Solving ODE in python

Question

In MATLAB I am solving this ode as:

B = [0 0 -5]'; 
y0 =[0 -5 0 5 0 0]';
f = @(t,y) [y(4:6); (q/m)*cross(y(4:6),B)];
[t,y] = ode45(f,[0 100],y0);

I've been reading previous answers about how to solve ODEs in Python and how to fit the MATLAB code to corresponding Python code but, when I try to solve the same ODE as:

f=lambda t,y: np.array([y(np.arange(4,6)),(q/m)*np.cross(y(np.arange(4,6)),B)])
sol =solve_ivp(f, np.array([0 100]), y0)

I get an error message(line 454, in solve_ivp solver = method(fun, t0, y0, tf, vectorized=vectorized, **options))

I also tried:

sol = integrate.odeint(f, np.array([0 100]), y0)

without any luck.

tdy · Accepted Answer · 2021-06-18 09:01:21Z

3

y(4:6) in MATLAB is not y(np.arange(4,6)) in Python.
- In MATLAB, y(4:6) gets the 4th to 6th elements from array y.
- In Python, that should be y[3:6] (square brackets; 3 instead of 4 because Python is 0-indexed; 6 instead of 5 because Python's right bound is non-inclusive).
In the MATLAB f, the vectors are joined vertically (semicolon), so in Python they should be stacked vertically with vstack().
Python lists require commas. In the sol line, [0 100] needs to be [0, 100].

You probably want something like this:

f = lambda t, y: np.vstack([y[3:6], (q/m) * np.cross(y[3:6], B)])
sol = solve_ivp(f, np.array([0, 100]), y0)

edited Jun 18, 2021 at 9:01

answered Jun 18, 2021 at 3:46

tdy

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