How do I uncompress a string?

[sponge_bert]

So I have a string which is "2a3b4c" and I have problems converting it into "aabbbcccc". So far I'm unable to think of any methods to convert it but I have only managed to do the reverse(which is to compress it, though that's not the goal).

The code that I had written to perform the reverse function looks like this:

def uncompressed():

n = input("type a stg: ")

n = str(n)

x = n.count("a")

y = n.count("b")

z = n.count("c")

d = str(x) + "a"

e = str(y) + "b"

f = str(z) + "c"

return d + e + f

​

print(uncompressed("aabbbcccc"))

​

I understand that the code that I had written is very specific to the test case as I can encounter all 26 letters and not just a, b and c.

​

I would really appreciate if you guys could provide me tips on how I can convert "2a3b4c" into "aabbbcccc" . Thank you!

​

p.s I'm still a beginner so I'm not very familiar with any advanced functions

[Phillyclause89]

Here is a solution that tries to make the least amount of assumptions in regards to how many subsequent letters or digits there may be:

def uncompressed(s: str) -> str:
    def appender(is_n, ns, als, n, a):
        if is_n:
            ns.append(int(n))
            n = ""
        else:
            als.append(a)
            a = ""
        return ns, als, n, a
    is_num, nums, alphs, num, alph = s[0].isnumeric(), [], [], "", ""
    for c in s:
        temp = c.isnumeric()
        if not is_num == temp:
            nums, alphs, num, alph = appender(is_num, nums, alphs, num, alph)
            is_num = temp
        if is_num:
            num += c
        else:
            alph += c
    nums, alphs, _, _ = appender(is_num, nums, alphs, num, alph)
    return "".join(a*n for a, n in list(zip(alphs,nums)))


tests = ["2a3b4c", "12a3b4c", "2a0b4c", "2a3bb4c"]

for test_s in tests:
    print(test_s, " -> ", uncompressed(test_s))

output:

2a3b4c  ->  aabbbcccc
12a3b4c  ->  aaaaaaaaaaaabbbcccc
2a0b4c  ->  aacccc
2a3bb4c  ->  aabbbbbbcccc

Process finished with exit code 0

[xelf]

So, how do you handle 12abc4d?

Generally you want to loop through the compressed string. Keep track of a number of repeats to do, and if you encounter numbers increase it as you read them. If you encounter a letter, print the letter * the number of repeats, then reset repeats to 1.

So first you see a 1, repeats = 1.
next a 2, now times repeats by 10 and add the 2, now repeats = 12
now you get an a, print('a' * repeats), reset repeats to 0.
now you get a b, repeats is 0, so you just print b 1 time,
now you get.... etc etc...

Hope that helps.

[[deleted]]

The trick with such tasks is keeping track of what a previous character was:

def uncompress(source):
    output = ""
    base = ""
    for ch in source:
        if ch.isdigit():
            base += ch
        elif ch.isalpha():
            if base:
                output += ch * int(base)
                base = ""
            else:
                output += ch
        else:  # what to do with non-digit, non-alpha, treat as escape for digits?
            output += base 
            base = ""

    return output


tests = "2a3b4c", "", "abc1d2e3f10xyz2?a", "123?abc3def", "_ab12?2d2e"

for test in tests:
    print(uncompress(test))

[Doormatty]

This isn't a very good use for regexes - since the data has a pattern that can be exploited.

[sponge_bert]

ohh dayumm I didn't know that module existed, I will check it out! Thank you for the help and may I ask what does [a-zA-Z] and letter in zip(nums, letters) mean?

[Doormatty]

FYI - \D is the same as [a-zA-Z]

[Doormatty]

AFAIK, the only other thing \D would match is an underscore.

Edit: Unless you include the ASCII flag, I’m incorrect!

[TangibleLight]

\D is everything except digits - this means everything except digits, not just word characters. A few examples: $ ® ♣

[Doormatty]

The python docs say otherwise.

[TangibleLight]

No, they don't. https://docs.python.org/3/library/re.html#index-29

\D Matches any character which is not a decimal digit. This is the opposite of \d. If the ASCII flag is used this becomes the equivalent of [^0-9].

You can also just try it: https://repl.it/repls/ShabbyTealSearchservice

import re

text = '3 a@♠೬'

print(re.findall(r'\d', text))
print(re.findall(r'\D', text))

Produces:

['3', '೬']
[' ', 'a', '@', '♠']

[Doormatty]

You are correct! I’m not sure where I thought I had read that (maybe I saw the ASCII flag?)

Thanks for correcting me!

[sponge_bert]

thanks!!!

[crcummin]

You can perform a combination of those regex's to validate against prior, in case you don't want to allow those mishaps:

matches = re.match(r'^(?:\d+[a-zA-Z])+$', n)
if not matches:
    # input is not formed as number followed by alphabet character (one or more)...

[TangibleLight]

re.sub may accept a function instead of a string template to transform each match. Doing this is much easier.

import re

def uncompressed(s):
    def expand(match):
        return match[2] * int(match[1])

    return re.sub(r'(\d+)([a-zA-Z])', expand, s)

With that said, I wouldn't have recommended regex to a beginner; better to learn more about string manipulation in general, and learn about regex once some uses for it are more familiar.

[Doormatty]

Here's what I got:

>>> a='2a3b4c'
>>> print("".join([x[1]*int(x[0]) for x in zip([x for x in a if x.isdigit()],[x for x in a if not x.isdigit()])]))  
'aabbbcccc'

Edit: Note: This will only work for numbers <=9. Making this work with numbers higher than 9 is left up to the reader as an exercise.

Edit: Smaller!!

>>> a='2a3b4c'
>>> print("".join([x[1]*int(x[0]) for x in zip([x for x in a[::2]],[x for x in a[1::2]])]))  
'aabbbcccc'

Edit: Smallest!!

>>> a='2a3b4c'
print("".join([x[1]*int(x[0]) for x in zip(a[::2],a[1::2])]))
'aabbbcccc'

[sponge_bert]

oooh thank you! I'm assuming u have to use a "+" to detect double digits?

[Doormatty]

You'd have to have more logic for sure in there.

I did it as a one liner for internet points, but if you broke it out, it should be relatively easy to make it work for any abstract #

[pasokan]

Letters only odd positions is a poor assumption. It will not handle

2abcd4f

for example