How to fix “TypeError: 'numpy.ndarray' object is not callable” in python

Question

I am trying to solve Newton 2nd law (F=ma) by numerical method using RK4 method. Below is the implementation of my code so far:

import numpy as np
import matplotlib.pyplot as plt

v0 = 1                  # Initial velocity
theta = 45              # Launch angle
t0 = 0                  # Start time
tf = 100                # End time
h = 1                   # Steps

Uy = v0 * np.sin(theta*np.pi/180) # Initial velocity in y component
Ux = v0 * np.cos(theta*np.pi/180) # Initial velocity in x component
t = np.arange(t0,tf,h)            # Time points

v = [Uy,Ux]
s = [Uy,Ux]

def velocity(v,t):
    g  = 9.81               # Acceleration of free fall
    
    Vy = v[0]               # Array for initial value
    Vx = v[1]               # Array for initial value
    
    dVy_dt = -g
    dVx_dt = 0
    
    return [dVy_dt,dVx_dt]

def position(s,t):
    g  = 9.81               # Acceleration of free fall
    
    Sy = s[0]               # Array for initial value
    Sx = s[1]               # Array for initial value
    
    dSy_dt = Sy - (g*t)
    dSx_dt = Sx * t
    
    return [dSy_dt,dSx_dt]

def RK4(func,t,h,y0):
    nt = t.size
    y = np.zeros(nt)
    y[0] = y0
    for n in range(0,nt - 1):
        k1 = func(t[n],y[n])
        k1 = func(t[n] + h/2,y[n] + h*k1/2)
        k3 = func(t[n] + h/2,y[n] + h*k2/2)
        k4 = func(t[n] + h,y[n] + h*k3)
        y[n+1] = y[n] + (h*k1/6 + h*k2/3 + h*k3/3 + h*k4/6)
        
    return y

Position_y = RK4(position(s,t)[0],t,h,s[0])
Position_x = RK4(position(s,t)[1],t,h,s[1])

I get the following error below:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-51-01727ad9bc4b> in <module>
     50     return y
     51 
---> 52 Position_y = RK4(position(s,t)[0],t,h,s[0])
     53 Position_x = RK4(position(s,t)[1],t,h,s[1])

<ipython-input-51-01727ad9bc4b> in RK4(func, t, h, y0)
     42     y[0] = y0
     43     for n in range(0,nt - 1):
---> 44         k1 = func(t[n],y[n])
     45         k1 = func(t[n] + h/2,y[n] + h*k1/2)
     46         k3 = func(t[n] + h/2,y[n] + h*k2/2)

TypeError: 'numpy.ndarray' object is not callable

I do not understand what the error means. Could the problem be my function call to RK4() is indexed wrongly? Could it be some other problem?

Answer 1

You are returning numpy array in function position(s,t) and trying to call it as a function which is giving error

One of the solution is store the result of position(s,t) in numpy array like [p1,p2]=position(s,t) and then use Position_y = RK4(p1,t,h,s[0])

same for Position_x

Good Luck

Answer 2

Looks like you are assuming NP array as function, position(s,t)[0] returns np array, and you are using that as a callable func in RK4. change last 2 lines as,

Position_y = RK4(position,t,h,s[0])
Position_x = RK4(position,t,h,s[1])