Replace a random instance of a substring in a string

Question

Say I have the string

s = 'x x x x x'

I want to randomly change one of the 'x' to y

s2 = 'x x y x x'

x and y will be multiple characters long.

If I just wanted to change the first instance of x, I would use string.replace, but how can I change a random instance?

Is this only for cases of strings with a sequence of one character type separated by spaces? — Dylan Lawrence
– Dylan Lawrence, Commented Nov 4, 2013 at 16:49
Are the tokens (xx, yy, ...) always separated by exactly one space? — Lukas Graf
– Lukas Graf, Commented Nov 4, 2013 at 16:49
They are not cleanly separated, it could be arbitrary characters eg sentences, with x and y being multiple words. — user2953349
– user2953349, Commented Nov 4, 2013 at 16:50
Then please give an example of the strings you're actually dealing with. — Lukas Graf
– Lukas Graf, Commented Nov 4, 2013 at 16:51

Jon Clements · Accepted Answer · 2013-11-04 20:39:57Z

4

You could use re.finditer to retrieve the start/end of all possible matches and do an appropriate replacement. This will cover variable length replacing, but does mean you need to be wary of re syntax for the frm argument.

import re
from random import choice

def replace_random(src, frm, to):
    matches = list(re.finditer(frm, src))
    replace = choice(matches)
    return src[:replace.start()] + to + src[replace.end():]

Example:

>>> [replace_random('x x x x x', r'\bx\b', 'y') for _ in range(10)]
['y x x x x', 'x x x x y', 'x x y x x', 'x y x x x', 'x x x y x', 'x x x y x', 'x x y x x', 'x y x x x', 'x x x x y', 'x x y x x']

edited Nov 4, 2013 at 20:39

answered Nov 4, 2013 at 16:58

Jon Clements

143k34 gold badges254 silver badges288 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

mgilson Over a year ago

You could always re.escape frmif you want it to behave like a str.replace.

Jon Clements Over a year ago

@mgilson Yep... for production purposes it should probably be documented that it's expected an actual regex pattern be supplied... unless the OP prefers adding another option which enables the function to behave in two modes... but that's probably just confusing...

Lukas Graf Over a year ago

I think if you would use needle, haystack and replacement as variable names it could also make the example a bit more clear - but maybe that's just my personal preference.

user2953349 Over a year ago

Exactly what I wanted, thank you. On the last line, it should be src, not s though.

Jon Clements Over a year ago

@user2953349 errr yes... not sure how I copied/pasted that wrongly, but corrected :) Thanks.

astreal · Accepted Answer · 2013-11-04 16:58:14Z

2

you can do

import random

def replace_random(string, str_a, str_b):
    rand = max(random.randint(0, string.count(str_a)), 1)
    return string.replace(str_a, str_b, rand).replace(str_b, str_a, rand - 1)

print replace_random('x x x x x', 'x', 'y')

answered Nov 4, 2013 at 16:58

astreal

3,5431 gold badge23 silver badges35 bronze badges

Collectives™ on Stack Overflow

Replace a random instance of a substring in a string

2 Answers 2

5 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related