Python – Sort by Units Digit in a List

When sorting a list by its units digit (last digit), we can use a custom key function with the sort() method. This function converts each number to a string and extracts the last character using negative indexing.

Using a Custom Key Function

The most straightforward approach is to define a helper function that extracts the units digit ?

def unit_sort(element):
    return str(element)[-1]

numbers = [716, 134, 343, 24742]

print("The list is:")
print(numbers)

numbers.sort(key=unit_sort)

print("The result is:")
print(numbers)

The list is:
[716, 134, 343, 24742]
The result is:
[24742, 343, 134, 716]

Using Lambda Function

For a more concise approach, we can use a lambda function instead of defining a separate function ?

numbers = [716, 134, 343, 24742, 89, 105]

print("Original list:", numbers)

# Sort by units digit using lambda
numbers.sort(key=lambda x: str(x)[-1])

print("Sorted by units digit:", numbers)

Original list: [716, 134, 343, 24742, 89, 105]
Sorted by units digit: [343, 134, 105, 716, 24742, 89]

Using Modulo Operator

Alternatively, we can use the modulo operator to get the units digit mathematically ?

numbers = [716, 134, 343, 24742, 89, 105]

print("Original list:", numbers)

# Sort by units digit using modulo
numbers.sort(key=lambda x: x % 10)

print("Sorted by units digit:", numbers)

Original list: [716, 134, 343, 24742, 89, 105]
Sorted by units digit: [343, 134, 105, 716, 24742, 89]

How It Works

The sorting process works as follows:

• The sort() method uses the key parameter to determine sorting criteria

• For each element, the key function extracts the units digit

• String method: str(element)[-1] converts to string and gets the last character

• Modulo method: element % 10 gives the remainder when divided by 10

• Elements are sorted based on these extracted digits

Comparison

Conclusion

Use the modulo operator x % 10 for better performance, or string indexing str(x)[-1] for readability. Lambda functions provide a concise solution for one-time sorting operations.