Python Concerning Nested Parentheses

Question

The problem I'm facing is being able to take an string like "((1+4)) + (2-1) - 3" and turn it in a list with [((1+4)), (2-1), 3] in it. This would also apply to other amount of parentheses. I tried doing it with indexes and counting parentheses with no luck. Here is some code I have so far:

final = []
while("(" in string):
final.append(string[string.index("("):string.index(")")+1])
left = string[:string.index("(")]
right = string[string.index(")")+1:]
string = string.replace("+", ";")
string = string.replace("-", ";")
string = string.split(";")
for item in string:
   if item.strip() != "":
     final.append(item)

needs to deal with stuff not nested in parentheses as well since this is a basic and plausible mathematical expression — The Dark Knight, Oct 11 '15 at 05:08
don't you want something more like [1+4, 2-1, 3], getting rid of parentheses altogether? — Guillaume, Oct 11 '15 at 05:09
naw, They get removed later on with other expression that print some other needed information. — The Dark Knight, Oct 11 '15 at 05:10
This is uncomfortably close to parsing HTML with regex, and we [know how that ends](http://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags). — TigerhawkT3, Oct 11 '15 at 05:39

score 0 · Answer 1 · edited May 23 '17 at 12:29

The regex (https://docs.python.org/2/library/re.html) gives you the shortest and possibility the fastest solution. But, if you prefer a solution that is easier to read and understand try this one:

def split_expr(s):
    l = []
    p =0
    expr = ''
    for c in s:
        if c=='(': p+=1
        if c==')': p-=1
        if (p==0) and (c in '+-*/'):
            l.append(expr)
            expr = ''
        else:
            expr += c
    else:
        l.append(expr)
    return l

You can also see this post (How to split but ignore separators in quoted strings, in python?). It can inspire you for some more elegant solutions.

Games Brainiac · Answer 2 · 2015-10-11T05:47:28.507

-1

You might want to use a regular expression for this one. This answer is probably not optimal, but take a look at how to use the re module:

>>> import re
>>> regex = re.compile(r"\s+[\+\*\-\/]\s+")
>>> s = "((1+4)) + (2-1) - 3"
>>> for sub in regex.findall(s):
...     s = s.replace(sub, " ")
>>> s
'((1+4)) (2-1) 3'
>>> s.split()
['((1+4))', '(2-1)', '3']

So, we compile a regular expression using re.compile, this way we can use the regular expression again and again without having to compile it each time. Then we use findall to findall the substrings that satisfy the regular expression, then we take each substring and use the replace function on a string to replace it with a single space. We then use split with the default delimiter set to " " to split the string.

EDIT: Because I totally forgot, you can use re.split, which does all of the above in one fell swoop.

>>> s = "((1+4)) + (2-1) - 3"
>>> regex.split(s)
['((1+4))', '(2-1)', '3']

Thanks @BurhanKhalid :)

edited Oct 11 '15 at 05:47

answered Oct 11 '15 at 05:39

Games Brainiac

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You might want to check with the OP on whether they basically want to just split on whitespace, but I get the feeling they don't. – TigerhawkT3 Oct 11 '15 at 05:45
@GamesBrainiac What if the `string` in this format `"((1 + 4)) + (2 - 1) - 3"`? – gsb-eng Oct 11 '15 at 05:46
@gsb-eng Then you'd need a more complex regex to get the work done, preferably with `\)(\s+[\+\-\*\/]\s+)(\))?`, and replace group 1. – Games Brainiac Oct 11 '15 at 05:50
You could also replace this regex with a simple `[item for item in s.split() if item not in '+-*/']`. I don't think it solves the right problem at all. – TigerhawkT3 Oct 11 '15 at 05:59
@TigerhawkT3 Then offer an alternative answer. – Games Brainiac Oct 11 '15 at 06:05

Python Concerning Nested Parentheses

2 Answers2