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Chapter 4

NumPy

NumPy is a fundamental Python package to efficiently practice data science. Learn to work with powerful tools in the NumPy array, and get started with data exploration.

slides_link
https://projector-video-pdf-converter.datacamp.com/735/chapter4.pdf

nb_of_exercises	title
5	Numpy

nb_of_exercises	title
5	2D Numpy Arrays

nb_of_exercises	title
3	Numpy: Basic Statistics

NumPy

type: VideoExercise
key: f4545baa53
xp: 50

@projector_key a0487c26210f6b71ea98f917734cea3a

Your First NumPy Array

type: NormalExercise
key: 84cab9d170
lang: python
xp: 100
skills:
  - 2

You're now going to dive into the world of baseball. Along the way, you'll get comfortable with the basics of numpy, a powerful package to do data science.

A list baseball has already been defined in the Python script, representing the height of some baseball players in centimeters. Can you add some code to create a numpy array from it?

@instructions

Import the numpy package as np, so that you can refer to numpy with np.
Use np.array() to create a numpy array from baseball. Name this array np_baseball.
Print out the type of np_baseball to check that you got it right.

@hint

import numpy as np will do the trick. Now, you have to use np.fun_name() whenever you want to use a numpy function.
np.array() should take on input baseball. Assign the result of the function call to np_baseball.
To print out the type of a variable x, simply type print(type(x)).

@pre_exercise_code

import numpy as np

@sample_code

# Import the numpy package as np


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

# Create a numpy array from baseball: np_baseball


# Print out type of np_baseball

@solution

# Import the numpy package as np
import numpy as np

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

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

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

@sct

predef_msg = "You don't have to change or remove the predefined variables."
Ex().has_import("numpy")
Ex().check_correct(
  check_object("np_baseball"),
  multi(
    check_object("baseball", missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
    check_function("numpy.array").check_args(0).has_equal_ast()
  )
)

Ex().has_printout(0)
success_msg("Great job!")

Baseball players' height

type: NormalExercise
key: e7e25a89ea
lang: python
xp: 100
skills:
  - 2

You are a huge baseball fan. You decide to call the MLB (Major League Baseball) and ask around for some more statistics on the height of the main players. They pass along data on more than a thousand players, which is stored as a regular Python list: height_in. The height is expressed in inches. Can you make a numpy array out of it and convert the units to meters?

height_in is already available and the numpy package is loaded, so you can start straight away (Source: stat.ucla.edu).

@instructions

Create a numpy array from height_in. Name this new array np_height_in.
Print np_height_in.
Multiply np_height_in with 0.0254 to convert all height measurements from inches to meters. Store the new values in a new array, np_height_m.
Print out np_height_m and check if the output makes sense.

@hint

Use np.array() and pass it height. Store the result in np_height_in.
To print out a variable x, type print(x) in the Python script.
Perform calculations as if np_height_in is a single number: np_height_in * conversion_factor is part of the answer.
To print out a variable x, type print(x) in the Python script.

@pre_exercise_code

import pandas as pd
mlb = pd.read_csv("https://assets.datacamp.com/course/intro_to_python/baseball.csv")
height_in = mlb['Height'].tolist()
import numpy as np

@sample_code

# Import numpy
import numpy as np

# Create a numpy array from height_in: np_height_in


# Print out np_height_in


# Convert np_height_in to m: np_height_m


# Print np_height_m

@solution

# Import numpy
import numpy as np

# Create a numpy array from height_in: np_height_in
np_height_in = np.array(height_in)

# Print out np_height_in
print(np_height_in)

# Convert np_height_in to m: np_height_m
np_height_m = np_height_in * 0.0254

# Print np_height_m
print(np_height_m)

@sct

Ex().has_import("numpy", same_as = False)

Ex().check_correct(
  has_printout(0),
  check_correct(
    check_object('np_height_in').has_equal_value(),
    check_function('numpy.array').check_args(0).has_equal_ast()
  )
)

Ex().check_correct(
  has_printout(1),
  check_object("np_height_m").has_equal_value(incorrect_msg = "Use `np_height_in * 0.0254` to calculate `np_height_m`.")
)

success_msg("Nice! In the blink of an eye, `numpy` performs multiplications on more than 1000 height measurements.")

NumPy Side Effects

type: MultipleChoiceExercise
key: 3662ff6637
lang: python
xp: 50
skills:
  - 2

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

Some lines of code have been provided for you. Try these out and select the one that would match this:

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

The numpy package is already imported as np.

@possible_answers

np.array([True, 1, 2, 3, 4, False])
np.array([4, 3, 0]) + np.array([0, 2, 2])
np.array([1, 1, 2]) + np.array([3, 4, -1])
np.array([0, 1, 2, 3, 4, 5])

@hint

Copy the different code chunks and paste them in the IPython Shell. Hit enter to run the code and see which output matches the one generated by np.array([True, 1, 2]) + np.array([3, 4, False]).

@pre_exercise_code

import numpy as np

@sct

msg1 = msg3 = msg4 = "Incorrect. Try out the different code chunks and see which one matches the target code chunk."
msg2 = "Great job! `True` is converted to 1, `False` is converted to 0."
Ex().has_chosen(2, [msg1, msg2, msg3, msg4])

Subsetting NumPy Arrays

type: NormalExercise
key: fcb2a9007b
lang: python
xp: 100
skills:
  - 2

Subsetting (using the square bracket notation on lists or arrays) works exactly the same with both lists and arrays.

This exercise already has two lists, height_in and weight_lb, loaded in the background for you. These contain the height and weight of the MLB players as regular lists. It also has two numpy array lists, np_weight_lb and np_height_in prepared for you.

@instructions

Subset np_weight_lb by printing out the element at index 50.
Print out a sub-array of np_height_in that contains the elements at index 100 up to and including index 110.

@hint

Make sure to wrap a print() call around your subsetting operations.
Use [100:111] to get the elements from index 100 up to and including index 110.

@pre_exercise_code

import pandas as pd
mlb = pd.read_csv("https://assets.datacamp.com/course/intro_to_python/baseball.csv")
height_in = mlb['Height'].tolist()
weight_lb = mlb['Weight'].tolist()

@sample_code

import numpy as np

np_weight_lb = np.array(weight_lb)
np_height_in = np.array(height_in)

# Print out the weight at index 50


# Print out sub-array of np_height_in: index 100 up to and including index 110

@solution

import numpy as np

np_weight_lb = np.array(weight_lb)
np_height_in = np.array(height_in)

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

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

@sct

Ex().has_import("numpy", same_as=False)
msg = "You don't have to change or remove the predefined variables."
Ex().multi(
    check_object("np_height_in", missing_msg=msg).has_equal_value(incorrect_msg = msg),
    check_object("np_weight_lb", missing_msg=msg).has_equal_value(incorrect_msg = msg)
)

Ex().has_printout(0)
Ex().has_printout(1)

success_msg("Nice! Time to learn something new: 2D NumPy arrays!")

2D NumPy Arrays

type: VideoExercise
key: 1241efac7a
xp: 50

@projector_key ae3238dcc7feb9adecfee0c395fc8dc8

Your First 2D NumPy Array

type: NormalExercise
key: 5cb045bb13
lang: python
xp: 100
skills:
  - 2

Before working on the actual MLB data, let's try to create a 2D numpy array from a small list of lists.

In this exercise, baseball is a list of lists. The main list contains 4 elements. Each of these elements is a list containing the height and the weight of 4 baseball players, in this order. baseball is already coded for you in the script.

@instructions

Use np.array() to create a 2D numpy array from baseball. Name it np_baseball.
Print out the type of np_baseball.
Print out the shape attribute of np_baseball. Use np_baseball.shape.

@hint

baseball is already coded for you in the script. Call np.array() on it and store the resulting 2D numpy array in np_baseball.
Use print() in combination with type() for the second instruction.
np_baseball.shape will give you the dimensions of the np_baseball. Make sure to wrap a print() call around it.

@pre_exercise_code

@sample_code

import numpy as np

baseball = [[180, 78.4],
            [215, 102.7],
            [210, 98.5],
            [188, 75.2]]

# Create a 2D numpy array from baseball: np_baseball


# Print out the type of np_baseball


# Print out the shape of np_baseball

@solution

import numpy as np

baseball = [[180, 78.4],
            [215, 102.7],
            [210, 98.5],
            [188, 75.2]]

# Create a 2D numpy array from baseball: np_baseball
np_baseball = np.array(baseball)

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

# Print out the shape of np_baseball
print(np_baseball.shape)

@sct

msg = "You don't have to change or remove the predefined variables."
Ex().check_object("baseball", missing_msg=msg).has_equal_value(incorrect_msg = msg)
Ex().has_import("numpy", same_as = False)

Ex().check_correct(
    multi(
        has_printout(0),
        has_printout(1)
    ),
    check_correct(
        check_object('np_baseball').has_equal_value(),
        check_function('numpy.array').check_args(0).has_equal_ast()
    )
)

success_msg("Great! You're ready to convert the actual MLB data to a 2D `numpy` array now!")

Baseball data in 2D form

type: NormalExercise
key: 5df25d0b7b
lang: python
xp: 100
skills:
  - 2

You realize that it makes more sense to restructure all this information in a 2D numpy array.

You have a Python list of lists. In this list of lists, each sublist represents the height and weight of a single baseball player. The name of this list is baseball and it has been loaded for you already (although you can't see it).

Store the data as a 2D array to unlock numpy's extra functionality.

@instructions

Use np.array() to create a 2D numpy array from baseball. Name it np_baseball.
Print out the shape attribute of np_baseball.

@hint

baseball is already available in the Python environment. Call np.array() on it and store the resulting 2D numpy array in np_baseball.
np_baseball.shape will give the dimensions of the np_baseball. Make sure to wrap a print()call around it.

@pre_exercise_code

import pandas as pd
baseball = pd.read_csv("https://assets.datacamp.com/course/intro_to_python/baseball.csv")[['Height', 'Weight']].to_numpy().tolist()
import numpy as np

@sample_code

import numpy as np

# Create a 2D numpy array from baseball: np_baseball
np_baseball = 

# Print out the shape of np_baseball

@solution

import numpy as np

# Create a 2D numpy array from baseball: np_baseball
np_baseball = np.array(baseball)

# Print out the shape of np_baseball
print(np_baseball.shape)

@sct

Ex().has_import("numpy", same_as = False)

Ex().check_correct(
    has_printout(0),
    check_correct(
        check_object('np_baseball').has_equal_value(),
        check_function('numpy.array').check_args(0).has_equal_ast()
    )
)

success_msg("Slick! Time to show off some killer features of multi-dimensional `numpy` arrays!")

Subsetting 2D NumPy Arrays

type: NormalExercise
key: aeca4977f0
lang: python
xp: 100
skills:
  - 2

If your 2D numpy array has a regular structure, i.e. each row and column has a fixed number of values, complicated ways of subsetting become very easy. Have a look at the code below where the elements "a" and "c" are extracted from a list of lists.

# numpy
import numpy as np
np_x = np.array(x)
np_x[:, 0]

The indexes before the comma refer to the rows, while those after the comma refer to the columns. The : is for slicing; in this example, it tells Python to include all rows.

@instructions

Print out the 50th row of np_baseball.
Make a new variable, np_weight_lb, containing the entire second column of np_baseball.
Select the height (first column) of the 124th baseball player in np_baseball and print it out.

@hint

You need row index 49 in the first instruction! More specifically, you'll want to use [49, :].
To select the entire second column, you'll need [:, 1].
For the last instruction, use [123, 0]; don't forget to wrap it all in a print() statement.

@pre_exercise_code

import pandas as pd
baseball = pd.read_csv("https://assets.datacamp.com/course/intro_to_python/baseball.csv")[['Height', 'Weight']].to_numpy().tolist()
import numpy as np

@sample_code

import numpy as np

np_baseball = np.array(baseball)

# Print out the 50th row of np_baseball


# Select the entire second column of np_baseball: np_weight_lb


# Print out height of 124th player

@solution

import numpy as np

np_baseball = np.array(baseball)

# Print out the 50th row of np_baseball
print(np_baseball[49,:])

# Select the entire second column of np_baseball: np_weight_lb
np_weight_lb = np_baseball[:,1]

# Print out height of 124th player
print(np_baseball[123, 0])

@sct

msg = "You don't have to change or remove the predefined variables."
Ex().multi(
    has_import("numpy", same_as = False),
    check_object("np_baseball", missing_msg=msg).has_equal_value(incorrect_msg = msg)
)

Ex().has_printout(0)

Ex().check_object('np_weight_lb').has_equal_value(incorrect_msg = "You can use `np_baseball[:,1]` to define `np_weight_lb`. This will select the entire first column.")

Ex().has_printout(1)

success_msg("This is going well!")

2D Arithmetic

type: NormalExercise
key: 1c2378b677
lang: python
xp: 100
skills:
  - 2

2D numpy arrays can perform calculations element by element, like numpy arrays.

np_baseball is coded for you; it's again a 2D numpy array with 3 columns representing height (in inches), weight (in pounds) and age (in years). baseball is available as a regular list of lists and updated is available as 2D numpy array.

@instructions

You managed to get hold of the changes in height, weight and age of all baseball players. It is available as a 2D numpy array, updated. Add np_baseball and updated and print out the result.
You want to convert the units of height and weight to metric (meters and kilograms, respectively). As a first step, create a numpy array with three values: 0.0254, 0.453592 and 1. Name this array conversion.
Multiply np_baseball with conversion and print out the result.

@hint

np_baseball + updated will do an element-wise summation of the two numpy arrays.
Create a numpy array with np.array(); the input is a regular Python list with three elements.
np_baseball * conversion will work, without extra work. Try out it! Make sure to wrap it in a print() call.

@pre_exercise_code

import pandas as pd
import numpy as np
baseball = pd.read_csv("https://assets.datacamp.com/course/intro_to_python/baseball.csv")[['Height', 'Weight', 'Age']].to_numpy().tolist()
n = len(baseball)
updated = np.array(pd.read_csv("https://assets.datacamp.com/course/intro_to_python/update.csv", header = None))
import numpy as np

@sample_code

import numpy as np

np_baseball = np.array(baseball)

# Print out addition of np_baseball and updated


# Create numpy array: conversion


# Print out product of np_baseball and conversion

@solution

import numpy as np

np_baseball = np.array(baseball)

# Print out addition of np_baseball and updated
print(np_baseball + updated)

# Create numpy array: conversion
conversion = np.array([0.0254, 0.453592, 1])

# Print out product of np_baseball and conversion
print(np_baseball * conversion)

@sct

Ex().has_import("numpy")

msg = "You don't have to change or remove the predefined variables."
Ex().check_object("np_baseball", missing_msg=msg).has_equal_value(incorrect_msg = msg)

Ex().has_printout(0)

Ex().check_correct(
    has_printout(1),
    check_correct(
        check_object('conversion').has_equal_value(),
        check_function('numpy.array', index = 1).check_args(0).has_equal_value()
    )    
)

success_msg("Great job! Notice how with very little code, you can change all values in your `numpy` data structure in a very specific way. This will be very useful in your future as a data scientist!")

NumPy: Basic Statistics

type: VideoExercise
key: 287995e488
xp: 50

@projector_key 34495ba457d74296794d2a122c9b6e19

Average versus median

type: NormalExercise
key: 509c588eb6
lang: python
xp: 100
skills:
  - 2

You now know how to use numpy functions to get a better feeling for your data.

The baseball data is available as a 2D numpy array with 3 columns (height, weight, age) and 1015 rows. The name of this numpy array is np_baseball. After restructuring the data, however, you notice that some height values are abnormally high. Follow the instructions and discover which summary statistic is best suited if you're dealing with so-called outliers. np_baseball is available.

@instructions

Create numpy array np_height_in that is equal to first column of np_baseball.
Print out the mean of np_height_in.
Print out the median of np_height_in.

@hint

Use 2D numpy subsetting: [:,0] is a part of the solution.
If numpy is imported as np, you can use np.mean() to get the mean of a NumPy array. Don't forget to throw in a print() call.
For the last instruction, use np.median().

@pre_exercise_code

import pandas as pd
np_baseball = pd.read_csv("https://assets.datacamp.com/course/intro_to_python/baseball.csv")[['Height', 'Weight', 'Age']].to_numpy()
np_baseball[slice(0, 1015, 50), 0] = np_baseball[slice(0, 1015, 50), 0]*1000
import numpy as np

@sample_code

import numpy as np

# Create np_height_in from np_baseball


# Print out the mean of np_height_in


# Print out the median of np_height_in

@solution

import numpy as np

# Create np_height_in from np_baseball
np_height_in = np_baseball[:,0]

# Print out the mean of np_height_in
print(np.mean(np_height_in))

# Print out the median of np_height_in
print(np.median(np_height_in))

@sct

Ex().has_import("numpy", same_as = False)

Ex().check_object("np_height_in").has_equal_value(incorrect_msg = "You can use `np_baseball[:,0]` to select the first column from `np_baseball`"),

Ex().check_correct(
    has_printout(0),
    check_function('numpy.mean').has_equal_value()
)

Ex().check_correct(
    has_printout(1),
    check_function('numpy.median').has_equal_value()
)

success_msg("An average height of 1586 inches, that doesn't sound right, does it? However, the median does not seem affected by the outliers: 74 inches makes perfect sense. It's always a good idea to check both the median and the mean, to get an idea about the overall distribution of the entire dataset.")

Explore the baseball data

type: NormalExercise
key: '4409948807'
lang: python
xp: 100
skills:
  - 2

Because the mean and median are so far apart, you decide to complain to the MLB. They find the error and send the corrected data over to you. It's again available as a 2D NumPy array np_baseball, with three columns.

The Python script in the editor already includes code to print out informative messages with the different summary statistics and numpy is already loaded as np. Can you finish the job? np_baseball is available.

@instructions

The code to print out the mean height is already included. Complete the code for the median height. Replace None with the correct code.
Use np.std() on the first column of np_baseball to calculate stddev. Replace None with the correct code.
Do big players tend to be heavier? Use np.corrcoef() to store the correlation between the first and second column of np_baseball in corr. Replace None with the correct code.

@hint

Use np.median() to calculate the median. Make sure to select to correct column first!
Subset the same column when calculating the standard deviation with np.std().
Use np_baseball[:, 0] and np_baseball[:, 1] to select the first and second columns; these are the inputs to np.corrcoef().

@pre_exercise_code

import pandas as pd
np_baseball = pd.read_csv("https://assets.datacamp.com/course/intro_to_python/baseball.csv")[['Height', 'Weight', 'Age']].to_numpy()
import numpy as np

@sample_code

avg = np.mean(np_baseball[:,0])
print("Average: " + str(avg))

# Print median height
med = ____
print("Median: " + str(med))

# Print out the standard deviation on height
stddev = ____
print("Standard Deviation: " + str(stddev))

# Print out correlation between first and second column
corr = ____
print("Correlation: " + str(corr))

@solution

avg = np.mean(np_baseball[:,0])
print("Average: " + str(avg))

# Print median height
med = np.median(np_baseball[:,0])
print("Median: " + str(med))

# Print out the standard deviation on height
stddev = np.std(np_baseball[:,0])
print("Standard Deviation: " + str(stddev))

# Print out correlation between first and second column
corr = np.corrcoef(np_baseball[:,0], np_baseball[:,1])
print("Correlation: " + str(corr))

@sct

msg = "You shouldn't change or remove the predefined `avg` variable."
Ex().check_object("avg", missing_msg=msg).has_equal_value(incorrect_msg=msg)

missing = "Have you used `np.median()` to calculate the median?"
incorrect = "To calculate `med`, pass the first column of `np_baseball` to `numpy.median()`. The example of `np.mean()` shows how it's done."
Ex().check_correct(
  check_object("med").has_equal_value(),
  check_function("numpy.median", index=0, missing_msg=missing).check_args(0).has_equal_value(incorrect_msg=incorrect)
)

missing = "Have you used `np.std()` to calculate the standard deviation?"
incorrect = "To calculate `stddev`, pass the first column of `np_baseball` to `numpy.std()`. The example of `np.mean()` shows how it's done."
Ex().check_correct(
  check_object("stddev").has_equal_value(),
  check_function("numpy.std", index=0, missing_msg=missing).check_args(0).has_equal_value(incorrect_msg=incorrect)
)

missing = "Have you used `np.corrcoef()` to calculate the correlation?"
incorrect1 = "To calculate `corr`, the first argument to `np.corrcoef()` should be the first column of `np_baseball`, similar to how did it before."
incorrect2 = "To calculate `corr`, the second argument to `np.corrcoef()` should be the second column of `np_baseball`. Instead of `[:,0]`, use `[:,1]` this time."
Ex().check_correct(
  check_object("corr").has_equal_value(),
  check_function("numpy.corrcoef", index=0, missing_msg=missing).multi(
    check_args(0, missing_msg=incorrect1).has_equal_value(incorrect_msg=incorrect1),
    check_args(1, missing_msg=incorrect2).has_equal_value(incorrect_msg=incorrect2)
  )
)

success_msg("Great! Time to use all of your new data science skills in the last exercise!")

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chapter4.md

chapter4.md

NumPy

Your First NumPy Array

Baseball players' height

NumPy Side Effects

Subsetting NumPy Arrays

2D NumPy Arrays

Your First 2D NumPy Array

Baseball data in 2D form

Subsetting 2D NumPy Arrays

2D Arithmetic

NumPy: Basic Statistics

Average versus median

Explore the baseball data

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chapter4.md

File metadata and controls

NumPy

Your First NumPy Array

Baseball players' height

NumPy Side Effects

Subsetting NumPy Arrays

2D NumPy Arrays

Your First 2D NumPy Array

Baseball data in 2D form

Subsetting 2D NumPy Arrays

2D Arithmetic

NumPy: Basic Statistics

Average versus median

Explore the baseball data