Counting array elements in Python

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Top 5 Answer for Counting array elements in Python

vote vote

94

The method len() returns the number of elements in the list.

Syntax:

len(myArray) 

Eg:

myArray = [1, 2, 3] len(myArray) 

Output:

3 

vote vote

90

len is a built-in function that calls the given container object's __len__ member function to get the number of elements in the object.

Functions encased with double underscores are usually "special methods" implementing one of the standard interfaces in Python (container, number, etc). Special methods are used via syntactic sugar (object creation, container indexing and slicing, attribute access, built-in functions, etc.).

Using obj.__len__() wouldn't be the correct way of using the special method, but I don't see why the others were modded down so much.

vote vote

80

If you have a multi-dimensional array, len() might not give you the value you are looking for. For instance:

import numpy as np a = np.arange(10).reshape(2, 5) print len(a) == 2 

This code block will return true, telling you the size of the array is 2. However, there are in fact 10 elements in this 2D array. In the case of multi-dimensional arrays, len() gives you the length of the first dimension of the array i.e.

import numpy as np len(a) == np.shape(a)[0] 

To get the number of elements in a multi-dimensional array of arbitrary shape:

import numpy as np size = 1 for dim in np.shape(a): size *= dim 
vote vote

60

Or,

myArray.__len__() 

if you want to be oopy; "len(myArray)" is a lot easier to type! :)

vote vote

54

Before I saw this, I thought to myself, "I need to make a way to do this!"

for tempVar in arrayName: tempVar+=1 

And then I thought, "There must be a simpler way to do this." and I was right.

len(arrayName)

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