Value error with numpy arrays (shapes)

Question

I keep getting a ValueError when working with numpy arrays and I can't figure out what's causing it, as it seems to be working correctly outside of my for loop. Here is my code:

import numpy as np

def x(t, x_0, w):
    return x_0*np.cos(w*t)
def x_prime(t, x_0, w):
    return -x_0*w*np.sin(w*t)

w = 1
x_0 = 1
h = 0.001
t = np.arange(0, 10, h)
y = np.array([[0, 0]]*len(t))

y[0] = [x_0, 0]
# The line below works correctly, but not inside my loop
print np.array([x_prime(1, x_0, w), -w**2 * x(1, x_0, w)])*h + y[0]
for i in range(1, len(t)):
    # Euler's method
    y[i] = y[i-1] + np.array([x_prime(t, x_0, w), -w**2 * x(t, x_0, w)]) * h

From the print line I get this output: [ 9.99158529e-01 -5.40302306e-04], so that seems to be working correctly. However, I'm getting this error at the y[i] line:

ValueError: operands could not be broadcast together with shapes (2,) (2,10000)

I'm not sure why, since my print statement earlier is basically doing the same thing, and y[i] should be the same shape. Does anyone know what the problem is?

fferri · Accepted Answer · 2018-04-23 22:19:42Z

1

In the print line the first argument of x()/x_prime() is a scalar (1).

In the y[i] line you pass t instead, which is a 10000-elements array, resulting in np.array([x_prime(t, x_0, w), -w**2 * x(t, x_0, w)]) being a (2,10000) matrix, hence the ValueError.

Perhaps what you want to do is:

y[i] = y[i-1] + np.array([x_prime(t[i], x_0, w), -w**2 * x(t[i], x_0, w)]) * h

answered Apr 23, 2018 at 22:19

fferri

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Comments

monkeyking9528 · Accepted Answer · 2018-04-23 22:21:14Z

0

No idea what your algorithm try to realize, but your seems to use all the t in one iteration, maybe my change can help you. import numpy as np

def x(t, x_0, w):
    return x_0*np.cos(w*t)
def x_prime(t, x_0, w):
    return -x_0*w*np.sin(w*t)

w = 1
x_0 = 1
h = 0.001
t = range(0, int(10/h))
y = np.array([[0, 0]]*len(t))

y[0,0] = x_0
y[0,1] = 0
# The line below works correctly, but not inside my loop
print(np.array([x_prime(1, x_0, w), -w**2 * x(1, x_0, w)])*h + y[0])
for i in range(1, len(t)):
    # Euler's method
    y[i] = y[i-1] + np.array([x_prime(t[i]*h, x_0, w), -w**2 * x(t[i]*h, x_0, w)]) * h

answered Apr 23, 2018 at 22:21

monkeyking9528

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1 Comment

monkeyking9528 Over a year ago

because i use range instead of np.arange, look carefully for the defintion, the range is (0~int(10/h)).

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