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  • 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
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