Break a math expression up into a list of its parts, separating the numbers from the operators

Question

I tried:

tokens = [t.strip() for t in re.split(r'((?:-?\d+)|[+\-*/()])', exp)]  
return [t for t in tokens if t != '']

But it gets the wrong result:

Expected :[3, '+', -4, '*', 5]

Actual   :['3', '+', '-4', '*', '5']

Please take some time reading about how to create a [Minimal, Complete, and Verifiable example](https://stackoverflow.com/help/mcve). — Vasilis G., Dec 01 '17 at 01:52
Possible duplicate of [Python: Check if a string represents an int, Without using Try/Except?](https://stackoverflow.com/questions/1265665/python-check-if-a-string-represents-an-int-without-using-try-except) — Tom Karzes, Dec 01 '17 at 01:59
@PatrickHaugh you're right: `[int(t) for t in tokens if re.match(r'^-?\d+$', t) else t]` — Nir Alfasi, Dec 01 '17 at 02:02
Possible duplicate of [Python: Extract numbers from a string](https://stackoverflow.com/questions/4289331/python-extract-numbers-from-a-string) — robinCTS, Dec 01 '17 at 02:44

score 1 · Answer 1 · answered Dec 01 '17 at 01:53

You need to cast the items in the list to ints where appropriate

def try_int(s):
    try:
        return int(s)
    except ValueError:
        return s

Then in your function, you can apply this to all the items in the return list

return [try_int(t) for t in tokens if t != '']

score 0 · Answer 2 · answered Dec 01 '17 at 01:45

0

Use a for loop without the syntactic sugar. Convert the str of the integers to int using string method isdigit().

Example:

>>> "4".isdigit()
>>> True

answered Dec 01 '17 at 01:45

jeevaa_v

2 Answers2