如何检查列表中是否存在字符序列？

Question

How do I check if a sequence of characters exists in a list?

I have a string with some characters that have sequences that reoccur. I know that strings are immutable so I turn the string into the list. However, I'm not sure how to iterate through the list, find the occurrence and change the first letter of the occurrence.

message: DDMCAXQVEKGYBNDDMZUH
Occurence is: DDM

list: ['D', 'D', 'M', 'C', 'A', 'X', 'Q', 'V', 'E', 'K', 'G', 'Y', 'B', 'N', 'D', 'D', 'M', 'Z', 'U', 'H']

What I have currently is simply turning the message into the list. I've tried different ways, which were unsuccessfully that's what I didn't post it. Not really asking you to write the code but at the least explain how to achieve this.

Answer 1

It's a lot easier to check if a string exists in another string since you can simply use the in operator:

if 'DDM' in message:
     # do something

But since your goal is to change the first letter of the occurrence, you can use the str.index method to obtain the index of the occurrence and then assemble a new string with slices of the current string and the new letter:

try:
    i = message.index('DDM')
    message = message[:i] + new_letter + message[i + 1:]
except ValueError:
    raise RuntimeError("Sequence 'DDM' not found in message.")

Answer 2

You can use re.sub() :

import re

s = 'DDMCAXQVEKGYBNDDMZUH'

re.sub(r'DDM', '$DM', s)
# $DMCAXQVEKGYBN$DMZUH

Answer 3

A simple solution with a for-loop would be:

msg = 'DDMCAXQVEKGYBNDDMZUH'
occ = 'DDM'

for i in range(len(msg)):
    if msg[i:i+len(occ)] == occ:
        msg = msg[:i] + 'x' + msg[i+1:]

resulting in xDMCAXQVEKGYBNxDMZUH

This also works with overlapping substrings. For example:

msg = 'AAABAA'
occ = 'AA'

will give xxABxA

Answer 4

The simplest way would be using string replace() function.

string.replace(s, old, new[, maxreplace])

Return a copy of string s with all occurrences of substring old replaced by new. If the optional argument maxreplace is given, the first maxreplace occurrences are replaced.

message = "DDMCAXQVEKGYBNDDMZUH"
print message.replace("DDM", "ABC", 1)

Replace function would replace the first occurrence of DDM in the message string.

output: ABCCAXQVEKGYBNDDMZUH

If I carefully read your question you want to search the first occurrence of DDM in your message and replace the first character of it. In that case use below:

message = "DDMCAXQVEKGYBNDDMZUH"
print message.replace("DDM", "ADM", 1)

output: ADMCAXQVEKGYBNDDMZUH