Evaluating a string of simple mathematical expressions [closed]

Answer 1

Python with regular expressions

Number of characters: 283

Fully obfuscated function:

import re
from operator import*
def c(e):
 O=dict(zip("+-/*()",(add,sub,truediv,mul)))
 a=[add,0];s=a
 for v,o in re.findall("(-?[.\d]+)|([+-/*()])",e):
  if v:s=[float(v)]+s
  elif o=="(":s=a+s
  elif o!=")":s=[O[o]]+s
  if v or o==")":s[:3]=[s[1](s[2],s[0])]
 return s[0]

Not obfuscated:

import re
from operator import *

def compute(s):
    operators = dict(zip("+-/*()", (add, sub, truediv, mul)))
    stack = [add, 0]
    for val, op in re.findall("(-?[.\d]+)|([+-/*()])", s):
        if val:
            stack = [float(val)] + stack
        elif op == "(":
            stack = [add, 0] + stack
        elif op != ")":
            stack = [operators[op]] + stack
        if val or op == ")":
            stack[:3] = [stack[1](stack[2], stack[0])]
    return stack[0]

I wanted to see if I cab beat the other Python solutions using regular expressions.

Couldn't.

The regular expression I'm using creates a list of pairs (val, op) where only one item in each pair is valid. The rest of the code is a rather standard stack based parser with a neat trick of replacing the top 3 cells in the stack with the result of the computation using Python list assignment syntax. Making this work with negative numbers required only two additional characters (-? in the regex).