How to reduce a string by another string in Python?-CodePudding

I would like to remove all characters from a first string s1 exactly the number of times they appear in another string s2, i.e. if s1 = "AAABBBCCCCCCD" and s2 = "ABBCCC" then the result should be s = "AABCCCD". (The order of the characters in the resulting string is actually irrelevant but it's a plus if it can be preserved.)

The following rather crude code can do this:

def reduce_string(s1, s2):
    s = s1
    for c in s2:
        if c in s:
            s = s.replace(c, "", 1)
    return(s)

# examples
reduce_string("AAABBBCCCCCCD", "ABBCCC")
reduce_string("AAABBBCCCCCCD", "ABBCCCE")

My question is, can the same be achieved by clever use of some built-in function or at least in a more elegant way? Thank you for all your answers!

CodePudding user response：

You can use counter objects. Subtract one against the other and join the remaining elements together.

from collections import Counter

s1 = "AAABBBCCCCCCD"
s2 = "ABBCCC"

counter = Counter(s1)
counter.subtract(Counter(s2))

result = ''.join(counter.elements())
print(result)

AABCCCD

As a one-liner:

print(''.join((Counter(s1) - Counter(s2)).elements()))

CodePudding user response：

There is a filterfalse function in itertools module that you should see. Consult the documentation here.

The filterfalse function returns elements from a iterable when the predicate is evaluated as False.

So, one possible solution might be:

import itertools

def reduce_string(s1, s2):

    def predicate(letter, param=list(s2)):
        if letter in param:
            param.remove(letter)
            return True
        return False

    result = itertools.filterfalse(predicate, s1)

    return ''.join(result)

reduce_string("AAABBBCCCCCCD", "ABBCCC")
reduce_string("AAABBBCCCCCCD", "ABBCCCE")

However, notice how my predicate function is a little trickier when it changes the second string.

The keyword argument param is evaluated when the function predicate is created inside the reduce_string scope as a list object.

Since the reference does not change but the elements inside param, I was able to change the second string for comparisons reasons.

Now, the question remains: is there a more elegant way to define predicate function?