[Tut] How to Count the Occurrences of a List Element

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How to Count the Occurrences of a List Element

In this article, you’ll learn how to count the occurrences of a selected List element in Python.

To make it more fun, we have the following running scenario:

A Teacher from Orchard Elementary would like a script created for the 4th-grade students called “Count-Me“. She would like this script to do the following:

• First, generate and display 10 random numbers on a single line.
  
  
• Next, generate and display one (1) random number to find.
  
  
• Prompt for the total occurrences found.
  
  
• Display a message validating the solution.
  

Question: How would we write the Python code to accomplish this task?

We can accomplish this task by one of the following options:

• Method 1: Use NumPy and
  Code:

  count()
  
  
  
• Method 2: Use operator
  Code:

  countOf()
  
  
• Method 3: Use a For Loop
  
  
• Method 4: Use a
  Code:

  Counter()

Preparation

Before any data manipulation can occur, one (1) new library will require installation.

• The NumPy library supports multi-dimensional arrays and matrices in addition to a collection of mathematical functions.
  

To install this library, navigate to an IDE terminal. At the command prompt (

Code:

$

), execute the code below. For the terminal used in this example, the command prompt is a dollar sign (

Code:

$

). Your terminal prompt may be different.

Code:

$ pip install numpy

Hit the

Code:

<Enter>

key on the keyboard to start the installation process.

If the installation was successful, a message displays in the terminal indicating the same.

Feel free to view the PyCharm installation guide for the required library.

• How to install NumPy on PyCharm
  

Add the following code to the top of each code snippet. This snippet will allow the code in this article to run error-free.

Code:

import numpy as np import random import operator from collections import Counter

 Note: The

Code:

counter

and

Code:

collections

libraries are built-in to Python and do not require installation.

Method 1: Use NumPy and count()

To count the total occurrences of an element inside a List, this example will use NumPyand the

Code:

count()

function.

Code:

the_list = list(np.random.choice(20, 20)) dup_num = the_list[random.randint(0, 19)] dup_count = the_list.count(dup_num) try: print(the_list) check = int(input(f'How man times does the number {dup_num} appear in the list? ')) if check == dup_count: print(f'Correct! The answer is {check}.') else: print(f'Sorry! Try again!') except ValueError: print(f'Incorrect value. Bye')

The previous code snippet performs the following steps:

• Our first line generates and saves 20 random numbers to
  Code:

  the_list
  .
  
  
• Next,
  Code:

  dup_num
  is created by generating and saving one (1) random number from
  Code:

  the_list
  .
  
  
• Finally, we determine how many occurrences of
  Code:

  dup_num
  were found using
  Code:

  count()
  .
  
  
• The result saves to
  Code:

  dup_count
  .
  

Inside the

Code:

try

statement,

Code:

the_list

is output to the terminal.

The user is prompted to enter the total number of occurrences. To confirm, the user presses the

Code:

<Enter>

key. The value entered is then compared to

Code:

dup_count

, and a message indicates the outcome.

 Note: Click here for details on the try/except statement.

Method 2: Use operator countOf()

To count the total occurrences of a specified element inside a List, this example will use the

Code:

countOf()

function.

Code:

the_list = [random.randrange(0, 20) for num in range(20)] dup_num = the_list[random.randint(0, 19)] dup_count = operator.countOf(the_list, dup_num) try: print(the_list) check = int(input(f'How man times does the number {dup_num} appear in the list? ')) if check == dup_count: print(f'Correct! The answer is {check}.') else: print(f'Sorry! Try again!') except ValueError: print(f'Incorrect value. Bye')

This code snippet performs the following steps:

• Our first line generates and saves 20 random numbers to
  Code:

  the_list
  .
  
  
• Next,
  Code:

  dup_num
  is created by generating and saving one (1) random number from
  Code:

  the_list
  .
  
  
• Finally, we determine how many occurrences of
  Code:

  dup_num
  were found using
  Code:

  operator.countOf()
  .
  
  
• The result saves to
  Code:

  dup_count
  .
  

Inside the

Code:

try

statement,

Code:

the_list

is output to the terminal.

The user is prompted to enter the total number of occurrences. To confirm, the user presses the

Code:

<Enter>

key.

The value entered is then compared to

Code:

dup_count

, and a message indicates the outcome.

Method 3: Use a For Loop

To count the total occurrences of a specified element inside a List, this example will use the For Loop.

Code:

the_list = [random.randrange(0, 20) for num in range(20)] dup_num = the_list[random.randint(0, 19)] dup_count = 0 for i in the_list: if i == dup_num: dup_count += 1 try: print(the_list) check = int(input(f'How man times does the number {dup_num} appear in the list? ')) if check == dup_count: print(f'Correct! The answer is {check}.') else: print(f'Sorry! Try again!') except ValueError: print(f'Incorrect value. Bye')

The previous code snippet performs the following steps:

• Our first line generates and saves 20 random numbers to
  Code:

  the_list
  .
  
  
• Next,
  Code:

  dup_num
  is created by generating and saving one (1) random number from
  Code:

  the_list
  .
  
  
• Finally, a For Loop is instantiated. Upon each Loop, the element is matched against
  Code:

  dup_num
  .
  
  
• If found,
  Code:

  dup_count
  is increased by one (1).
  

Inside the

Code:

try

statement,

Code:

the_list

is output to the terminal.

The user is prompted to enter the total number of occurrences. To confirm, the user presses the

Code:

<Enter>

key.

The value entered is then compared to

Code:

dup_count

, and a message indicates the outcome.

Method 4: Counter()

To count the total occurrences of a specified element inside a List, this example will use the

Code:

Counter()

initializer method.

Code:

the_list = [random.randrange(0, 20) for num in range(20)] dup_num = the_list[random.randint(0, 19)] d = Counter(the_list) dup_count = d[dup_num] try: print(the_list) check = int(input(f'How man times does the number {dup_num} appear in the list? ')) if check == dup_count: print(f'Correct! The answer is {check}.') else: print(f'Sorry! Try again!') except ValueError: print(f'Incorrect value. Bye')

The previous code snippet performs the following steps:

• Our first line generates and saves 20 random numbers to
  Code:

  the_list
  .
  
  
• Next,
  Code:

  dup_num
  is created by generating and saving one (1) random number from
  Code:

  the_list
  .
  
  
• Finally, a For Loop is instantiated. Upon each Loop, an element is matched against
  Code:

  dup_num
  .
  
  
• If found,
  Code:

  dup_count
  is increased by one (1).
  

Inside the

Code:

try

statement,

Code:

the_list

is output to the terminal.

The user is prompted to enter the total number of occurrences. To confirm, the user presses the

Code:

<Enter>

key.

The value entered is then compared to

Code:

dup_count

, and a message indicates the outcome.

Summary

These four (4) methods of counting occurrences of a specified element inside a List should give you enough information to select the best one for your coding requirements.

Good Luck & Happy Coding!