How to use NumPy Array Slicing?

Question

Anubhav Kumar · Answer

What is Slicing?
Slicing lets you extract a 
portion of an array — like substring slicing in Python.
Syntax:
array[start : end : step]
start → index to begin (inclusive)end → index to stop (exclusive)step → how many to skip
1. Slicing a 1-D Array
Example:
import numpy as np
arr = np.array([10, 20, 30, 40, 50, 60])
a) Basic slicing
print(arr[1:4])
Output:
[20 30 40]
b) Without start
print(arr[:3])
Output:
[10 20 30]
c) Without end
print(arr[3:])
Output:
[40 50 60]
d) With step
print(arr[0:6:2])
Output:
[10 30 50]
e) Reverse slicing
print(arr[::-1])
Output:
[60 50 40 30 20 10]
2. Slicing a 2-D Array
arr2 = np.array([[10, 20, 30],
                 [40, 50, 60],
                 [70, 80, 90]])
a) Slice specific row
print(arr2[1, :])   # Entire 2nd row
Output:
[40 50 60]
b) Slice specific column
print(arr2[:, 1])   # Entire 2nd column
Output:
[20 50 80]
c) Slice a sub-matrix
print(arr2[0:2, 1:3])
Output:
[[20 30]
 [50 60]]
This extracts rows 0–1 and columns 1–2.
3. Slicing With Steps (2D)
Example: Every 2nd row and column
print(arr2[::2, ::2])
Output:
[[10 30]
 [70 90]]
4. Slicing Higher-Dimensional Arrays (3D)
If you have a 3D array (like an image):
arr3 = np.arange(27).reshape(3, 3, 3)
Get 1st "layer":
print(arr3[0, :, :])
Get middle column of every layer:
print(arr3[:, :, 1])
Summary Cheatsheet
Operation
Example
Meaning
a[start:end]
a[1:4]
From 1 to 3
a[:end]
a[:3]
From 0 to 2
a[start:]
a[2:]
From 2 to end
a[::step]
a[::2]
Every 2nd element
a[::-1]
Reverse array
and 
a[row, col]
a[1,2]
Element at row 1 col 2
a[rows, cols]
a[0:2, 1:3]
Sub-matrix
and

Operation	Example	Meaning
`a[start:end]`	`a[1:4]`	From 1 to 3
`a[:end]`	`a[:3]`	From 0 to 2
`a[start:]`	`a[2:]`	From 2 to end
`a[::step]`	`a[::2]`	Every 2nd element
`a[::-1]`	Reverse array
`a[row, col]`	`a[1,2]`	Element at row 1 col 2
`a[rows, cols]`	`a[0:2, 1:3]`	Sub-matrix

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How to use NumPy Array Slicing?

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1 Answers

1. Slicing a 1-D Array

Example:

a) Basic slicing

b) Without start

c) Without end

d) With step

e) Reverse slicing

2. Slicing a 2-D Array

a) Slice specific row

b) Slice specific column

c) Slice a sub-matrix

3. Slicing With Steps (2D)

Example: Every 2nd row and column

4. Slicing Higher-Dimensional Arrays (3D)

Summary Cheatsheet

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