5 Best Ways to Find the Intersection of Two Strings in Python

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πŸ’‘ Problem Formulation: This article discusses how to find the common characters between two strings in Pythonβ€”a situation you might encounter while comparing tags in metadata, searching for shared interests in user profiles, or looking at DNA sequence similarities. For instance, given the two strings "apple" and "ample", the desired output would be "aple".

Method 1: Using Set Intersection

The set intersection method entails converting both strings into sets and then using the intersection() method or & operator to retrieve common elements. As sets do not contain duplicates and are unordered, this method returns unique common characters without any specified order.

Here’s an example:

str1 = "apple"
str2 = "ample"
intersection = set(str1) & set(str2)
print("".join(intersection))

Output:

aple

This code snippet converts each string to a set, uses the intersection operator to find common elements, and then joins them back into a string. The output is the unordered set of characters that appear in both strings, resulting in "aple".

Method 2: Using list comprehension

List comprehension in Python can be used for filtering common elements in two strings by iterating through one string and checking if each character is present in the other string, while also ensuring that each character is added only once to avoid duplicates.

Here’s an example:

str1 = "apple"
str2 = "ample"
intersection = ''.join([c for c in str1 if c in str2 and str1.count(c) == str2.count(c)])
print(intersection)

Output:

aple

This snippet iterates over str1 and includes a character in the result if it is also found in str2 and the number of occurrences in both strings match. The result is the string "aple", showing common characters with the same count in both input strings.

Method 3: Using filter function

The filter function applies a lambda function over an iterable, such as a string, to filter out only the elements that meet a certain condition. This method works well for finding common characters by applying the condition of the element being present in the other string.

Here’s an example:

str1 = "apple"
str2 = "ample"
intersection = "".join(filter(lambda c: c in str2, str1))
print(intersection)

Output:

aple

This example uses the filter() function with a lambda that returns the characters in str1 that are also in str2. It then joins them into a string, returning "aple" as the intersected characters.

Method 4: Using Counter from collections

The Counter class from the collections module can be used to create a frequency map of characters in each string, and then one can use the & operator between two Counters to get the intersection, considering the minimum count of characters present in both strings.

Here’s an example:

from collections import Counter
str1 = "apple"
str2 = "ample"
intersection = Counter(str1) & Counter(str2)
print("".join(intersection.elements()))

Output:

aple

In this snippet, Counter objects for both strings are created. The intersection operator finds the minimum count for common elements. The elements() method then iterates through the intersection, yielding the common characters, and the join() method returns the result "aple".

Bonus One-Liner Method 5: Functional Approach with Reduce

The reduce() function from the functools module is a powerful tool that can be used to apply a function cumulatively to an iterable’s items. When paired with set intersection, it provides an elegant one-liner for intersecting multiple strings.

Here’s an example:

from functools import reduce
str1 = "apple"
str2 = "ample"
intersection = reduce(lambda s1, s2: s1 & s2, (set(str1), set(str2)))
print("".join(intersection))

Output:

aple

This code applies the reduce() function to a tuple of sets created from str1 and str2, using the lambda function to perform the intersection, and the final chain of join() to convert the resulting set into a string, outputting "aple".

Summary/Discussion

  • Method 1: Set Intersection. Simple and concise. Does not preserve character order or count.
  • Method 2: List Comprehension. Extremely flexible. Can be verbose for complex conditions.
  • Method 3: Filter Function. Functional programming style. Less intuitive for beginners.
  • Method 4: Counter from collections. Accounts for character counts. Slightly more overhead.
  • Method 5: Functional Approach with Reduce. Elegant for multiple sets. Can be hard to read for those not familiar with functional programming.