*t)-5.9439*exp(-1.457*t)
end function dv

subroutine Euler(h,x,v)
implicit none
real :: v,x,h,dv
integer :: i

x(i+1) = x(i) + v(i)*h
v(i+1) = v(i) + dv(i)*h

end subroutine Euler

## Answers and Replies

Staff Emeritus
Homework Helper
Code:
 tags when attaching source files.

IDK about your error, but the program has some serious errors in implementation.

For example, in the main program, you have a DO WHILE construct calculating values for the arrays t,x, and v.
The problem is, the array index i is never defined or calculated, either inside or outside the DO WHILE construct.

The condition for termination (t(i) <= n) can't be met because what value of the t array should be used?  There are no statements which define any of the values of the t array except for t(1) = 0.0.

Mentor
When I try to compile, it comes up with an error "Unclassifiable statement at (1)".
I don't know either about this error. What compiler are you using?

To add to what SteamKing said, here are some more flagrant errors:

do while(t(i) <= n)
h = t(i+1) - t(i)
t(i) has never been defined. And usually, you would do this the other way around: you define h, and then increment t.

write(6,'(4(e12.5,3x))') t(i), x(i), v(i)

t(i)=t(i+1)
x(i)=x(i+1)
v(i)=v(i+1)
If you're only going to print out t, x, and v, there is no need to store them in an array.

function dx(t)
implicit none
real dx, t
dx = exp(-0.9*t)*(-2.5*exp(0.557*t)-2.8*exp(-0.557*t))
end function dx
This function is never used.

dv=-0.2941*exp(-0.343*t)-5.9439*exp(-1.457*t)
This is an extremely strange equation to see in combination with the Euler method. You seem to already know what ##v(t)## is. What problem are you actually trying to solve?

subroutine Euler(h,x,v)
implicit none
real :: v,x,h,dv
integer :: i

x(i+1) = x(i) + v(i)*h
v(i+1) = v(i) + dv(i)*h

end subroutine Euler
x and v are not arrays in the subroutine, so they shouldn't have any index. dv is not declared as external. dv takes a real as an argument, not an integer. i is never defined.