5 Best Ways to Convert a Python Set of Strings to a List of Lists

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πŸ’‘ Problem Formulation: In Python, converting data structures is a common task that can optimize storage and improve accessibility of the data. Specifically, developers might encounter the need to transform a set of strings into a list of lists, where each string from the set becomes a list within a list. For instance, given a set {'apple', 'banana', 'cherry'}, the desired output would be [['apple'], ['banana'], ['cherry']].

Method 1: Using a List Comprehension

A list comprehension in Python is a concise way to generate lists. It can iterate over each element of a set and create inner lists for each element. This approach is often preferred for its readability and speed.

Here’s an example:

fruits_set = {'apple', 'banana', 'cherry'}
fruits_ll = [[fruit] for fruit in fruits_set]

The output will be:

[['banana'], ['apple'], ['cherry']]

This code snippet uses a list comprehension to iterate over each element in the set fruits_set, wrapping each string in a new list and then including those lists in the outer list fruits_ll.

Method 2: Using the map() Function

The map() function allows for the application of a function to every item in an iterable, such as a set. When used with a lambda function that simply wraps an item in a list, map() can effectively convert a set of strings to a list of lists.

Here’s an example:

fruits_set = {'apple', 'banana', 'cherry'}
fruits_ll = list(map(lambda x: [x], fruits_set))

The output will be:

[['banana'], ['apple'], ['cherry']]

In this example, we map a lambda function over the fruits_set, which creates a new list for each string. We then convert the map object to a list to obtain fruits_ll.

Method 3: For Loop Append

The for loop is a fundamental control flow statement that can be used to iterate over each element in a set and append a single-item list of that element to a new list.

Here’s an example:

fruits_set = {'apple', 'banana', 'cherry'}
fruits_ll = []
for fruit in fruits_set:
    fruits_ll.append([fruit])

The output will be:

[['banana'], ['apple'], ['cherry']]

Here, we initialize an empty list fruits_ll and loop over the set fruits_set, appending a new list containing the current fruit to fruits_ll.

Method 4: Using a Generator Expression with list()

A generator expression is similar to a list comprehension but is more memory efficient. When passed to the list() function, it can generate the list of lists on-the-fly without creating an intermediate list in memory.

Here’s an example:

fruits_set = {'apple', 'banana', 'cherry'}
fruits_ll = list([fruit] for fruit in fruits_set)

The output will be:

[['banana'], ['apple'], ['cherry']]

The generator expression ([fruit] for fruit in fruits_set) is passed to the list() function, which consumes the generator to form fruits_ll.

Bonus One-Liner Method 5: Using the * Operator

Python’s unpacking operator * can be used within a list literal to unpack elements of an iterable into a new list structure where each item is again a list.

Here’s an example:

fruits_set = {'apple', 'banana', 'cherry'}
fruits_ll = [[*fruits_set]]

The output will be:

[['banana', 'apple', 'cherry']]

Note that this method does not create a list of lists in the same way as the other methodsβ€”instead, it creates a list that contains a single list with all the elements of the set. This might not be suitable if individual lists are required for each element.

Summary/Discussion

  • Method 1: List Comprehension. Neat and pythonic. Very readable. However, may not be the most memory-efficient for large sets.
  • Method 2: map() Function. Functional programming style. Can be less readable to those unfamiliar with lambda functions. Efficiently handled by Python internally.
  • Method 3: For Loop Append. Explicit and clear on what is happening. Can be slightly more verbose. Easy to understand for beginners.
  • Method 4: Generator Expression. Memory efficient for large datasets. Slightly less readable due to it being less common than a list comprehension.
  • Method 5: Unpacking Operator * One-liner. Not useful if separate sublists for each string are needed; may require further manipulation to achieve the desired structure.