How do I split a string containing a math expression into a list?

Question

How do I tokenize the string:

"2+24*48/32"

Into a list:

['2', '+', '24', '*', '48', '/', '32']

• Jim: You want to split a string into a list, but you don't want to use .split() because it returns a list? You're contradicting yourself. If you don't want a list, then what is it you do want?
• readonly: @Jim: I think Jibmo means that split() only allows you to specify one delimiter, so he would have to call it once for '+', once for '-', once for '/', etc...
• Jibmo: sorry for the bad explanation, what I meant is that split will return a list, which means for the second split, I now need to iterate over strings within a list. syntaxly incorrect example.. string = "2+2-2" list = string.split(+) returns ['2', '+', '2-2'] now i need to iterate over 3 strings
• Allen: You should mention that you're working on a program that needs to be able to evaluate these strings as arithmetic expressions. Jerub's answer covers that, but that's because he's a mindreader.
• Brian Cain: Why not just use SymPy? It should do what you're trying to achieve.

Answer 1

It just so happens that the tokens you want split are already Python tokens, so you can use the built-in tokenize module. It's almost a one-liner; this program:

from io import StringIO
from tokenize import generate_tokens
STRING = 1
print(
 list(
 token[STRING]
 for token in generate_tokens(StringIO("2+24*48/32").readline)
 if token[STRING]
 )
)

produces this output:

['2', '+', '24', '*', '48', '/', '32']

• Kiv: Great answer, I didn't realize this module existed :)
• roskakori: Instead or manually assigning STRING=1 you could use the constant from the token module by doing a from token import STRING. This is particular useful if you need several token constants.
• Victor S: why would such a complicated answer be rated so high? It's a pretty simple question. Whatever happened to finding the cleanest, most concise answer?

Answer 2

You can use split from the re module.

re.split(pattern, string, maxsplit=0, flags=0)

Split string by the occurrences of pattern. If capturing parentheses are used in pattern, then the text of all groups in the pattern are also returned as part of the resulting list.

Example code:

import re
data = re.split(r'(\D)', '2+24*48/32')

---

\D

When the UNICODE flag is not specified, \D matches any non-digit character; this is equivalent to the set [^0-9].

Answer 3

>>> import re
>>> re.findall(r'\d+|\D+', '2+24*48/32=10')
['2', '+', '24', '*', '48', '/', '32', '=', '10']

Matches consecutive digits or consecutive non-digits.

Each match is returned as a new element in the list.

Depending on the usage, you may need to alter the regular expression. Such as if you need to match numbers with a decimal point.

>>> re.findall(r'[0-9\.]+|[^0-9\.]+', '2+24*48/32=10.1')
['2', '+', '24', '*', '48', '/', '32', '=', '10.1']

Answer 4

This looks like a parsing problem, and thus I am compelled to present a solution based on parsing techniques.

While it may seem that you want to 'split' this string, I think what you actually want to do is 'tokenize' it. Tokenization or lexxing is the compilation step before parsing. I have amended my original example in an edit to implement a proper recursive decent parser here. This is the easiest way to implement a parser by hand.

import re
patterns = [
 ('number', re.compile('\d+')),
 ('*', re.compile(r'\*')),
 ('/', re.compile(r'\/')),
 ('+', re.compile(r'\+')),
 ('-', re.compile(r'\-')),
]
whitespace = re.compile('\W+')
def tokenize(string):
 while string:
 # strip off whitespace
 m = whitespace.match(string)
 if m:
 string = string[m.end():]
 for tokentype, pattern in patterns:
 m = pattern.match(string)
 if m:
 yield tokentype, m.group(0)
 string = string[m.end():]
def parseNumber(tokens):
 tokentype, literal = tokens.pop(0)
 assert tokentype == 'number'
 return int(literal)
def parseMultiplication(tokens):
 product = parseNumber(tokens)
 while tokens and tokens[0][0] in ('*', '/'):
 tokentype, literal = tokens.pop(0)
 if tokentype == '*':
 product *= parseNumber(tokens)
 elif tokentype == '/':
 product /= parseNumber(tokens)
 else:
 raise ValueError("Parse Error, unexpected %s %s" % (tokentype, literal))
 return product
def parseAddition(tokens):
 total = parseMultiplication(tokens)
 while tokens and tokens[0][0] in ('+', '-'):
 tokentype, literal = tokens.pop(0)
 if tokentype == '+':
 total += parseMultiplication(tokens)
 elif tokentype == '-':
 total -= parseMultiplication(tokens)
 else:
 raise ValueError("Parse Error, unexpected %s %s" % (tokentype, literal))
 return total
def parse(tokens):
 tokenlist = list(tokens)
 returnvalue = parseAddition(tokenlist)
 if tokenlist:
 print 'Unconsumed data', tokenlist
 return returnvalue
def main():
 string = '2+24*48/32'
 for tokentype, literal in tokenize(string):
 print tokentype, literal
 print parse(tokenize(string))
if __name__ == '__main__':
 main()

Implementation of handling of brackets is left as an exercise for the reader. This example will correctly do multiplication before addition.

• Jibmo: I'm reading up on tokenizing now to understand it. So I'm not able too say where the problem is though I think it's in the fact that this script will eval * and / at the same time, which is incorrect. 8/2*2 this string should print a result of 2, but it prints a result of 8.
• Jibmo: excuse me im wrong, always took bomdas literally turns out multiplication and division are equal in order of predecnce and whichever is occurs first is evaluated first
• Air: In tokenize: Why use re to remove whitespace over a built-in string function?

Answer 5

This is a parsing problem, so neither regex not split() are the "good" solution. Use a parser generator instead.

I would look closely at pyparsing. There have also been some decent articles about pyparsing in the Python Magazine.

Answer 6

s = "2+24*48/32"

p = re.compile(r'(\W+)')

p.split(s)

Answer 7

Regular expressions:

>>> import re
>>> splitter = re.compile(r'([+*/])')
>>> splitter.split("2+24*48/32")

You can expand the regular expression to include any other characters you want to split on.

Answer 8

Another solution to this would be to avoid writing a calculator like that altogether. Writing an RPN parser is much simpler, and doesn't have any of the ambiguity inherent in writing math with infix notation.

import operator, math
calc_operands = {
 '+': (2, operator.add),
 '-': (2, operator.sub),
 '*': (2, operator.mul),
 '/': (2, operator.truediv),
 '//': (2, operator.div),
 '%': (2, operator.mod),
 '^': (2, operator.pow),
 '**': (2, math.pow),
 'abs': (1, operator.abs),
 'ceil': (1, math.ceil),
 'floor': (1, math.floor),
 'round': (2, round),
 'trunc': (1, int),
 'log': (2, math.log),
 'ln': (1, math.log),
 'pi': (0, lambda: math.pi),
 'e': (0, lambda: math.e),
}
def calculate(inp):
 stack = []
 for tok in inp.split():
 if tok in self.calc_operands:
 n_pops, func = self.calc_operands[tok]
 args = [stack.pop() for x in xrange(n_pops)]
 args.reverse()
 stack.append(func(*args))
 elif '.' in tok:
 stack.append(float(tok))
 else:
 stack.append(int(tok))
 if not stack:
 raise ValueError('no items on the stack.')
 return stack.pop()
 if stack:
 raise ValueError('%d item(s) left on the stack.' % len(stack))
calculate('24 38 * 32 / 2 +')

• Jerub: Why don't you just go implement forth, it'll only be 5 more lines!

Answer 9

>>> import re
>>> my_string = "2+24*48/32"
>>> my_list = re.findall(r"-?\d+|\S", my_string)
>>> print my_list
['2', '+', '24', '*', '48', '/', '32']

This will do the trick. I have encountered this kind of problem before.

Answer 10

This doesn't answer the question exactly, but I believe it solves what you're trying to achieve. I would add it as a comment, but I don't have permission to do so yet.

I personally would take advantage of Python's maths functionality directly with exec:

expression = "2+24*48/32"
exec "result = " + expression
print result
38

• P Daddy: Forgive me if I'm wrong, but wouldn't it be preferable to use result = eval(expression)?
• Timotheos: Indeed it would; my apologies.

Answer 11

i'm sure Tim meant

splitter = re.compile(r'([\D])'). 

if you copy exactly what he has down you only get the digits not the operators.

Answer 12

Here is a good way that I always use when splitting str with different special characters. However, this code does not work with _, if there is a _ in the str you want to split, you might need to do another split one more time.

import re
 
 
# initializing string 
data = "2+24*48/32"
 
# printing original string 
print("The original string is : " + data) 
 
# Using re.findall() 
# Splitting characters in String 
res = re.findall(r"[\w']+", data)
 
# printing result 
print("The list after performing split functionality : " + str(res))