Array Indexing Basics
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Questions and Answers

What is the index of the first element in a Python array?

  • n-1
  • 0 (correct)
  • -1
  • 1

    • What is the purpose of negative indexing in Python arrays?

  • To access every other element in the array
  • To access elements from the end of the array (correct)
  • To access the middle element of the array
  • To access elements from the start of the array

    • What is the default step value in Python array slicing?

  • 1 (correct)
  • 0
  • n-1
  • 2

    • How do you access the element at the second row and third column of a 2D array arr?

    <p>arr[1, 2]</p> Signup and view all the answers

    What happens when you try to access an element in a Python array with an index that is out of bounds?

    <p>It raises an <code>IndexError</code></p> Signup and view all the answers

    What is the purpose of slicing in Python arrays?

    <p>To extract a subset of elements from the array</p> Signup and view all the answers

    What is the syntax for extracting every other element from the second to the sixth element of an array arr?

    <p>arr[1:6:2]</p> Signup and view all the answers

    What is the index of the last element in a Python array of length n?

    <p>n-1</p> Signup and view all the answers

    What does arr[1:5:2] return?

    <p>Elements from index 1 to 4 with a step of 2</p> Signup and view all the answers

    What does arr[:3] return?

    <p>Elements from index 0 to 2</p> Signup and view all the answers

    What does arr[:, 1] return?

    <p>The entire 2nd column</p> Signup and view all the answers

    What does arr[arr > 3] return?

    <p>Elements from arr where the corresponding element is greater than 3</p> Signup and view all the answers

    What does arr[::] return?

    <p>A copy of the entire array</p> Signup and view all the answers

    What does arr[0, :] return?

    <p>The entire 1st row</p> Signup and view all the answers

    What does arr[[True, False, True, False]] return?

    <p>Elements at indices where the corresponding boolean is True</p> Signup and view all the answers

    What does arr[1::2] return?

    <p>Every other element from the array starting from the 2nd element</p> Signup and view all the answers

    What does arr[[-1, -2, -3]] return?

    <p>The elements at indices -1, -2, and -3 from the array</p> Signup and view all the answers

    What does arr[0, 1:] return?

    <p>The entire 1st row of the array from the 2nd column</p> Signup and view all the answers

    Study Notes

    Array Indexing

    Basic Indexing

    • In Python, arrays are 0-indexed, meaning the first element has an index of 0.
    • Indexing starts from the left, with the first element having an index of 0, and the last element having an index of n-1, where n is the length of the array.

    Positive Indexing

    • Positive indexing is used to access elements from the start of the array.
    • Example: arr[0] accesses the first element of the array arr.

    Negative Indexing

    • Negative indexing is used to access elements from the end of the array.
    • Example: arr[-1] accesses the last element of the array arr.
    • Example: arr[-2] accesses the second last element of the array arr.

    Slicing

    • Slicing is used to extract a subset of elements from the array.
    • Syntax: arr[start:stop:step]
    • start: The index of the first element to include in the slice (inclusive).
    • stop: The index of the last element to include in the slice (exclusive).
    • step: The increment between elements (default is 1).
    • Example: arr[1:3] extracts the second and third elements of the array arr.
    • Example: arr[1:6:2] extracts every other element from the second to the sixth element of the array arr.

    Multi-Dimensional Arrays

    • In Python, multi-dimensional arrays are arrays of arrays.
    • Indexing for multi-dimensional arrays is done using comma-separated indices.
    • Example: arr[0, 1] accesses the element at the first row and second column of the 2D array arr.

    Out-of-Bounds Indexing

    • If an index is out of bounds (i.e., greater than or equal to the length of the array), a IndexError is raised.
    • Example: arr[10] raises an IndexError if the length of arr is less than 11.

    Array Indexing

    Basic Indexing

    • Arrays in Python are 0-indexed, meaning the first element has an index of 0.
    • Indexing starts from the left, with the first element having an index of 0, and the last element having an index of n-1, where n is the length of the array.

    Positive Indexing

    • Positive indexing is used to access elements from the start of the array.
    • The first element of an array can be accessed using arr[0].

    Negative Indexing

    • Negative indexing is used to access elements from the end of the array.
    • The last element of an array can be accessed using arr[-1].
    • The second last element of an array can be accessed using arr[-2].

    Slicing

    • Slicing is used to extract a subset of elements from the array.
    • The syntax for slicing is arr[start:stop:step].
    • The start parameter defines the index of the first element to include in the slice (inclusive).
    • The stop parameter defines the index of the last element to include in the slice (exclusive).
    • The step parameter defines the increment between elements (default is 1).
    • Example: arr[1:3] extracts the second and third elements of the array arr.
    • Example: arr[1:6:2] extracts every other element from the second to the sixth element of the array arr.

    Multi-Dimensional Arrays

    • In Python, multi-dimensional arrays are arrays of arrays.
    • Indexing for multi-dimensional arrays is done using comma-separated indices.
    • Example: arr[0, 1] accesses the element at the first row and second column of the 2D array arr.

    Out-of-Bounds Indexing

    • If an index is out of bounds (i.e., greater than or equal to the length of the array), a IndexError is raised.
    • Example: arr[10] raises an IndexError if the length of arr is less than 11.

    Array Indexing

    • Array indexing in Python uses square brackets [] and the index of the element.
    • There are two types of indexing: positive and negative.

    Positive and Negative Indexing

    • Positive indexing starts from 0 and increments by 1.
    • Negative indexing starts from -1 and decrements by 1.

    Slicing

    • Slicing extracts a subset of elements from an array using a colon :.
    • arr[start:stop] returns elements from start to stop-1.
    • arr[start:] returns elements from start to the end of the array.
    • arr[:stop] returns elements from the beginning to stop-1.
    • arr[:] returns a copy of the entire array.

    Indexing with Step

    • Indexing with step extracts every n-th element from an array using arr[start:stop:step].
    • arr[::2] returns every 2nd element from the beginning to the end.
    • arr[1::2] returns every 2nd element starting from the 2nd element.

    Multi-indexing

    Multi-dimensional Arrays

    • Multi-dimensional arrays can be indexed using commas , to separate indices.
    • arr[0, 1] returns the element at the 1st row and 2nd column.
    • arr[0, :] returns the entire 1st row.
    • arr[:, 1] returns the entire 2nd column.

    Boolean Array Indexing

    • Boolean array indexing uses a boolean array to select elements from another array.
    • arr[arr &gt; 5] returns elements from arr where the corresponding element in arr is greater than 5.

    Fancy Indexing

    • Fancy indexing uses an array of indices to select elements from another array.
    • arr[[0, 2, 4]] returns elements at indices 0, 2, and 4 from arr.
    • arr[[True, False, True]] returns elements from arr where the corresponding element in the boolean array is True.

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    Description

    Learn about the basics of array indexing in Python, including 0-indexing, positive and negative indexing, and how to access elements in an array.

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