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

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

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[Tut] How to Count the Occurrences of a List Element - xSicKxBot - 04-16-2022

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!