Understanding python slice syntax

Question

N = 100 # number of points per class
D = 2 # dimensionality
K = 3 # number of classes
X = np.zeros((N*K,D))
y = np.zeros(N*K, dtype='uint8')
for j in xrange(K):
  ix = range(N*j,N*(j+1))
  r = np.linspace(0.0,1,N) # radius
  t = np.linspace(j*4,(j+1)*4,N) + np.random.randn(N)*0.2 # theta
  X[ix] = np.c_[r*np.sin(t), r*np.cos(t)]
  y[ix] = j
fig = plt.figure()
plt.scatter(X[:, 0], X[:, 1], c=y, s=40, cmap=plt.cm.Spectral)
plt.xlim([-1,1])
plt.ylim([-1,1])
plt.show()

This was the code and it's from here: https://cs.stanford.edu/people/karpathy/cs231nfiles/minimal_net.html

The only thing I don't understand here is this line:

plt.scatter(X[:, 0], X[:, 1], c=y, s=40, cmap=plt.cm.Spectral)

How can we do that (X[:, 0]) with lists and what this operation does?

X is NumPy array, not a list. X[:, 0] simply slices the second axis. Try X.shape / X.ndim to check the shape of your array. — jpp
– jpp, Commented Jun 24, 2018 at 20:26

Jonah · Accepted Answer · 2018-06-24 20:29:33Z

1

X is not a builtin python list. It's a numpy array. Have a look at the documentation for zeros https://docs.scipy.org/doc/numpy-1.14.0/reference/generated/numpy.zeros.html

and indexing arrays: https://docs.scipy.org/doc/numpy-1.13.0/reference/arrays.indexing.html

answered Jun 24, 2018 at 20:29

Jonah

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