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Ultimate NumPy
course content

Course Content

Ultimate NumPy

Ultimate NumPy

1. NumPy Basics
Introduction to NumPyBasic Array CreationCreating Higher Dimensional ArraysCreation Functions for 1D ArraysCreation Functions for 2D ArraysGeneral Array Creation FunctionsRandom Arrays
2. Indexing and Slicing
Basic IndexingMultidimensional IndexingSlicingSlicing in 2D ArraysInteger Array IndexingInteger Array Indexing in 2D ArraysBoolean IndexingMore about Comparisons Boolean Indexing in 2D ArraysAssigning Values to Indexed ElementsAssigning Values to Indexed Subarrays
3. Commonly used NumPy Functions
Sorting ArraysSorting 2D ArraysCopying ArraysReshaping ArraysFlattening ArraysArray Concatenation
4. Math with NumPy
BroadcastingBasic Mathematical OperationsStatistical OperationsBasic Linear Algebra with NumPy

bookSlicing

Slicing in Python refers to retrieving elements from one index to another within a sequence. In this chapter, however, we will focus on slicing in NumPy arrays.

Slicing in 1D Arrays

The general syntax for slicing in 1D arrays is as follows: array[start:end:step]. Let’s take a more detailed look at it:

  • start is the index at which to start slicing;
  • end is the index at which slicing ends (the index itself is not included);
  • step specifies the increments between the indices (default is 1).

Here is an example to clarify everything (purple squares represent the elements retrieved from slicing):

Note

Since we did not explicitly specify step, it defaults to a value of 1.

Here is the code for this example:


              
123456789101112

            
import numpy as np
array = np.array([5, 10, 2, 8, 9, 1, 0, 4])
print(f'Initial array: {array}')
print('-' * 47)
# Slicing from the element at index 2 to the element at index 4 exclusive
print(array[2:4])
print('-' * 47)
# Slicing from the first element to the element at index 5 exclusive
print(array[:5])
print('-' * 47)
# Slicing from the element at index 5 to the last element inclusive
print(array[5:])
copy

Omitting Start, End, and Step

As you can see, we can often omit the start, end, step, or even all of them at the same time. For example, step can be omitted when we want it to be equal to 1. start and end can be omitted in the following scenarios:

  1. Omitting start:
    • Slicing from the first element (step is positive);
    • Slicing from the last element (step is negative).
  2. Omitting end:
    • Slicing to the last element inclusive (step is positive);
    • Slicing to the first element inclusive (step is negative).

Let's take a look at a few more examples (the black arrow indicates that the elements are taken in reverse order):

Here is the corresponding code:


              
123456789101112131415

            
import numpy as np
array = np.array([5, 10, 2, 8, 9, 1, 0, 4])
print(f'Initial array: {array}')
print('-' * 47)
# Slicing from the first element to the last element inclusive with step=2
print(array[::2])
print('-' * 47)
# Slicing from the element at index 4 to the element at index 2 exclusive (step=-1)
print(array[4:2:-1])
print('-' * 47)
# Slicing from the last element to the first element inclusive (reversed array)
print(array[::-1])
print('-' * 47)
# Slicing from the first element to the last inclusive (the same as our array)
print(array[:])
copy

Task

You are analyzing the daily sales data of a small retail store. The sales for the past week are stored in a NumPy array, with each element representing the sales for a specific day. Create a slice of weekly_sales containing the sales data for every second day, starting from the second day (Tuesday), and store it in alternate_day_sales (use a positive index for start and do not specify end).

Switch to desktopSwitch to desktop for real-world practiceContinue from where you are using one of the options below
Everything was clear?

How can we improve it?

Thanks for your feedback!

Section 2. Chapter 3
toggle bottom row

bookSlicing

Slicing in Python refers to retrieving elements from one index to another within a sequence. In this chapter, however, we will focus on slicing in NumPy arrays.

Slicing in 1D Arrays

The general syntax for slicing in 1D arrays is as follows: array[start:end:step]. Let’s take a more detailed look at it:

  • start is the index at which to start slicing;
  • end is the index at which slicing ends (the index itself is not included);
  • step specifies the increments between the indices (default is 1).

Here is an example to clarify everything (purple squares represent the elements retrieved from slicing):

Note

Since we did not explicitly specify step, it defaults to a value of 1.

Here is the code for this example:


              
123456789101112

            
import numpy as np
array = np.array([5, 10, 2, 8, 9, 1, 0, 4])
print(f'Initial array: {array}')
print('-' * 47)
# Slicing from the element at index 2 to the element at index 4 exclusive
print(array[2:4])
print('-' * 47)
# Slicing from the first element to the element at index 5 exclusive
print(array[:5])
print('-' * 47)
# Slicing from the element at index 5 to the last element inclusive
print(array[5:])
copy

Omitting Start, End, and Step

As you can see, we can often omit the start, end, step, or even all of them at the same time. For example, step can be omitted when we want it to be equal to 1. start and end can be omitted in the following scenarios:

  1. Omitting start:
    • Slicing from the first element (step is positive);
    • Slicing from the last element (step is negative).
  2. Omitting end:
    • Slicing to the last element inclusive (step is positive);
    • Slicing to the first element inclusive (step is negative).

Let's take a look at a few more examples (the black arrow indicates that the elements are taken in reverse order):

Here is the corresponding code:


              
123456789101112131415

            
import numpy as np
array = np.array([5, 10, 2, 8, 9, 1, 0, 4])
print(f'Initial array: {array}')
print('-' * 47)
# Slicing from the first element to the last element inclusive with step=2
print(array[::2])
print('-' * 47)
# Slicing from the element at index 4 to the element at index 2 exclusive (step=-1)
print(array[4:2:-1])
print('-' * 47)
# Slicing from the last element to the first element inclusive (reversed array)
print(array[::-1])
print('-' * 47)
# Slicing from the first element to the last inclusive (the same as our array)
print(array[:])
copy

Task

You are analyzing the daily sales data of a small retail store. The sales for the past week are stored in a NumPy array, with each element representing the sales for a specific day. Create a slice of weekly_sales containing the sales data for every second day, starting from the second day (Tuesday), and store it in alternate_day_sales (use a positive index for start and do not specify end).

Switch to desktopSwitch to desktop for real-world practiceContinue from where you are using one of the options below
Everything was clear?

How can we improve it?

Thanks for your feedback!

Section 2. Chapter 3
toggle bottom row

bookSlicing

Slicing in Python refers to retrieving elements from one index to another within a sequence. In this chapter, however, we will focus on slicing in NumPy arrays.

Slicing in 1D Arrays

The general syntax for slicing in 1D arrays is as follows: array[start:end:step]. Let’s take a more detailed look at it:

  • start is the index at which to start slicing;
  • end is the index at which slicing ends (the index itself is not included);
  • step specifies the increments between the indices (default is 1).

Here is an example to clarify everything (purple squares represent the elements retrieved from slicing):

Note

Since we did not explicitly specify step, it defaults to a value of 1.

Here is the code for this example:


              
123456789101112

            
import numpy as np
array = np.array([5, 10, 2, 8, 9, 1, 0, 4])
print(f'Initial array: {array}')
print('-' * 47)
# Slicing from the element at index 2 to the element at index 4 exclusive
print(array[2:4])
print('-' * 47)
# Slicing from the first element to the element at index 5 exclusive
print(array[:5])
print('-' * 47)
# Slicing from the element at index 5 to the last element inclusive
print(array[5:])
copy

Omitting Start, End, and Step

As you can see, we can often omit the start, end, step, or even all of them at the same time. For example, step can be omitted when we want it to be equal to 1. start and end can be omitted in the following scenarios:

  1. Omitting start:
    • Slicing from the first element (step is positive);
    • Slicing from the last element (step is negative).
  2. Omitting end:
    • Slicing to the last element inclusive (step is positive);
    • Slicing to the first element inclusive (step is negative).

Let's take a look at a few more examples (the black arrow indicates that the elements are taken in reverse order):

Here is the corresponding code:


              
123456789101112131415

            
import numpy as np
array = np.array([5, 10, 2, 8, 9, 1, 0, 4])
print(f'Initial array: {array}')
print('-' * 47)
# Slicing from the first element to the last element inclusive with step=2
print(array[::2])
print('-' * 47)
# Slicing from the element at index 4 to the element at index 2 exclusive (step=-1)
print(array[4:2:-1])
print('-' * 47)
# Slicing from the last element to the first element inclusive (reversed array)
print(array[::-1])
print('-' * 47)
# Slicing from the first element to the last inclusive (the same as our array)
print(array[:])
copy

Task

You are analyzing the daily sales data of a small retail store. The sales for the past week are stored in a NumPy array, with each element representing the sales for a specific day. Create a slice of weekly_sales containing the sales data for every second day, starting from the second day (Tuesday), and store it in alternate_day_sales (use a positive index for start and do not specify end).

Switch to desktopSwitch to desktop for real-world practiceContinue from where you are using one of the options below
Everything was clear?

How can we improve it?

Thanks for your feedback!

Slicing in Python refers to retrieving elements from one index to another within a sequence. In this chapter, however, we will focus on slicing in NumPy arrays.

Slicing in 1D Arrays

The general syntax for slicing in 1D arrays is as follows: array[start:end:step]. Let’s take a more detailed look at it:

  • start is the index at which to start slicing;
  • end is the index at which slicing ends (the index itself is not included);
  • step specifies the increments between the indices (default is 1).

Here is an example to clarify everything (purple squares represent the elements retrieved from slicing):

Note

Since we did not explicitly specify step, it defaults to a value of 1.

Here is the code for this example:


              
123456789101112

            
import numpy as np
array = np.array([5, 10, 2, 8, 9, 1, 0, 4])
print(f'Initial array: {array}')
print('-' * 47)
# Slicing from the element at index 2 to the element at index 4 exclusive
print(array[2:4])
print('-' * 47)
# Slicing from the first element to the element at index 5 exclusive
print(array[:5])
print('-' * 47)
# Slicing from the element at index 5 to the last element inclusive
print(array[5:])
copy

Omitting Start, End, and Step

As you can see, we can often omit the start, end, step, or even all of them at the same time. For example, step can be omitted when we want it to be equal to 1. start and end can be omitted in the following scenarios:

  1. Omitting start:
    • Slicing from the first element (step is positive);
    • Slicing from the last element (step is negative).
  2. Omitting end:
    • Slicing to the last element inclusive (step is positive);
    • Slicing to the first element inclusive (step is negative).

Let's take a look at a few more examples (the black arrow indicates that the elements are taken in reverse order):

Here is the corresponding code:


              
123456789101112131415

            
import numpy as np
array = np.array([5, 10, 2, 8, 9, 1, 0, 4])
print(f'Initial array: {array}')
print('-' * 47)
# Slicing from the first element to the last element inclusive with step=2
print(array[::2])
print('-' * 47)
# Slicing from the element at index 4 to the element at index 2 exclusive (step=-1)
print(array[4:2:-1])
print('-' * 47)
# Slicing from the last element to the first element inclusive (reversed array)
print(array[::-1])
print('-' * 47)
# Slicing from the first element to the last inclusive (the same as our array)
print(array[:])
copy

Task

You are analyzing the daily sales data of a small retail store. The sales for the past week are stored in a NumPy array, with each element representing the sales for a specific day. Create a slice of weekly_sales containing the sales data for every second day, starting from the second day (Tuesday), and store it in alternate_day_sales (use a positive index for start and do not specify end).

Switch to desktopSwitch to desktop for real-world practiceContinue from where you are using one of the options below
Section 2. Chapter 3
Switch to desktopSwitch to desktop for real-world practiceContinue from where you are using one of the options below
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