Arrays - indexing and slicing

NumPy - The Basics

1 min read

Published Sep 22 2025

NumPyPython

Indexing → access a single element using its position.
Slicing → access a range of elements using start:stop:step.
Supports multi-dimensional arrays - [row, col] slicing for 2D or [depth, row, column] for 3D etc.
Negative indices count from the end (-1 is the last element).
Using : selects the entire axis.

1D Array Examples:

import numpy as np

arr = np.array([10, 20, 30, 40, 50])

# Positive Indexing

print("arr[2] =", arr[2])

# → 30

# Negative Indexing

print("arr[-3] =", arr[-3])

# → 30

# Slicing start:stop

print("arr[1:4] =", arr[1:4])

# → [20 30 40]

print("arr[-4:-1] =", arr[-4:-1])

# → [20 30 40]

# Slicing with step

print("arr[0:5:2] =", arr[0:5:2])

# → [10 30 50]

print("arr[-5:-1:2] =", arr[-5:-1:2])

# → [10 30]

# From index to end

print("arr[2:] =", arr[2:])

# → [30 40 50]

print("arr[-3:] =", arr[-3:])

# → [30 40 50]

# From start to index

print("arr[:3] =", arr[:3])

# → [10 20 30]

print("arr[:-2] =", arr[:-2])

# → [10 20 30]

2D Array Examples:

arr2d = np.array([[1, 2, 3],

[4, 5, 6],

[7, 8, 9]])

# Single element

print("arr2d[1,2] =", arr2d[1,2])

# → 6

print("arr2d[-2,-1] =", arr2d[-2,-1])

# → 6

# Row slicing

print("arr2d[1,:] =", arr2d[1,:])

# → [4 5 6]

print("arr2d[-2,:] =", arr2d[-2,:])

# → [4 5 6]

# Column slicing

print("arr2d[:,2] =", arr2d[:,2])

# → [3 6 9]

print("arr2d[:,-1] =", arr2d[:,-1])

# → [3 6 9]

# Subarray

print("arr2d[0:2,1:3] =", arr2d[0:2,1:3])

# → [[2 3]

# [5 6]]

print("arr2d[-3:-1,-2:] =", arr2d[-3:-1,-2:])

# → [[2 3]

# [5 6]]

# Step in slicing

print("arr2d[::2,::2] =", arr2d[::2,::2])

# → [[1 3]

# [7 9]]

3D Array Examples:

import numpy as np

arr3d = np.array([

[[ 1, 2, 3], [ 4, 5, 6]],

[[ 7, 8, 9], [10, 11, 12]],

[[13, 14, 15], [16, 17, 18]]

])

# Accessing a single element

print("Element at depth 1, row 0, col 2:", arr3d[1,0,2])

# → 9

print("Element with negative indexing:", arr3d[-2,0,-1])

# → 9

# Accessing a whole “layer” (2D slice)

print("Layer 0:\n", arr3d[0])

# → [[1 2 3]

# [4 5 6]]

print("Last layer with negative indexing:\n", arr3d[-1])

# → [[13 14 15]

# [16 17 18]]

# Accessing a row across layers

print("First row across all layers:\n", arr3d[:,0,:])

# → [[ 1 2 3]

# [ 7 8 9]

# [13 14 15]]

# Accessing a column across layers

print("Second column across all layers:\n", arr3d[:,:,1])

# → [[ 2 5]

# [ 8 11]

# [14 17]]

# Subarray / slicing with steps

print("Every other layer, all rows, last two columns:\n", arr3d[::2, :, 1:])

# → [[[ 2 3]

# [ 5 6]]

# [[14 15]

# [17 18]]]

Arrays - indexing and slicing

NumPy - The Basics

1 min read

Published Sep 22 2025

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