Axes3D: Numpy arrays error while trying to build 3D plot

Question

Let's start from the beginning. This how I getting x and y values:

from mpl_toolkits.mplot3d import Axes3D
fig = plt.figure()
ax = fig.gca(projection='3d') # get current axis

w0 = np.arange(30, 80, 1) # x - values
w1 = np.arange(-3, 3, 0.1) # y - values

X, Y = np.meshgrid(w0, w1) # no idea why to do that

Because I don't know how to avoid loops I go this way to calculate Z values:

    sizeWo = len(w0)
    sizeW1 = len(w1)
    Z = np.zeros((sizeWo, sizeW1))

    for i in xrange(0,sizeWo):
        for j in xrange(0,sizeW1):
            Z[i,j] = errorLose(w0[i], w1[j])

surf = ax.plot_surface(X, Y, Z) # that lines generates the error (ValueError: shape mismatch: objects cannot be broadcast to a single shape)

Even this code generates the same error:

surf = ax.plot_surface(w0, w1, Z) shape mismatch: objects cannot be broadcast to a single shape
plt.show()

What is wrong here and how to make it work?

errorLose function takes two values and calculates error using dataframe data behind:

def errorLose(w0,w1):
    return np.sum((data.Height - (w0 + w1 *data.Weight))**2)

This is how you can get the same data. This the link to the csv file:

data = pd.read_csv('weights_heights.csv', index_col='Index')

Community · Accepted Answer · 2017-05-23 12:07:40Z

11

You're very close. The issue is that you are creating Z to be sizeWo x sizeW1 but meshgrid creates X and Y which are size sizeW1 x sizeW0. You can either setup Z differently:

Z = np.zeros((sizeW1, sizeWo))

for i in xrange(0,sizeWo):
    for j in xrange(0,sizeW1):
        Z[j,i] = errorLose(w0[i], w1[j])

Or you could keep Z the same and simply pass the transpose of Z to your plot constructor

surf = ax.plot_surface(X, Y, np.transpose(Z))

Update

The reason that you need to use meshgrid is that your x and y coordinates are only 1D arrays initially and plot_surface expects X and Y to be 2D arrays. What meshgrid does is it creates 2D arrays from the two inputs with every permutation of the two. This answer contains a more detailed discussion of meshgrid.

edited May 23, 2017 at 12:07

CommunityBot

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answered Apr 16, 2016 at 19:54

Suever

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3 Comments

Suever Over a year ago

@Rocketq Sorry, that was a typo on my part. Updated now.

Rocketq Over a year ago

Wow, this is fantastic) But why do we need to meshgrid?

Suever Over a year ago

@Rocketq I added a little discussion on that at the end as well as a link to a potentially relevant answer

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