Python - how to replace string in a list?

Question

The script I'm writing generates a list called "trt" full of strings. Some of those strings may need to be replaced before the list is processed further. For instance, string "5" needs to be replaced with "4", but without messing with strings such as "-5" or "5*". The solution must not change the order of strings in the list "trt" or enter any new character or blank space in the list.

I've already tried to do it like this:

trt = [word.replace('5','4') for word in trt]

but it is not a good solution, as it affects "5*" and makes the script misbehave.

@RohitJain He doesn't need regex, he's replacing strings in a list, not substrings in a string. — rlms
– rlms, Commented Aug 17, 2013 at 19:59

Pavel Anossov · Accepted Answer · 2013-08-17 19:56:01Z

9

If I understood you correctly, you want this:

trt = ['4' if word == '5' else word for word in trt]

If you need to do a lot of these replacements, you might want to define a map:

replacements = {
    '5': '4',
    'a': 'b',
}
trt = [replacements.get(word, word) for word in trt]

get looks up word in the replacements dict and returns word itself if it doesn't find such a key, or the corresponding value if it does.

answered Aug 17, 2013 at 19:56

Pavel Anossov

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user2684432 Over a year ago

You, sir, are full of win.

rlms · Accepted Answer · 2013-08-17 19:57:51Z

1

word.replace('5','4') replaces each 5 in the string word with a 4. This is not what you want to do (I don't think).

One solution:

for index, value in enumerate(trt):
    if value == "5":
        trt[index] = "4"

A more functional solution:

 trt = [i if i != "5" else "4" for i in trt]

answered Aug 17, 2013 at 19:57

rlms

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1 Comment

answerSeeker Over a year ago

the first solution makes more sense haha

Grant Shannon · Accepted Answer · 2023-01-17 11:48:15Z

0

Here is a one-line lambda:

newlst = list(map(lambda x: '4' if x in ['5'] else x,lst))

Assume this is your list:

lst = ['-5','5*','5','-5','5']

Then:

newlst = list(map(lambda x: '4' if x in ['5'] else x,lst))

Returns:

['-5', '5*', '4', '-5', '4']

answered Jan 17, 2023 at 11:48

Grant Shannon

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