Intro to Python for Data Science Learning 6

NumPy

From:https://campus.datacamp.com/courses/intro-to-python-for-data-science/chapter-4-numpy?ex=1

  • Your First NumPy Array

# Create list baseball
baseball = [180, 215, 210, 210, 188, 176, 209, 200]

# Import the numpy package as np
import numpy as np

# Create a numpy array from baseball: np_baseball
np_baseball = np.array(baseball)

# Print out type of np_baseball
print(type(np_baseball))

  • Baseball players' height

# height is available as a regular list

height =  [74, 74, 72, 72, 73, 69, 69, 71, 76, 71, 73, 73, 74, 74, 69, 70, 73, 75, 78, 79, 76, 74, 76, 72, 71, 75, 77, 74, 73, 74, 78, 73, 75, 73, 75, 75, 74, 69, 71, 74, 73, 73, 76, 74, 74, 70, 72, 77, 74, 70, 73, 75, 76, 76, 78, 74, 74, 76, 77, 81, 78, 75, 77, 75, 76, 74, 72, 72, 75, 73, 73, 73, 70, 70, 70, 76, 68, 71, 72, 75, 75, 75, 75, 68, 74, 78, 71, 73, 76, 74, 74, 79, 75, 73, 76, 74, 74, 73, 72, 74, 73, 74, 72, 73, 69, 72, 73, 75, 75, 73, 72, 72, 76, 74, 72, 77, 74, 77, 75, 76, 80, 74, 74, 75, 78, 73, 73, 74, 75, 76, 71, 73, 74, 76, 76, 74, 73, 74, 70, 72, 73, 73, 73, 73, 71, 74, 74, 72, 74, 71, 74, 73, 75, 75, 79, 73, 75, 76, 74, 76, 78, 74, 76, 72, 74, 76, 74, 75, 78, 75, 72, 74, 72, 74, 70, 71, 70, 75, 71, 71, 73, 72, 71, 73, 72, 75, 74, 74, 75, 73, 77, 73, 76, 75, 74, 76, 75, 73, 71, 76, 75, 72, 71, 77, 73, 74, 71, 72, 74, 75, 73, 72, 75, 75, 74, 72, 74, 71, 70, 74, 77, 77, 75, 75, 78, 75, 76, 73, 75, 75, 79, 77, 76, 71, 75, 74, 69, 71, 76, 72, 72, 70, 72, 73, 71, 72, 71, 73, 72, 73, 74, 74, 72, 75, 74, 74, 77, 75, 73, 72, 71, 74, 77, 75, 75, 75, 78, 78, 74, 76, 78, 76, 70, 72, 80, 74, 74, 71, 70, 72, 71, 74, 71, 72, 71, 74, 69, 76, 75, 75, 76, 73, 76, 73, 77, 73, 72, 72, 77, 77, 71, 74, 74, 73, 78, 75, 73, 70, 74, 72, 73, 73, 75, 75, 74, 76, 73, 74, 75, 75, 72, 73, 73, 72, 74, 78, 76, 73, 74, 75, 70, 75, 71, 72, 78, 75, 73, 73, 71, 75, 77, 72, 69, 73, 74, 72, 70, 75, 70, 72, 72, 74, 73, 74, 76, 75, 80, 72, 75, 73, 74, 74, 73, 75, 75, 71, 73, 75, 74, 74, 72, 74, 74, 74, 73, 76, 75, 72, 73, 73, 73, 72, 72, 72, 72, 71, 75, 75, 74, 73, 75, 79, 74, 76, 73, 74, 74, 72, 74, 74, 75, 78, 74, 74, 74, 77, 70, 73, 74, 73, 71, 75, 71, 72, 77, 74, 70, 77, 73, 72, 76, 71, 76, 78, 75, 73, 78, 74, 79, 75, 76, 72, 75, 75, 70, 72, 70, 74, 71, 76, 73, 76, 71, 69, 72, 72, 69, 73, 69, 73, 74, 74, 72, 71, 72, 72, 76, 76, 76, 74, 76, 75, 71, 72, 71, 73, 75, 76, 75, 71, 75, 74, 72, 73, 73, 73, 73, 76, 72, 76, 73, 73, 73, 75, 75, 77, 73, 72, 75, 70, 74, 72, 80, 71, 71, 74, 74, 73, 75, 76, 73, 77, 72, 73, 77, 76, 71, 75, 73, 74, 77, 71, 72, 73, 69, 73, 70, 74, 76, 73, 73, 75, 73, 79, 74, 73, 74, 77, 75, 74, 73, 77, 73, 77, 74, 74, 73, 77, 74, 77, 75, 77, 75, 71, 74, 70, 79, 72, 72, 70, 74, 74, 72, 73, 72, 74, 74, 76, 82, 74, 74, 70, 73, 73, 74, 77, 72, 76, 73, 73, 72, 74, 74, 71, 72, 75, 74, 74, 77, 70, 71, 73, 76, 71, 75, 74, 72, 76, 79, 76, 73, 76, 78, 75, 76, 72, 72, 73, 73, 75, 71, 76, 70, 75, 74, 75, 73, 71, 71, 72, 73, 73, 72, 69, 73, 78, 71, 73, 75, 76, 70, 74, 77, 75, 79, 72, 77, 73, 75, 75, 75, 73, 73, 76, 77, 75, 70, 71, 71, 75, 74, 69, 70, 75, 72, 75, 73, 72, 72, 72, 76, 75, 74, 69, 73, 72, 72, 75, 77, 76, 80, 77, 76, 79, 71, 75, 73, 76, 77, 73, 76, 70, 75, 73, 75, 70, 69, 71, 72, 72, 73, 70, 70, 73, 76, 75, 72, 73, 79, 71, 72, 74, 74, 74, 72, 76, 76, 72, 72, 71, 72, 72, 70, 77, 74, 72, 76, 71, 76, 71, 73, 70, 73, 73, 72, 71, 71, 71, 72, 72, 74, 74, 74, 71, 72, 75, 72, 71, 72, 72, 72, 72, 74, 74, 77, 75, 73, 75, 73, 76, 72, 77, 75, 72, 71, 71, 75, 72, 73, 73, 71, 70, 75, 71, 76, 73, 68, 71, 72, 74, 77, 72, 76, 78, 81, 72, 73, 76, 72, 72, 74, 76, 73, 76, 75, 70, 71, 74, 72, 73, 76, 76, 73, 71, 68, 71, 71, 74, 77, 69, 72, 76, 75, 76, 75, 76, 72, 74, 76, 74, 72, 75, 78, 77, 70, 72, 79, 74, 71, 68, 77, 75, 71, 72, 70, 72, 72, 73, 72, 74, 72, 72, 75, 72, 73, 74, 72, 78, 75, 72, 74, 75, 75, 76, 74, 74, 73, 74, 71, 74, 75, 76, 74, 76, 76, 73, 75, 75, 74, 68, 72, 75, 71, 70, 72, 73, 72, 75, 74, 70, 76, 71, 82, 72, 73, 74, 71, 75, 77, 72, 74, 72, 73, 78, 77, 73, 73, 73, 73, 73, 76, 75, 70, 73, 72, 73, 75, 74, 73, 73, 76, 73, 75, 70, 77, 72, 77, 74, 75, 75, 75, 75, 72, 74, 71, 76, 71, 75, 76, 83, 75, 74, 76, 72, 72, 75, 75, 72, 77, 73, 72, 70, 74, 72, 74, 72, 71, 70, 71, 76, 74, 76, 74, 74, 74, 75, 75, 71, 71, 74, 77, 71, 74, 75, 77, 76, 74, 76, 72, 71, 72, 75, 73, 68, 72, 69, 73, 73, 75, 70, 70, 74, 75, 74, 74, 73, 74, 75, 77, 73, 74, 76, 74, 75, 73, 76, 78, 75, 73, 77, 74, 72, 74, 72, 71, 73, 75, 73, 67, 67, 76, 74, 73, 70, 75, 70, 72, 77, 79, 78, 74, 75, 75, 78, 76, 75, 69, 75, 72, 75, 73, 74, 75, 75, 73]

# Import numpy
import numpy as np

# Create a numpy array from height: np_height
np_height = np.array(height)

# Print out np_height
print(np_height)

# Convert np_height from inches to meters: np_height_m
np_height_m = np_height * 0.0254

# Print np_height_m
print(np_height_m)

  • Baseball player's BMI

# height and weight are available as a regular lists

# Import numpy
import numpy as np

# Create array from height with correct units: np_height_m
np_height_m = np.array(height) * 0.0254

# Create array from weight with correct units: np_weight_kg
np_weight_kg = np.array(weight) * 0.453592

# Calculate the BMI: bmi
bmi = np_weight_kg/np_height_m ** 2

# Print out bmi
print(bmi)

  • Lightweight baseball players

To subset both regular Python lists and numpy arrays, you can use square brackets:

x = [4 , 9 , 6, 3, 1]
x[1]
import numpy as np
y = np.array(x)
y[1]

For numpy specifically, you can also use boolean numpy arrays:

high = y > 5
y[high]

# height and weight are available as a regular lists

# Import numpy
import numpy as np

# Calculate the BMI: bmi
np_height_m = np.array(height) * 0.0254
np_weight_kg = np.array(weight) * 0.453592
bmi = np_weight_kg / np_height_m ** 2

# Create the light array
light = bmi < 21

# Print out light
print(light)

# Print out BMIs of all baseball players whose BMI is below 21
print(bmi[light])

  • NumPy Side Effects

As Filip explained before, numpy is great for doing vector arithmetic. If you compare its functionality with regular Python lists, however, some things have changed.

First of all, numpy arrays cannot contain elements with different types. If you try to build such a list, some of the elements' types are changed to end up with a homogeneous list. This is known astype coercion.

Second, the typical arithmetic operators, such as +, -, * and / have a different meaning for regular Python lists and numpy arrays.

Have a look at this line of code:

np.array([True, 1, 2]) + np.array([3, 4, False])

Output:

   array([4, 5, 2])

  • Subsetting NumPy Arrays

Python lists and numpy arrays sometimes behave differently,but subsetting (using the square bracket notation on lists or arrays) works exactly the same. 

x = ["a", "b", "c"]
x[1]

np_x = np.array(x)
np_x[1]

# height and weight are available as a regular lists

# Import numpy
import numpy as np

# Store weight and height lists as numpy arrays
np_weight = np.array(weight)
np_height = np.array(height)

# Print out the np_weight at index 50
print(np_weight[50])

# Print out sub-array of np_height: index 100 up to and including index 110
print(np_height[100:111])

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原文地址：https://www.cnblogs.com/keepSmile/p/7752611.html