How to Check If a List Has a Certain Length in Python

Introduction

In this lab, you will learn how to check if a list has a certain length in Python. This involves understanding the concept of list length and utilizing the built-in len() function to determine the number of elements in a list.

You will practice using len() with lists containing different data types, such as numbers and strings, and then comparing the result with a desired length to validate data or perform conditional operations. This lab provides hands-on experience with fundamental list manipulation techniques in Python.

Skills Graph

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Learn About List Length

In this step, you will learn about the concept of list length in Python. Understanding how to determine the length of a list is fundamental for many programming tasks, such as iterating through lists, validating data, and performing calculations based on the number of elements.

A list's length is simply the number of elements it contains. Python provides a built-in function called len() that makes it easy to find the length of any list. Let's start by creating a simple list and then using len() to find its length.

First, open the VS Code editor in the LabEx environment. Create a new file named list_length.py in the ~/project directory.

## Create a list of numbers
numbers = [1, 2, 3, 4, 5]

## Use the len() function to find the length of the list
length = len(numbers)

## Print the length of the list
print(length)

Save the file. Now, execute the Python script using the python command in the terminal:

python ~/project/list_length.py

You should see the following output:

This output indicates that the list numbers contains 5 elements.

Now, let's try with a list of strings:

## Create a list of strings
fruits = ["apple", "banana", "cherry"]

## Use the len() function to find the length of the list
length = len(fruits)

## Print the length of the list
print(length)

Save the changes to list_length.py and run the script again:

python ~/project/list_length.py

The output should be:

As you can see, len() works with lists containing different types of data.

Use len() Function

In the previous step, you learned the basic concept of list length and how to use the len() function. In this step, we will explore more practical applications of the len() function with different types of lists and scenarios.

The len() function is versatile and can be used with lists containing various data types, including numbers, strings, and even other lists. Let's look at some examples to illustrate this.

Continue using the list_length.py file you created in the ~/project directory.

First, let's create a list containing a mix of data types:

## Create a list with mixed data types
mixed_list = [1, "hello", 3.14, True]

## Find the length of the mixed list
length = len(mixed_list)

## Print the length of the list
print(length)

Save the changes to list_length.py and run the script:

python ~/project/list_length.py

The output should be:

This shows that len() correctly counts the number of elements, regardless of their data type.

Now, let's try a list containing other lists (nested lists):

## Create a nested list
nested_list = [[1, 2], [3, 4, 5], [6]]

## Find the length of the nested list
length = len(nested_list)

## Print the length of the list
print(length)

Save the changes to list_length.py and run the script again:

python ~/project/list_length.py

The output should be:

In this case, len() counts the number of sublists within the nested_list. It doesn't count the individual elements within the sublists.

Finally, let's consider an empty list:

## Create an empty list
empty_list = []

## Find the length of the empty list
length = len(empty_list)

## Print the length of the list
print(length)

Save the changes to list_length.py and run the script:

python ~/project/list_length.py

The output should be:

An empty list has a length of 0.

These examples demonstrate the flexibility and utility of the len() function in determining the size of various lists in Python.

Compare with Desired Length

In this step, you will learn how to compare the length of a list with a desired length using conditional statements. This is a common task in programming when you need to validate data or perform different actions based on the size of a list.

We'll use the len() function in conjunction with if statements to check if a list's length matches a specific value.

Continue using the list_length.py file you've been working on in the ~/project directory.

Let's create a list and then check if its length is equal to a desired value:

## Create a list of names
names = ["Alice", "Bob", "Charlie"]

## Desired length
desired_length = 3

## Check if the length of the list is equal to the desired length
if len(names) == desired_length:
    print("The list has the desired length.")
else:
    print("The list does not have the desired length.")

Save the changes to list_length.py and run the script:

python ~/project/list_length.py

The output should be:

The list has the desired length.

Now, let's modify the list and see what happens:

## Create a list of names
names = ["Alice", "Bob"]

## Desired length
desired_length = 3

## Check if the length of the list is equal to the desired length
if len(names) == desired_length:
    print("The list has the desired length.")
else:
    print("The list does not have the desired length.")

Save the changes to list_length.py and run the script again:

python ~/project/list_length.py

The output should now be:

The list does not have the desired length.

You can also use other comparison operators, such as > (greater than), < (less than), >= (greater than or equal to), and <= (less than or equal to), to compare the length of the list with the desired length.

For example:

## Create a list of numbers
numbers = [1, 2, 3, 4]

## Minimum length required
min_length = 3

## Check if the length of the list is greater than or equal to the minimum length
if len(numbers) >= min_length:
    print("The list meets the minimum length requirement.")
else:
    print("The list does not meet the minimum length requirement.")

Save the changes to list_length.py and run the script:

python ~/project/list_length.py

The output should be:

The list meets the minimum length requirement.

This step demonstrates how to use the len() function in conjunction with conditional statements to compare the length of a list with a desired length, allowing you to perform different actions based on the size of the list.

Summary

In this lab, you learned about the concept of list length in Python and how to determine it using the len() function. You created Python scripts to find the length of lists containing numbers and strings, demonstrating the versatility of len().

The lab involved creating a list_length.py file in the ~/project directory, writing Python code to define lists and use len() to calculate their length, and then executing the script using the python command in the terminal to verify the output.