How to Slice a NumPy Array: Syntax and Examples

To slice a numpy array, use the syntax array[start:stop:step] where start is the index to begin, stop is the index to end (exclusive), and step is the interval between elements. This lets you select parts of the array easily and efficiently.

Syntax

The basic syntax for slicing a NumPy array is array[start:stop:step].

• start: The index where the slice starts (inclusive). Defaults to 0 if omitted.
• stop: The index where the slice ends (exclusive). Defaults to the array length if omitted.
• step: The step size or interval between elements. Defaults to 1 if omitted.

You can omit any of these parts to use their default values.

python

import numpy as np

# Syntax pattern
array = np.array([0, 1, 2, 3, 4, 5])
slice_example = array[start:stop:step]

Example

This example shows how to slice a NumPy array to get different parts:

python

import numpy as np

arr = np.array([10, 20, 30, 40, 50, 60, 70])

# Slice from index 1 to 4 (4 excluded)
slice1 = arr[1:4]

# Slice from start to index 3
slice2 = arr[:3]

# Slice from index 3 to end
slice3 = arr[3:]

# Slice with step 2
slice4 = arr[::2]

print('Slice 1:', slice1)
print('Slice 2:', slice2)
print('Slice 3:', slice3)
print('Slice 4:', slice4)

Output

Slice 1: [20 30 40]
Slice 2: [10 20 30]
Slice 3: [40 50 60 70]
Slice 4: [10 30 50 70]

Common Pitfalls

Common mistakes when slicing NumPy arrays include:

• Using stop index that is out of range (NumPy handles this gracefully by stopping at the array end).
• Confusing inclusive and exclusive indices: stop is not included in the slice.
• Forgetting that negative indices count from the end of the array.
• Using a step of zero, which causes an error.

python

import numpy as np

arr = np.array([1, 2, 3, 4, 5])

# Wrong: step cannot be zero (raises error)
# slice_wrong = arr[::0]

# Right: negative step to reverse
slice_right = arr[::-1]

print('Reversed array:', slice_right)

Output

Reversed array: [5 4 3 2 1]

Quick Reference

Slice Syntax	Description	Example	Result
array[start:stop]	Elements from start to stop-1	arr[1:4]	[element1, element2, element3]
array[:stop]	Elements from start to stop-1	arr[:3]	[element0, element1, element2]
array[start:]	Elements from start to end	arr[3:]	[element3, element4, ...]
array[::step]	Elements with step interval	arr[::2]	[element0, element2, element4, ...]
array[::-1]	Reversed array	arr[::-1]	[last, ..., first]

Key Takeaways

Use array[start:stop:step] to slice NumPy arrays with clear start, stop, and step values.

Remember stop index is exclusive and negative indices count from the end.

Omitting start, stop, or step uses default values: 0, array length, and 1 respectively.

A step of zero is invalid and causes an error; use negative step to reverse arrays.

Slicing works on multi-dimensional arrays by specifying slices for each axis.