5 Best Ways to Check if One String Can Be Converted to Another With Given Constraints in Python

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πŸ’‘ Problem Formulation: You have two strings str1 and str2, and you need to determine whether it is possible to convert str1 into str2 following certain rules or constraints. For example, if our conversion rule is to replace characters or add any number of characters, could "tree" be converted to "treetop"? The desired output for this case would be a boolean value True because "tree" can be converted to "treetop" by adding characters.

Method 1: Length Check and Character Count

This method involves checking if the length of the target string is greater than or equal to the length of the source string, followed by a character frequency comparison. It’s assumed that characters can be inserted at any position, but not deleted or replaced.

Here’s an example:

from collections import Counter

def can_convert(str1, str2):
    if len(str2)  count2[char]:
            return False
    return True

print(can_convert("tree", "treetop"))

Output: True

This code snippet defines a function can_convert() that first verifies if str2 is not shorter than str1, then it compares character frequencies using the Counter class from the collections module. It ensures that str2 has at least as many occurrences of each character as str1.

Method 2: Sort and Compare

Under the assumption that we can rearrange the characters in the strings, this method checks if sorting both strings results in identical strings, therefore making them convertible to one another without any insertions or deletions.

Here’s an example:

def can_convert_sorted(str1, str2):
    return sorted(str1) == sorted(str2)

print(can_convert_sorted("listen", "silent"))

Output: True

The function can_convert_sorted() simply sorts both input strings and checks for equality. This means that one can be converted to the other by rearrangement if the sorted strings match.

Method 3: Dynamic Programming for Edit Distance

This method determines whether one string can be converted into another by inserting, deleting, or replacing characters. It does so by calculating the minimum number of operations required to convert one string into another, which is known as edit distance.

Here’s an example:

def min_operations(str1, str2): 
    dp = [[0] * (len(str2) + 1) for _ in range(len(str1) + 1)]

    for i in range(len(str1) + 1): 
        for j in range(len(str2) + 1): 
            if i == 0: 
                dp[i][j] = j   
            elif j == 0: 
                dp[i][j] = i   
            elif str1[i-1] == str2[j-1]: 
                dp[i][j] = dp[i-1][j-1]
                dp[i][j] = 1 + min(dp[i][j-1], dp[i-1][j], dp[i-1][j-1])
    return dp[-1][-1]

def can_convert_dp(str1, str2, max_cost):
    return min_operations(str1, str2) <= max_cost

print(can_convert_dp("kitten", "sitting", 3))

Output: True

The function min_operations() computes the edit distance using a dynamic programming approach, filling out a 2D array where the final value represents the minimum operations required. The enclosing function can_convert_dp() takes a maximum allowed number of operations (max_cost) to determine if conversion is possible under this limit.

Method 4: Using a Sliding Window with Constraints

This method applies when we have constraints like only being able to change characters within a certain window size. It involves scanning across the source string in a window and trying to match it against the target string.

Here’s an example:

def can_convert_window(str1, str2, window_size):
    for i in range(len(str1) - window_size + 1):
        if str1[i:i+window_size] in str2:
            return True
    return False

print(can_convert_window("theremin", "hereandthere", 5))

Output: True

In the can_convert_window() function, a sliding window of a fixed size moves over str1, checking at each position if the windowed substring is present in str2. If found, it suggests conversion is possible under the given constraint.

Bonus One-Liner Method 5: Using Built-In String Methods

If our constraint is to determine if the source string can be a substring of the target, this one-liner uses the built-in in operator to check for the presence of str1 in str2.

Here’s an example:

can_convert_substring = lambda str1, str2: str1 in str2

print(can_convert_substring("burn", "suburban"))

Output: True

This one-liner defines a lambda function can_convert_substring(), which returns True if str1 is part of str2, suggesting that str1 can be converted to str2 by addition of characters before and/or after str1.


  • Method 1: Length Check and Character Count. Strengths: Simple and it handles additions of characters well. Weaknesses: Does not contemplate character deletions or replacements.
  • Method 2: Sort and Compare. Strengths: Very straightforward and identifies if strings can be made identical by rearrangement. Weaknesses: Does not consider insertions or deletions, only rearrangements.
  • Method 3: Dynamic Programming for Edit Distance. Strengths: The most comprehensive, can handle insertions, deletions, and replacements. Weaknesses: More complex and slower for long strings.
  • Method 4: Using a Sliding Window with Constraints. Strengths: Good for checking conversions with specific windowed patterns. Weaknesses: Limited to the defined window size, doesn’t check beyond that pattern.
  • Bonus Method 5: Using Built-In String Methods. Strengths: Extremely concise. Works well when checking if a string can become a substring of another. Weaknesses: It’s applicable only for very specific constraints and is not generalizable.