Python Example Method for Counting Number of Occurrences of List Elements

1. Introduction

When using Python, the following scenarios usually appear:

array = [1, 2, 3, 3, 2, 1, 0, 2]

Gets the number of occurrences of elements in array

For example, in the above list: 0 appears once, 1 appears twice, 2 appears three times, and 3 appears twice.

In this paper, several methods of obtaining the number of elements in Python are described. Click to get the complete code.

2. Methodology

There are many ways to get the number of occurrences of elements. Here I propose the following five methods for reference. The following code, the parameters passed in are

array = [1, 2, 3, 3, 2, 1, 0, 2]

2.1 Counter method

This method can quickly get the number of elements appearing in list, and can refer to the official documents

from collections import Counter
def counter(arr):
  return Counter(arr).most_common(2) #  Returns the two most frequently occurring numbers 

#  Results: [(2, 3), (1, 2)]

2.2 count in list, getting the number of occurrences of each element

def single_list(arr, target):
  return arr.count(target)

# target=2 Results: 3

2.3 count in list, getting the number of occurrences of all elements

Returns 1 dict

def all_list(arr):
  result = {}
  for i in set(arr):
    result[i] = arr.count(i)
  return result

#  Results: {0: 1, 1: 2, 2: 3, 3: 2}

2.4 Numpy Fancy Index, which gets the number of occurrences of each element

def single_np(arr, target):
  arr = np.array(arr)
  mask = (arr == target)
  arr_new = arr[mask]
  return arr_new.size

# target=2 Results: 3

2.5 Numpy Fancy Index, which gets the number of occurrences of all elements

Returns 1 dict

def all_np(arr):
  arr = np.array(arr)
  key = np.unique(arr)
  result = {}
  for k in key:
    mask = (arr == k)
    arr_new = arr[mask]
    v = arr_new.size
    result[k] = v
  return result

#  Results: {0: 1, 1: 2, 2: 3, 3: 2}

3. Summary

These are several methods that I summarized for Python to get the number of elements.

It is worth mentioning that all elements of list I use are integers

array = [1, 2, 3, 3, 2, 1, 0, 2]

If list contains other types of elements, such as

array = [1, 2, 3, 3, 2, 1, 'a', 'bc', 0.1]

In this case, when you need to get the number of occurrences of a or 1, the call form of the function in 2.4 should be: target='a' / target='1'