replacing a substring in a string: Python

Question

I am trying to continuously replace the substrings within a string based on user input but my string.replace syntax seems to replace the whole string with the substring from user input. here is the code:

import re
secret_word  = 'COMPUTER'    
clue = len(secret_word) * '-'   # this step gives the user the nos of characters in secret_word
user_guess = input("Type a single letter here, then press enter: ")
user_guess = user_guess.upper()
if user_guess in secret_word:
    index = [match.start() for match in re.finditer(user_guess, secret_word)] # this finds the index of the user guess in secret_word                                                     
    print(index)
    for i in index:
        clue  = clue.replace(clue[i], user_guess)
        print("The word now looks like this: "+ clue)

I am not sure why it is not replacing only the substrings.

Can u show us a sample input & expected output.

sushanth
– sushanth

2020-05-23 11:17:04 +00:00
Commented May 23, 2020 at 11:17 — sushanth
– sushanth, Commented May 23, 2020 at 11:17

Sylvaus · Accepted Answer · 2020-05-23 11:17:28Z

1

The cause is the line clue = clue.replace(clue[i], user_guess). clue[i] will always be equal to '*' at the beginning so the replace function will replace all the characters by the user_guess.

One solution is to change clue to be a list instead of a string clue = len(secret_word) * ['-'] and replace the replace operation by clue[i] = user_guess

Do not forget to update the print operation: clue becomes "".join(clue) in print("The word now looks like this: "+ clue)

answered May 23, 2020 at 11:17

Sylvaus

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Comments

ClimateUnboxed · Accepted Answer · 2022-01-16 07:50:25Z

1

secret_word  = 'COMPUTER'
clue = str(len(secret_word) * '-')   # this step gives the user the nos of characters in secret_word
user_guess = input("Type a single letter here, then press enter: ")
user_guess = user_guess.upper()

if user_guess in secret_word:
    index = user_guess.find(user_guess) # this finds the index of the user guess in secret_word
    clue = clue[:index] + user_guess + clue[index+1:]
    print("The word now looks like this: " + clue)

Try this, you don't need a regular expression.

edited Jan 16, 2022 at 7:50

ClimateUnboxed

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answered May 23, 2020 at 11:32

Sachin Singh

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2 Comments

O. Edward Over a year ago

Thanks, Sachin for this response. There is one problem i still face though. This solution only works if the substring being replaced occurs only once in the string. How would one write the code for it to update all instances of a particular substring within a string. For example, with your solution, if secret_word is 'HAPPY', when the user inputs 'P', only the first occurence of P would be updated in the variable 'clue'.

Sachin Singh Over a year ago

@O.Edward seems you have accepted the answer do you still want me to solve it again?

gutsytechster · Accepted Answer · 2020-05-23 11:26:26Z

In python, when you use str.replace(x, substitution), it replaces every occurrence of x with the substituion in the string str.

In the beginning the variable clue holds the -------- string, because of which your replace method is called as clue.replace('-', U) considering, user provided the input u, which in turn replace the every occurence of - i.e. the whole string clue to UUUUUUUU.

One way to achieve this would be to change your code to something like this:

import re
secret_word = 'COMPUTER'
clue = len(secret_word) * '-'   # this step gives the user the nos of characters in secret_word
user_guess = input("Type a single letter here, then press enter: ")
user_guess = user_guess.upper()
if user_guess in secret_word:
    index = [match.start() for match in re.finditer(user_guess, secret_word)]  # this finds the index of the user guess in secret_word
    print(index)
    for i in index:
        clue = clue[:i] + user_guess + clue[i:]
        print("The word now looks like this: "+ clue)

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