Indexing and Slicing Arrays

Indexing and slicing are the ways you reach into a NumPy array to read or change specific elements — indexing grabs a single value by its position, while slicing pulls out a whole range with a start:stop:step pattern.

Learn Indexing and Slicing Arrays in our free NumPy course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick…

Part of the free Numpy course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

In this lesson you'll select single elements, use negative indices, slice 1D and 2D arrays, carve out sub-grids, and assign new values straight into a slice.

A one-dimensional array works just like a Python list: each element has a position called an index , and indices start at 0 . Put the index in square brackets to read one value.

Negative indices count backwards from the end, so arr[-1] is the last element and arr[-2] is the second-to-last — handy when you don't know the length.

A slice selects a range of elements using start:stop:step . The start is included, the stop is excluded, and step controls how many you skip.

Expected output: [1 2 3] , [0 1 2] , [5 6 7] , [0 2 4 6] , and the reversed [7 6 5 4 3 2 1 0] .

For a two-dimensional array you give two indices: arr[row, col] . Use a single index like arr[1] to grab a whole row , and a colon like arr[:, 0] to grab a whole column (the colon means "all rows").

Expected output: 1 , 6 , [4 5 6] , [1 4 7] , and 9 .

Combine slices on both axes to cut out a rectangular sub-grid : arr[0:2, 1:3] takes rows 0–1 and columns 1–2. Because slices are views , you can also assign to them — arr[0:2] = 0 overwrites everything in those rows at once.

Expected output: the sub-grid [[2 3] [6 7]] , then [ 0 0 30 40 50] .

Replace each ___ so the program prints the last element, a middle slice, and the first column.

Expected output: 45 , [15 25 35] , [1 3] . (Answers: -1 , 4 , 0 .)

❌ IndexError: index 5 is out of bounds for axis 0 with size 5

You used an index that's past the end. A size-5 array has valid indices 0 through 4.

✅ Fix: use arr[-1] for the last element, or check arr.shape .

❌ Expected one column but got a single number

Writing arr[0] on a 2D array returns the whole first row , not a column.

✅ Fix: use arr[:, 0] — the colon keeps all rows and picks column 0.

From a 4×4 grid, slice out the inner 2×2 block and then zero out the top row.

Lesson 6 complete — you can navigate any array!

You can now index single elements, use negative indices, slice ranges with steps, reach into rows and columns of 2D arrays, carve out sub-grids, and assign values straight through a slice.

🚀 Up next: Boolean & Fancy Indexing — select elements by condition instead of position.

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