Guided Notes

Indexing & Slicing

Accessing elements, negative indexes, 2-D indexing, slices with steps, and selecting rows and columns.

Indexing 1-D arrays

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Access an element by its index in square brackets. Like Python lists, NumPy counts from 0:

arr = np.array([10, 20, 30, 40]) arr[0] → 10 arr[2] + arr[3] → 70

Negative indexes count from the end: arr[-1] is 40, the last element.

Indexes past the end raise an IndexError — an array of 4 elements has valid indexes 0–3 (or −1 to −4).