CODEWITHSUNDEEP
pythonarraysnumpy
dustin
dustin

Reputation: 4406

splitting an array in Python

I have an array that is (219812,2) but I need to split to 2 (219812).

I keep getting the error ValueError: operands could not be broadcast together with shapes (219812,2) (219812)

How can I accomplish?

As you can see, I need to take the two separate solutions from u = odeint and multiple them.

def deriv(u, t):
    return array([ u[1], u[0] - np.sqrt(u[0]) ])

time = np.arange(0.01, 7 * np.pi, 0.0001)
uinit = array([ 1.49907, 0])
u = odeint(deriv, uinit, time)

x = 1 / u * np.cos(time)
y = 1 / u * np.sin(time)

plot(x, y)
plt.show()

Upvotes: 1

Views: 534

Answers (3)

jorgeca
jorgeca

Reputation: 5522

To extract the ith column of a 2D array, use arr[:, i].

You could also unpack the array (it works row wise, so you need to transpose u so that it has shape (2, n)), using u1, u2 = u.T.

By the way, star imports aren't great (except maybe in the terminal for interactive use), so I added a couple of np. and plt. to your code, which becomes:

def deriv(u, t):
    return np.array([ u[1], u[0] - np.sqrt(u[0]) ])

time = np.arange(0.01, 7 * np.pi, 0.0001)
uinit = np.array([ 1.49907, 0])
u = odeint(deriv, uinit, time)

x = 1 / u[:, 0] * np.cos(time)
y = 1 / u[:, 1] * np.sin(time)

plt.plot(x, y)
plt.show()

It also seems like a logarithmic plot looks nicer.

Upvotes: 3

Joran Beasley
Joran Beasley

Reputation: 113940

u1,u2 = odeint(deriv, uinit, time)

maybe ?

Upvotes: 1

Amber
Amber

Reputation: 526543

It sounds like you want to index into the tuple:

foo = (123, 456)
bar = foo[0] # sets bar to 123
baz = foo[1] # sets baz to 456

So in your case, it sounds like what you want to do might be...

u = odeint(deriv, uinit, time)

x = 1 / u[0] * np.cos(time)
y = 1 / u[1] * np.sin(time)

Upvotes: 1

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