bc.py

import sys

# Constants
OPERATORS         = ['*', '/', '+', '-']
DECIMAL_SYMBOLS   = [',', '.']
SIGN_SYMBOLS      = ['-', '+']
PARENTHESIS_OPEN  = '('
PARENTHESIS_CLOSE = ')'
PRECEDENCE        = [[PARENTHESIS_OPEN], ['*', '/'], ['+', '-']]

# State variables
should_exit = False

# Utility functions
def subtraction(a, b):
    if b > 0:
        return subtraction(a, b - 1) - 1
    else:
        return a

def addition(a, b):
    if b > 0:
        return addition(a, b - 1) + 1
    else:
        return a

def multiplication(a, b):
    if (b > 0):
        return addition(multiplication(a, b - 1), a)
    else:
        return 0

def division(a, b):
    if (a > b):
        return 1 + division(subtraction(a, b), b)
    else:
        return 1

OPERATOR_FUNCS = {'-': subtraction, '+': addition, '*': multiplication, '/': division}

def halt_error(s):
    print('invalid expression: %s' % s)
    sys.exit()

def is_operator(s):
    return s in OPERATORS

def tokenize(str):
    tokens = []

    c = []                                         # Temporary storage for operands
    s = []                                         # Set of tokens
    for cr in str:
        if cr.isnumeric():
            c.append(cr)                           # Keep track of operand
        elif cr == PARENTHESIS_OPEN or cr == PARENTHESIS_CLOSE or is_operator(cr):
            s.append(c)                            # Push operand
            c = []                                 # Empty storage
            s.append(cr)                           # Push operator
        elif cr in DECIMAL_SYMBOLS:
            halt_error('no support for floating point arithmetic')

    s.append(c)                                    # Push operand

    return list(map(''.join, [v for v in s if v])) # Remove empty lists, join each inner list

# Returns the intersection of a and b
def intersect(a, b):
    return [v for v in a if v in b]

# Returns the nth index of v in l
def nth_index(l, v, n):
    i = -1
    for j in range(n):
        i = l.index(v, i + 1)
    return i

def execute(expr):
    for group in PRECEDENCE:
        while len(intersect(expr, group)) > 0:
            symbol = intersect(expr, group)[0]                # Fetch the first operator in the intersection of the precedence group and the expression
            if (symbol == PARENTHESIS_OPEN):
                s = expr.index(PARENTHESIS_OPEN)              # Find open parenthesis

                try:
                    e = nth_index(expr, PARENTHESIS_CLOSE, expr.count(PARENTHESIS_OPEN))
                    sub_expr = expr[s+1:e]                    # Containing expression
                    new_expr = execute(sub_expr)              # New expression resulting from recursive call
                    rem_expr_a = expr[e+1:]                   # Remaining expression after
                    rem_expr_b = expr[:s]                     # Remaining expression before
                    expr = rem_expr_b + new_expr + rem_expr_a # Reconstruct expression based on new result
                except ValueError:
                    halt_error('could not find matching parenthesis ")"')
            elif (is_operator(symbol)):
                s = expr.index(symbol)                        # Find operator
                a = int(expr[s-1])                          # First operand
                b = int(expr[s+1])                          # Second operand
                new_expr = [OPERATOR_FUNCS[symbol](a, b)]     # New expression resulting from operator implementation
                rem_expr_a = expr[s+2:]                       # Remaining expression after
                rem_expr_b = expr[:s-1]                       # Remaining expression before
                expr = rem_expr_b + new_expr + rem_expr_a     # Reconstruct expression based on new result
    return expr

def bc(str):
    tokens = tokenize(str)
    return execute(tokens).pop()

#Main logic
while not should_exit:
    s = input('')
    print(bc(s))