basic slicing and advanced indexing in numpy
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Загружено: 2025-06-13
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Okay, let's dive into NumPy slicing and indexing. This is a fundamental aspect of working with NumPy arrays effectively, allowing you to access and manipulate specific parts of your data without unnecessary copying. We'll cover basic slicing, advanced indexing (including integer and boolean indexing), and discuss some important considerations.
*1. NumPy Arrays: A Quick Recap*
Before we get into slicing and indexing, let's just remind ourselves about NumPy arrays:
*Homogeneous Data:* NumPy arrays store elements of the same data type (e.g., all integers, all floats). This makes operations highly efficient.
*Multi-dimensional:* Arrays can have one or more dimensions (1D, 2D, 3D, etc.).
*`ndarray` object:* NumPy arrays are represented by the `ndarray` object.
*Shape:* An array's shape tells you the size of each dimension (e.g., `(3, 4)` for a 2D array with 3 rows and 4 columns).
*Indexing starts at 0:* Like Python lists, indexing begins at 0.
*2. Basic Slicing*
Basic slicing uses the colon (`:`) operator to select a range of elements. It's similar to slicing Python lists, but with added flexibility for multi-dimensional arrays.
*Syntax:* `array[start:stop:step]`
`start`: The index to start the slice (inclusive). If omitted, defaults to 0.
`stop`: The index to stop the slice (exclusive). If omitted, defaults to the end of the dimension.
`step`: The increment between elements in the slice. If omitted, defaults to 1.
*2.1 Slicing 1D Arrays*
*2.2 Slicing 2D Arrays*
When working with 2D (or higher-dimensional) arrays, you provide a slice for each dimension, separated by commas.
*Important Note: Views vs. Copies*
A crucial concept is that basic slicing in NumPy creates *views*, not copies. This means the sliced array shares the same data buffer as the original array. Modifying a slice will modify the original array.
If you need a true copy of the slice, use the `.copy()` method ...
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