5 Best Ways to Check if a String Can Be Formed from Another String Using Given Constraints in Python

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πŸ’‘ Problem Formulation: Imagine you have two strings: ‘source’ and ‘target’. The task is to determine whether the ‘target’ string can be formed from the characters present in the ‘source’ string, following certain constraints such as character count, order preservation, etc. For example, given a ‘source’ of “aabbcc” and a ‘target’ of “abc”, we want our function to return True, indicating that ‘target’ can indeed be created from ‘source’.

Method 1: Using Collection Counter

This method employs the collections.Counter class from Python’s standard library to compare the frequency of each character in both strings. If the ‘target’ string’s character frequencies are all lesser than or equal to those in the ‘source’ string, it implies that one can form the ‘target’ from the ‘source’.

Here’s an example:

from collections import Counter

def can_form_from_source(source, target):
    source_counter = Counter(source)
    target_counter = Counter(target)
    return all(target_counter[char] <= source_counter[char] for char in target_counter)

print(can_form_from_source('aabbcc', 'abc'))

Output: True

The function can_form_from_source uses Counter to count character occurrences in each string, then compares these counts to determine if ‘target’ can be formed from ‘source’. The all() function assures all characters in ‘target’ meet the criteria.

Method 2: Sorting and Iterative Comparison

This technique involves sorting both strings and then iteratively comparing each character from the ‘target’ string to the ‘source’ string characters. This method assumes that the ‘target’ characters must appear in the same order in the ‘source’.

Here’s an example:

def can_form_from_source_sorted(source, target):
    source_sorted, target_sorted = sorted(source), sorted(target)
    iter_source = iter(source_sorted)
    return all(char in iter_source for char in target_sorted)

print(can_form_from_source_sorted('aabbcc', 'bca'))

Output: True

By sorting both strings, we then create an iterator for the sorted ‘source’ and use a generator expression to verify each character in the sorted ‘target’ is present in the ‘source’ iterator. The function returns True if ‘target’ can be formed from ‘source’.

Method 3: Using a Dictionary

A dictionary can be used to count character occurrences in the ‘source’ string. The function then loops over the ‘target’ string, decrementing the count for each character. If any character’s count drops below zero, the function will return False.

Here’s an example:

def can_form_from_source_dict(source, target):
    source_dict = {}
    for char in source:
        source_dict[char] = source_dict.get(char, 0) + 1
    for char in target:
        if char not in source_dict or source_dict[char] == 0:
            return False
        source_dict[char] -= 1
    return True

print(can_form_from_source_dict('aabbcc', 'bcab'))

Output: True

The function can_form_from_source_dict creates a dictionary to track characters counts from ‘source’. It validates character availability while traversing ‘target’, ensuring the ‘target’ can be formed.

Method 4: Linear Search

Using a linear search, we can iterate over the ‘source’ string only once to check for the presence and correct sequence of ‘target’ characters.

Here’s an example:

def can_form_from_source_linear(source, target):
    it = iter(source)
    return all(char in it for char in target)

print(can_form_from_source_linear('aabbcc', 'bca'))

Output: True

This function can_form_from_source_linear utilizes an iterator over the β€˜source’ string and verifies that each ‘target’ characters appear sequentially within the source iterator. This checks both availability and order without sorting.

Bonus One-Liner Method 5: Using Set and Subtraction

A concise one-liner that leverages set subtraction to ensure all ‘target’ characters are in the ‘source’. Doesn’t account for character counts or order.

Here’s an example:

can_form_from_source_set = lambda s, t: not set(t) - set(s)

print(can_form_from_source_set('aabbcc', 'bca'))

Output: True

The one-liner function can_form_from_source_set uses set subtraction to ascertain that the ‘source’ contains all characters in the ‘target’. However, it does not consider the frequency of characters or their order.


  • Method 1: Collection Counter. Efficient in comparison of character counts. Handles duplicate characters effectively. Relies on Counter object overhead.
  • Method 2: Sorting and Iterative Comparison. Ensures character order. Potentially inefficient due to sorting. Simple concept easy to understand.
  • Method 3: Using a Dictionary. Effective custom implementation of counting. Allows for checking order. More code than using Counter.
  • Method 4: Linear Search. Fast for ordered character checking. Avoids sorting. Not effective when ‘target’ characters have higher counts than in ‘source’.
  • Bonus Method 5: Set and Subtraction. Quick and concise for basic presence checking. Ignores character count and order, which could be a limitation in some use cases.