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NumPy

NumPy#

NumPy is a fundamental library for computation in Python.

Additional resources:

import numpy as np

Python list#

Lets’s compare regular Python lists and NumPy arrays.

# A list is created with [.., ..]
myvals = [1.0, 2.0, 1.5]
myvals

[1.0, 2.0, 1.5]

type(myvals)

list

Numpy 1D array (vector)#

myvals_np = np.array([1.2, 3.0, 4.0])

myvals_np

array([1.2, 3. , 4. ])

type(myvals_np)

numpy.ndarray

myvals_np.dtype

dtype('float64')

myvals_np.sum()

8.2

Indexing#

x = np.array([1.0,1.5, 2.0, 5.3]) 
x

array([1. , 1.5, 2. , 5.3])

x[1]

1.5

x[-1]

5.3

x[1] = 2.0 # modify the second value in the array
x

array([1. , 2. , 2. , 5.3])

Slicing#

x[:2]

array([1., 2.])

Inline exercise

  1. Create an array x with three values: 1, 2, 3

  2. What is is the data type of x?

  3. Create a new array: y = x/2

  4. What is the data type of y?`

Math operations#

Python is a general purpose language not designed with numerical computing in mind.

However, NumPy is designed for numerical computing!

[1.2, 4.5] + [2.3, 4.3] # is this the result you expected??

[1.2, 4.5, 2.3, 4.3]

np.array([1.2, 4.5]) + np.array([2.3, 4.3]) 

array([3.5, 8.8])

np.array([1.2, 4.5]) * np.array([2.3, 4.3]) 

array([ 2.76, 19.35])

Note for Matlab users, all operators such as * are element wise

np.array([1.2, 4.5]) @ np.array([2.3, 4.3]) # in case you actually wanted to do a dot product

22.11

x = np.arange(5, 100, 5)
x

array([ 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85,
       90, 95])

x.dtype # Integers!

dtype('int64')

x + 1 # add 1!

array([ 6, 11, 16, 21, 26, 31, 36, 41, 46, 51, 56, 61, 66, 71, 76, 81, 86,
       91, 96])

x = x + 3.0 # add a float to some integers, can we do that?
x

array([ 8., 13., 18., 23., 28., 33., 38., 43., 48., 53., 58., 63., 68.,
       73., 78., 83., 88., 93., 98.])

x.dtype # but now it became floats!

dtype('float64')

xr = np.random.random(10)
xr

array([0.51138884, 0.76122309, 0.35516621, 0.43512108, 0.8520334 ,
       0.89361148, 0.27111809, 0.52157156, 0.32692597, 0.98700368])

xr.mean()

0.5915163400545689

xr.std()

0.24670587350573767

xr.max()

0.9870036824983236

xr - xr.mean()

array([-0.0801275 ,  0.16970675, -0.23635013, -0.15639526,  0.26051706,
        0.30209514, -0.32039825, -0.06994478, -0.26459037,  0.39548734])

xn = np.random.normal(loc=5.0, scale=2.0, size=100)
xn[30] = 99.0

mu = xn.mean()
sigma = xn.std()

xn[xn < mu - 3*sigma]

array([], dtype=float64)

xn[xn > mu + 3*sigma]

array([99.])

Missing values (delete values)#

NumPy has support for missing values.

y = np.random.random(10)
y

array([0.9159014 , 0.43457129, 0.96962508, 0.79153184, 0.24476932,
       0.68118923, 0.32401229, 0.840004  , 0.22681353, 0.02227421])

y[5:] = np.nan
y

array([0.9159014 , 0.43457129, 0.96962508, 0.79153184, 0.24476932,
              nan,        nan,        nan,        nan,        nan])

y.mean()

nan

np.nanmean(y)

0.6712797883584728

y * np.pi

array([2.87738912, 1.36524597, 3.04616704, 2.48667062, 0.76896551,
              nan,        nan,        nan,        nan,        nan])

Boolean indexing#

z = np.random.normal(loc=0.0, scale=3.0, size=10)

z_sorted = np.sort(z)
z_sorted

array([-3.0696741 , -2.12224557, -1.7728654 , -1.26239075, -0.62741967,
       -0.38210881,  0.99777942,  1.05879158,  1.52975617,  3.58838654])

z<0.0

array([ True, False, False,  True, False,  True, False,  True,  True,
        True])

z_sorted<0.0

array([ True,  True,  True,  True,  True,  True, False, False, False,
       False])

z_sorted[z_sorted<0.0]

array([-3.0696741 , -2.12224557, -1.7728654 , -1.26239075, -0.62741967,
       -0.38210881])

z_sorted[z_sorted<0.0] = 0.0

z_sorted

array([0.        , 0.        , 0.        , 0.        , 0.        ,
       0.        , 0.99777942, 1.05879158, 1.52975617, 3.58838654])

np.where(z<0.0)

(array([0, 3, 5, 7, 8, 9]),)

xn = np.random.normal(loc=5.0, scale=2.0, size=100)

xn[30] = 99.0 # outlier

median = np.median(xn)
sigma = xn.std()

sigma # sample std affected by outlier

9.543150104037277

xn[xn > median + 3*sigma] # but 1 abnormally high value

array([99.])

xn[xn > median + 3*sigma] = np.nan

np.nanstd(xn) # much closer to the true std==2.0

1.9395730668307385

2D arrays#

X = np.array([
              [0.0, 1.0, 2.0],
              [3.0, 4.0, 5.0]
])
    

X

array([[0., 1., 2.],
       [3., 4., 5.]])

X.shape

(2, 3)

nrows = X.shape[0]
nrows

2

ncols = X.shape[1]
ncols

3

X[0,0]

0.0

X[1,1]

4.0

X[-1,-1]

5.0

X[0,:]

array([0., 1., 2.])

X[0]

array([0., 1., 2.])

X.mean()

2.5

colmeans = X.mean(axis=0)
colmeans

array([1.5, 2.5, 3.5])

colmeans.shape

(3,)

rowmeans = X.mean(axis=1)
rowmeans

array([1., 4.])

X - colmeans

array([[-1.5, -1.5, -1.5],
       [ 1.5,  1.5,  1.5]])

# X - rowmeans    # executing this will fail

NumPy broadcasting (detailed explanation of how arrays can be used in expressions)

R = rowmeans[:, np.newaxis] # add a new dimension to create a 2D array
R

array([[1.],
       [4.]])

np.expand_dims(rowmeans, 1) # same result

array([[1.],
       [4.]])

X.shape

(2, 3)

R.shape

(2, 1)

X - R

array([[-1.,  0.,  1.],
       [-1.,  0.,  1.]])

Reshaping#

x = X.flatten()

x

array([0., 1., 2., 3., 4., 5.])

x.reshape(2,3)

array([[0., 1., 2.],
       [3., 4., 5.]])
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