Multiple string replacements in one string

Question

I have got a string alphabet = "abcdefghijklmn" and would like to replace each vowel in the string with values from a substitution matrix matrix = { 'a' : 'FIRST VOWEL', 'e' : 'SECOND VOWEL', 'i' : 'THIRD VOWEL' }. I guess this is a case for recursion, so I tried the following function:

import re

def iterrepl(string):
    m = re.search(r'[aeiou]',string)
    if m:
         string = string[:m.start()]+matrix.get(m.group())+string[m.end():]
         iterrepl(string)
    else:
         return(string)

matrix = { 'a' : 'FIRST VOWEL', 'e' : 'SECOND VOWEL', 'i' : 'THIRD VOWEL' }
alphabet = "abcdefghijklmn"

print(iterrepl(alphabet))

The final result, however, is None, whereas I was hoping for FIRST VOWELbcdSECOND VOWELfghTHIRD VOWELjklmn. When I add a print statement before calling iterrepl within the function, I can see that the result of the last but one recursion is the desired one:

FIRST VOWELbcdefghijklmn
FIRST VOWELbcdSECOND VOWELfghijklmn
FIRST VOWELbcdSECOND VOWELfghTHIRD VOWELjklmn
None

I cannot figure out what is wrong with the end of the recursion and would appreciate any advice!

You need at least return iterrepl(string) otherwise what's it returning? — TessellatingHeckler
– TessellatingHeckler, Commented Feb 26, 2020 at 0:10
A generally more straightforward approach without recursion would be to use "re.sub" with a function as replacement. — Michael Butscher
– Michael Butscher, Commented Feb 26, 2020 at 0:14
Or let str.translate do the work for you. trans=str.maketrans({ 'a' : 'FIRST VOWEL', 'e' : 'SECOND VOWEL', 'i' : 'THIRD VOWEL' });print("abcdefghijklmn".translate(trans)) — tdelaney
– tdelaney, Commented Feb 26, 2020 at 0:18

cdlane · Accepted Answer · 2020-02-26 06:00:28Z

Assuming recursion is, as you imply, your choice, and not a requirement, then you're making this harder than necessary as Python has builtin functions for this:

matrix = {'a': 'FIRST VOWEL', 'e': 'SECOND VOWEL', 'i': 'THIRD VOWEL'}

alphabet = "abcdefghijklmn"

translation_table = str.maketrans(matrix)

print(alphabet.translate(translation_table))

OUTPUT

> python3 test1.py
FIRST VOWELbcdSECOND VOWELfghTHIRD VOWELjklmn
>

Or, we could simply do:

matrix = {'a': 'FIRST VOWEL', 'e': 'SECOND VOWEL', 'i': 'THIRD VOWEL'}

alphabet = "abcdefghijklmn"

for old, new in matrix.items():
    alphabet = alphabet.replace(old, new)

print(alphabet)

OUTPUT

> python3 test2.py
FIRST VOWELbcdSECOND VOWELfghTHIRD VOWELjklmn
>

Or if it must be recursive, then perhaps something like:

matrix = {'a': 'FIRST VOWEL', 'e': 'SECOND VOWEL', 'i': 'THIRD VOWEL'}

alphabet = "abcdefghijklmn"

def replace_vowels(string):

    def replace_vowels_recursive(string, dictionary):
        if dictionary:
            (vowel, replacement), *rest = dictionary.items()

            return replace_vowels_recursive(string.replace(vowel, replacement), dict(rest))

        return string

    return replace_vowels_recursive(string, matrix)

print(replace_vowels(alphabet))

OUTPUT

> python3 test3.py
FIRST VOWELbcdSECOND VOWELfghTHIRD VOWELjklmn
>

G Frank · Accepted Answer · 2020-02-26 03:42:44Z

Your question is not very clear, however if your desired return from the function is "FIRST VOWELbcdSECOND VOWELfghTHIRD VOWELjklmn" then try this:

def iterrepl(string):

    string_arr = []
    for x in range(0, len(string)):
        string_arr.append(string[x:x+1]) #enter every char into a list.

        for y in range(0, len(matrix)):
            if string_arr[x] == matrix.keys()[y]: #search list for match.
                string_arr[x] = matrix[matrix.keys()[y]] #found. replace list item and break loop.
                break

    return str(''.join(string_arr)) #convert back to string.


matrix = { 'a' : 'FIRST VOWEL', 'e' : 'SECOND VOWEL', 'i' : 'THIRD VOWEL' }
alphabet = "abcdefghijklmn"

print(iterrepl(alphabet))

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