Python parser for Calculator [duplicate]

Question

This question already has answers here:

Evaluating a mathematical expression in a string (14 answers)

Closed 9 years ago.

I am trying to write a parser which takes expressions as a input from file.

expressions can be A=B=10 or B=(C-A)-4 etc.

What i have tried so far is . I am reading a file IP.txt

import re

opert = '+-/*()_='
fileName = "input.txt"
f = open(fileName,'r')

variableDict = {}
lines = f.readlines()

for i in lines:

    for x in  re.finditer(r'[A-Z_]\w*', i):
        print x.group() # prints list containing all the alphabets.

    for z in  re.finditer(r'[0-9]\d*', i):
        print z.group() # prints list containing all the numbers.

    for c in i:
        if c in opert:
            print c # prints all the operators.

   # '_' has special meaning. '_' can only be used before numbers only like _1 or _12 etc
   #And i have parsed this also using
       print re.findall(r'[_][0-9]\d+',i) # prints the _digits combination.

Now the problem is i have struck at how should i proceed with expression evaluation. First some rule which i must mention about above inputs are. No line should be greater then 50 characters. Left most operator will always be '=' assignment operator. '=' always Preceded by variables[A-Z],operators are {'+','-','/','*','_'}, digits {0-9}.

How should i first extract the first variable then push it into python list then '=' operator,then either '(','A-Z' push it into stack and so on

Could someone help me with this problem. I am overwhelmed with problem..

If any one is not able to understand the description please goto this link

Answer 1

So, you asked about the stack problem, which of course you need for evaluation. I would do something like this:

import re #1
stack = [] #2 FIX: NOT NECESSARY (since fourth line returns a list anyway)
inputstr = "A=B=C+26-(23*_2 )-D" #3

stack =  re.findall(r'(?:[A-Z])|(?:[0-9]+)|(?:[/*+_=\(\)-])', inputstr) #4

while len(stack): #5
    print stack.pop() #6

First three lines are some init stuff only. After that, I would make a stack with regex in the fourth line. (?:[A-Z]) matches variable, (?:[0-9]+) matches number (which may have more than one digit) and (?:[/*+_=\(\)-]) matches all the operators. Braces are escaped, and - is on the end, so you don't have to escape it.

Fifth and sixth line prints the stack.

I used (?: ...) because I don't want to match either group. Hard to explain - just try to run it without ?: and you will see the effect.