How to Find Length of Array in NumPy: Simple Guide

To find the length of a NumPy array, use the len() function which returns the size of the first dimension. Alternatively, use array.size for total elements or array.shape to see all dimensions.

Syntax

The main ways to find the length of a NumPy array are:

• len(array): Returns the length of the first dimension (number of rows for 2D arrays).
• array.size: Returns the total number of elements in the array.
• array.shape: Returns a tuple showing the size of each dimension.

python

len(array)
array.size
array.shape

Example

This example shows how to find the length of a 1D and 2D NumPy array using len(), size, and shape.

python

import numpy as np

# 1D array
arr1 = np.array([10, 20, 30, 40])
print('Length of arr1:', len(arr1))  # 4
print('Total elements in arr1:', arr1.size)  # 4
print('Shape of arr1:', arr1.shape)  # (4,)

# 2D array
arr2 = np.array([[1, 2, 3], [4, 5, 6]])
print('Length of arr2:', len(arr2))  # 2 (number of rows)
print('Total elements in arr2:', arr2.size)  # 6
print('Shape of arr2:', arr2.shape)  # (2, 3)

Output

Length of arr1: 4
Total elements in arr1: 4
Shape of arr1: (4,)
Length of arr2: 2
Total elements in arr2: 6
Shape of arr2: (2, 3)

Common Pitfalls

Many beginners expect len() to give the total number of elements in multi-dimensional arrays, but it only returns the size of the first dimension. To get total elements, use array.size. Also, array.shape helps understand the array's dimensions.

python

import numpy as np

arr = np.array([[1, 2], [3, 4], [5, 6]])

# Wrong: expecting total elements
wrong_length = len(arr)  # returns 3, number of rows

# Right: total elements
correct_length = arr.size  # returns 6

print(f'Wrong length (len): {wrong_length}')
print(f'Correct total elements (size): {correct_length}')

Output

Wrong length (len): 3
Correct total elements (size): 6

Quick Reference

Method	Description	Example Output
len(array)	Length of first dimension	len(np.array([[1,2],[3,4]])) → 2
array.size	Total number of elements	np.array([[1,2],[3,4]]).size → 4
array.shape	Tuple of dimensions	np.array([[1,2],[3,4]]).shape → (2, 2)

Key Takeaways

Use len() to get the size of the first dimension of a NumPy array.

Use array.size to find the total number of elements in the array.

Use array.shape to understand the dimensions of the array.

Remember that len() does not give total elements for multi-dimensional arrays.

Check array dimensions with shape before deciding which length method to use.