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Fortran 90, Euler's method help

  1. May 4, 2008 #1


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    Gold Member

    I'm trying to make a program that can approximate a differential equation via the Euler's method.
    Here is my program :
    Program diff
    implicit none

    Real :: t_0, x_0, t_f, k_j, h
    Real :: t,x
    Integer :: n,j

    Write(*,*)'Chose t_0 and x_0'
    Read(*,*)t_0, x_0
    Write(*,*)'Chose t_f'

    Write(*,*)'Chose n'

    Do j=1,n

    end do

    Real Function f(t,x)
    real, intent(in) :: x,t
    end function

    end program

    Here I am trying to approximate the solutions of the following differential equation : [tex]x'=2x[/tex] and [tex]x(0)=1[/tex] in the interval [0,1], using a step h of 0.1. So in my program, I input t_0=0, X_0=1, t_f=1 and n=10.
    I know where is the problem of my program. It's in the "do" part. And more exactly here I think "k_j=f(t,x)". For example, the first term should be k_1=f(0,1) so it must be equal to 2. But the program doesn't do it. It doesn't understand that f(t,x) should be evaluated in t=0, x=1 for the first term. How can I fix this? I'm thinking about another "do" part, inside of the one I already have... But can't figure it out. Thanks for your help.
  2. jcsd
  3. May 4, 2008 #2


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    Homework Helper

    Hi fluidistic,

    It looks to me like you never set the variables equal to the initial values. The is, you had the program ask for t_0 and x_0, but you never actually used those values (except for t_0 to calculate h).

    Try putting

    Code (Text):

    before the do loop.
  4. May 4, 2008 #3


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    Gold Member

    Thanks alphysicist! That works great now.
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