[SOLVED] Calculations using a list with operations with numbers python

Calculations using a list with operations with numbers python

I am trying to use a list that has operations attached to them, but i can't get the numbers out of the list. Here is the list.

mylist=["+1", "-2", "*2"]

I was wondering if there was a way to put them together as a float, and then do the operation so it would do 1-2*2 and output the answer. Smaller the better. thanks :) the expected output is -2.

Solution

You can use a dict to map symbols to operations (that are all defined in operator) and regular expressions to parse the strings in order to avoid evil eval. This applies the operations in the list from left to right, regardless of mathematical operator precedence:

import re
from operator import add, sub, mul, truediv  # use 'floordiv' for integer division

def calc(lst):
    ops = {'+': add, '-': sub, '*': mul, '/': truediv}
    result = 0
    for x in lst:
        op, num = re.match(r'([+\-\*/])(\d+)', x).groups()
        result = ops[op](result, int(num))
    return result

>>> calc(['+1', '-2', '*2'])
-2
>>> calc(['+1', '-2', '*2', '+7', '*3'])
15