splitting an array in Python

Question

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()

jorgeca · Accepted Answer · 2013-04-11 16:36:04Z

3

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.

answered Apr 11, 2013 at 16:36

jorgeca

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4 Comments

jorgeca Over a year ago

@dustin Make sure you're not computing/plotting something else. This shows how to "split" an array, which I think answers your question. We won't be able to help you solve your other question without more information.

jorgeca Over a year ago

@dustin Do you want plt.plot(time, u[:,0])?

askewchan Over a year ago

@dustin There are possibly a few other problems with your script, and if you are interested in getting it to work, you should ask another question about the actual problem you're interested in solving. One thing I notice is that your deriv equation accepts a t argument and never uses it. Also it is splitting the arrays by row, not by column.

dustin Over a year ago

@askewchan numerically solving odes Here is a link to another method of solving it but still no luck.

Amber · Accepted Answer · 2013-04-11 15:56:03Z

1

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)

answered Apr 11, 2013 at 15:56

Amber

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2 Comments

dustin Over a year ago

that returned the same error I am getting: ValueError: operands could not be broadcast together with shapes (2) (219812)

Amber Over a year ago

It would be useful to mention that error in your original post.

Joran Beasley · Accepted Answer · 2013-04-11 15:59:35Z

1

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

maybe ?

answered Apr 11, 2013 at 15:59

Joran Beasley

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1 Comment

Jaime Over a year ago

@dustin Then try u1, u2 = odeint(deriv, uinit, time).T.

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