5 Best Ways to Sort Strings in Custom Order Using Python

💡 Problem Formulation: Sorting strings in a default lexicographical order is straightforward in Python, however, sometimes the requirement is to sort the characters in a string based on a custom-defined sequence. For instance, given a custom order ‘dbca’ and input string ‘abcd’, the desired output is ‘dbca’.

Method 1: Using a Custom Function and sorted()

This method involves creating a custom function that returns a sorting key based on the index of characters in a custom order. It leverages Python’s sorted() function which sorts any iterable. The custom function maps each character to its custom order index, and sorted() uses these indices to perform the sorting.

Here’s an example:

def custom_order_sort(s, order):
    order_index = {char: index for index, char in enumerate(order)}
    return ''.join(sorted(s, key=lambda x: order_index.get(x, 26)))

s = 'abcd'
order = 'dbca'
print(custom_order_sort(s, order))

Output: dbca

The code snippet defines a function custom_order_sort which takes a string and a custom order string. An order index dictionary is created for fast lookup. This dictionary maps characters to their respective index in the custom order. The sorted() method uses a lambda function that returns the index of each character as per this custom order, hence sorting the string accordingly.

Method 2: Using the key Parameter with a List

This technique utilizes a list that represents the order in which characters should be sorted. The key parameter of the sorted() function is used with a lambda function that references the character indices in the custom order list directly. This avoids the need for a dictionary lookup.

Here’s an example:

custom_order = ['d', 'b', 'c', 'a']
s = 'abcd'
sorted_string = ''.join(sorted(s, key=lambda x: custom_order.index(x)))

print(sorted_string)

Output: dbca

The example uses a list custom_order where the order of elements defines the custom sorting priority. The built-in sorted() function takes a string s and sorts it according to the indices of its characters in the custom_order list. The sorted characters are then joined to form the sorted string.

Method 3: Using a Comparison Function with functools.cmp_to_key

While Python 3 does not support the cmp parameter in its sorting functions, the functools.cmp_to_key function can be used to convert a comparison function into a key function. One defines a custom comparison function which compares elements based on their order in a custom order definition and passes this to the sorting function.

Here’s an example:

import functools

def compare_items(a, b, ordering):
    return ordering.index(a) - ordering.index(b)

s = 'abcd'
ordering = 'dbca'
sorted_string = ''.join(sorted(s, key=functools.cmp_to_key(lambda x,y: compare_items(x,y,ordering))))

print(sorted_string)

Output: dbca

This method relies on providing a custom comparison logic which is encapsulated in the compare_items function. It uses the cmp_to_key utility from the functools module to convert this comparison function into a key function which can be used by sorted(). It is then used to sort the string in the custom order defined.

Method 4: Using a Callable Class as Key Function

Another technique involves creating a callable class that upon instantiation, takes the custom order, and implements the __call__ method to provide sorting keys. This way, the class instance itself can be used as a key function when calling the sorted() function.

Here’s an example:

class OrderKey:
    def __init__(self, order):
        self.order = order

    def __call__(self, char):
        return self.order.index(char)

s = 'abcd'
custom_order = 'dbca'
order_key = OrderKey(custom_order)
sorted_string = ''.join(sorted(s, key=order_key))

print(sorted_string)

Output: dbca

In this code, the OrderKey class is created with a method __call__ that makes instances callable. An order_key object is created with the desired custom order. When sorting the string s, this object provides the key for each character based on its index in the custom order list.

Bonus One-Liner Method 5: Inline Lambda Expression

A one-liner version using a lambda expression directly in the sorted() function call can be used for simple sorting tasks, eliminating the need for auxiliary functions or classes. However, this approach is not optimal for long strings or multiple sorting operations, as it calculates the index repeatedly.

Here’s an example:

custom_order = 'dbca'
s = 'abcd'
sorted_string = ''.join(sorted(s, key=lambda x: custom_order.index(x)))

print(sorted_string)

Output: dbca

This example demonstrates a one-liner solution where the sorted() function takes a lambda function as the key. The lambda function looks up the index of each character directly in the custom order string. Note that this can be less efficient for large strings due to the repeated index method calls.

Summary/Discussion

• Method 1: Custom Function with sorted(). Flexible and efficient. Suitable for strings with a high number of sort operations. Can incur overhead in creating a lookup dictionary.
• Method 2: List Index Reference in the key Parameter. Simplistic and easy to understand. There is, however, a performance cost for the index lookup on each comparison.
• Method 3: Comparison Function with functools.cmp_to_key. Useful for complex sort conditions and compatibility with Python 2-style comparison functions. Can be more difficult to understand and maintain.
• Method 4: Callable Class as Key Function. Object-oriented approach. The class can encapsulate complex sorting logic. Might be overkill for simpler sort operations.
• Method 5: One-Liner Inline Lambda. Quick and dirty, best for simple and one-off sort operations. Not efficient for repeated use or on large datasets.