Help with Python RK4: Solving Error Plotting 1st Order ODE

[gulaman]

i have been attempting to plot a first order ODE with pyhton's rk4

i can't display the graph and i have been having this error:

Traceback (most recent call last):
File "C:\Documents and Settings\AP155user38\Desktop\gladys python codes\differential equations and runge-kutta\1ODE.py", line 20, in <module>
Y[i+1] = rk4(t, Y, f)
File "c:\python25\lib\site-packages\matplotlib-0.98.3.0001-py2.5-win32.egg\matplotlib\mlab.py", line 928, in rk4
yout = np.zeros( (len(t),), np.float_)
TypeError: object of type 'function' has no len()

i need help in understanding this so i could move on with solving the error. :)

i hope you guys could help me out.

here's my code:

from scipy import *
from pylab import *


h = 0.1 #time step
ti = 0. #start
tf = 10. #end
t = arange(ti, tf, h) #time array

N = len(t) #number of time steps
Y = zeros(N) #stores value of y at each time step

Y[0] = 0. #initial condition

def f(t, Y):
dYdt = -9.81 - 0.5*Y
return dYdt

for i in range(N-1):
Y[i+1] = rk4(t, Y, f)

plot(t,Y)
show()

thanx.

gulaman
Quezon City, Philippines

 

[Tanner65]

I don't use Python but I had "learned" it enough to pass a class.

I think you're getting the error because you're not passing any arguments to f for rk4 to use.

Or

The value of t or Y is a null value.

Hope that helps. :-/

 

[quicknote]

Hello Gulaman,

Thank you for reaching out for help with your Python RK4 code. I understand that you are trying to plot a first order ODE and are encountering an error when trying to display the graph. The error you are receiving seems to be related to the length of your time array, t. The error message states that the object of type 'function' (in this case, your function f) has no length. This is because in your code, you are passing the function f as the second argument in your call to rk4, when it should be the third argument.

The third argument in the rk4 function should be the time step, h. So your code should look like this:

from scipy import *
from pylab import *

h = 0.1 #time step
ti = 0. #start
tf = 10. #end
t = arange(ti, tf, h) #time array

N = len(t) #number of time steps
Y = zeros(N) #stores value of y at each time step

Y[0] = 0. #initial condition

def f(t, Y):
dYdt = -9.81 - 0.5*Y
return dYdt

for i in range(N-1):
Y[i+1] = rk4(t, Y, h, f) #passing h as the third argument

plot(t,Y)
show()

I hope this helps to solve your error and allows you to continue working on your project. Best of luck with your Python coding!