6. Computation on Arrays: Broadcasting

Introducing Broadcasting

# In[1]
a=np.array([0,1,2])
M=np.ones((3,3))
M+a

# Out[1]
array([[1., 2., 3.],
       [1., 2., 3.],
       [1., 2., 3.]])

• the one-dimensional array a is stretched(broadcasted), across the second dimension in order to match the shape of M.

# In[2]
a=np.arange(3)
b=np.arange(3)[:,np.newaxis]
print(a)
print(b)
print(a+b)

# Out[2]
[0 1 2]
[[0]
 [1]
 [2]]
[[0 1 2]
 [1 2 3]
 [2 3 4]]

• a and b both stretched or broadcasted to match a common shape, and the result is a two-dimensional array.

Rules of Broadcasting

• Rule 1 : If the two arrays differ in their number of dimensions, the shape of the one with fewer dimensions is padded with ones on its leading(left) side.
• Rule 2 : If the shape of the two arrays does not match in any dimension, the array with shape equal to 1 in that dimension is stretched to match the other shape.
• Rule 3 : If in any dimension the sizes disagree and neither is equal to 1, an error is raised.

Broadcasting Example 1

# In[3]
M=np.ones((2,3))
a=np.arange(3)

• M.shape is (2,3) , and a.shape is (3,)
• We see by rule 1 that the array a has fewer dimensions, so we pad it on the left with ones.
  So, M.shape remains (2,3), a.shape becomes (1,3)
• By rule 2, we see that the first dimension disagrees, so we stretch this dimension to match.
  So, M.shape remains (2,3), a.shape becomes (2,3)

# In[4]
M+a

# Out[4]
array([[1.,2.,3.],
       [1.,2.,3.]])

Broadcasting Example 2

# In[5]
a=np.arange(3).reshape((3,1))
b=np.arange(3)

• a.shape is (3,1), and b.shape is (3,)
• By rule 1, a.shape remains (3,1), and b.shape becomes (1,3)
• By rule 2, a.shape becomes (3,3), and b.shape becomes (3,3)

# In[6]
a+b

# Out[6]
array([[0,1,2],
       [1,2,3],
       [2,3,4]])

Broadcasting Example 3

# In[7]
M=np.ones((3,2))
a=np.arange(3)

• M.shape is (3,2), and a.shape is (3,)
• By rule 1, M.shape remains (3,2), and a.shape becomes (1,3)
• By rule 2, M.shape remains (3,2), and a.shape becomes (3,3)
• By rule3, the final shape do not match, so these two arrays are incompatible.

Broadcasting in Practice

Centering an array

# In[8]
X=np.random.random((10,3))
print(X)

# Out[8]
[[0.79578526 0.06970127 0.80572102]
 [0.49596132 0.4203202  0.46907811]
 [0.14083824 0.66032281 0.86455548]
 [0.06037715 0.83184264 0.54172137]
 [0.38316786 0.05267514 0.70413834]
 [0.05020395 0.78665839 0.7274787 ]
 [0.47849237 0.98020416 0.44380548]
 [0.58073628 0.97996138 0.40468001]
 [0.25097966 0.39015983 0.79417086]
 [0.39169738 0.96715734 0.56671287]]

# In[9]
Xmean=X.mean(0) # X.mean('axis')
Xmean

# Out[9]
array([0.36282395, 0.61390031, 0.63220622])

• We can center the X array by subtracting the mean

# In[10]
X_centered=X-Xmean
X_centered.mean(0)

# Out[10]
array([ 6.66133815e-17, -4.44089210e-17, -6.66133815e-17])

Plotting a two-dimensional function

# In[11]
x=np.linspace(0,5,50)
y=np.linspace(0,5,50)[:,np.newaxis]
z=np.sin(x)**10+np.cos(10+y*x)*np.cos(x)

---

# In[12]
%matplotlib inline
import matplotlib.pyplot as plt
plt.imshow(z,origin='lower',extent=[0,5,0,5],cmap='viridis')
plt.colorbar();

• We can see the result that is a compelling visualization of the two-dimensional function above the image.

---

References:

1) https://jakevdp.github.io/PythonDataScienceHandbook/02.05-computation-on-arrays-broadcasting.html