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Questions and Answers
What is the default starting index in slicing notation?
What is the purpose of advanced indexing?
What is the result of the slice arr[1:5:2]?
How do you access the first element of a NumPy array arr?
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What is the result of the advanced indexing arr[[1, 3, 5]]?
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What is the purpose of boolean arrays in advanced indexing?
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Study Notes
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 arrayarr.
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.
-
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Description
Learn how to access and manipulate individual elements and subsets of NumPy arrays using indexing and slicing notation.
