Guillaume Redoulès

Counting values in an array

  2. Python
  3. Basics

Using lists

If you want to count the number of occurences of an element in a list you can use the .count() function of the list object

arr = [1,2,3,3,4,5,3,6,7,7]
print(f'Array : {arr}\n')

print(f'The number 3 appears {arr.count(3)} times in the list')
print(f'The number 7 appears {arr.count(7)} times in the list')
print(f'The number 4 appears {arr.count(4)} times in the list')

Array : [1, 2, 3, 3, 4, 5, 3, 6, 7, 7]

The number 3 appears 3 times in the list
The number 7 appears 2 times in the list
The number 4 appears 1 times in the list

Using collections

you can get a dictonnary of the number of occurences of each elements in a list thanks to the collections object like this

import collections

collections.Counter(arr)

Counter({1: 1, 2: 1, 3: 3, 4: 1, 5: 1, 6: 1, 7: 2})

Using numpy

You can have a simular result with numpy by hacking the unique function 

import numpy as np
arr = np.array(arr)
unique, counts = np.unique(arr, return_counts=True)
dict(zip(unique, counts))

{1: 1, 2: 1, 3: 3, 4: 1, 5: 1, 6: 1, 7: 2}