NumPy Array Indexing and Slicing

Indexing

• NumPy arrays can be indexed using square brackets [] and integers.
• Indexing allows access to individual elements of an array.
• Indexing is 0-based, meaning the first element is at index 0.
• Example: arr[0] accesses the first element of the array arr.

Slicing Notation

• Slicing allows access to a subset of elements in an array.
• Slicing notation: arr[start:stop:step]
  • start: starting index (inclusive), default is 0.
  • stop: ending index (exclusive), default is the end of the array.
  • step: increment between elements, default is 1.
• Examples:
  • arr[1:5]: gets elements at indices 1, 2, 3, and 4.
  • arr[1:5:2]: gets elements at indices 1 and 3.
  • arr[:5]: gets elements at indices 0, 1, 2, 3, and 4.
  • arr[1:]: gets elements at indices 1 to the end of the array.

Advanced Indexing

• Advanced indexing allows access to elements using arrays of indices.
• Advanced indexing can be used to access elements in any dimension.
• Examples:
  • arr[[1, 3, 5]]: gets elements at indices 1, 3, and 5.
  • arr[[1, 3], [2, 4]]: gets elements at indices (1, 2) and (3, 4) in a 2D array.
• Boolean arrays can also be used for advanced indexing.
  • arr[arr > 5]: gets elements that are greater than 5.
  • arr[(arr > 5) & (arr < 10)]: gets elements that are greater than 5 and less than 10.

Indexing

• NumPy arrays utilize square brackets [] for indexing, enabling access to specific elements.
• Indexing starts at 0, where the first element is positioned at index 0.
• Example for accessing the first element: arr[0].

Slicing Notation

• Slicing is employed to retrieve a range or subset of elements from an array.
• Slicing syntax follows the format arr[start:stop:step]:
  • start: the index to begin from (inclusive), defaults to 0.
  • stop: the index to end at (exclusive), defaults to the end of the array.
  • step: the interval between indices, with a default value of 1.
• Examples of slicing:
  • arr[1:5] retrieves elements at indices 1 through 4.
  • arr[1:5:2] accesses elements at index 1 and 3.
  • arr[:5] extracts the first five elements (indices 0 to 4).
  • arr[1:] yields all elements from index 1 to the end.

Advanced Indexing

• Advanced indexing permits selection of elements using multiple indices arrays.
• Applicable to any dimension of arrays, enhancing flexibility.
• Examples of advanced indexing:
  • arr[[1, 3, 5]] collects elements from indices 1, 3, and 5.
  • arr[[1, 3], [2, 4]] selects elements from coordinates (1, 2) and (3, 4) in a 2D array.
• Boolean arrays can serve for advanced indexing:
  • arr[arr > 5] fetches elements exceeding the value of 5.
  • arr[(arr > 5) & (arr < 10)] retrieves elements between 5 and 10, both limits exclusive.