Coding dojo in Minsk session result for 2019-02-24

reverse_polish.py

"""
See https://www.codewars.com/kata/reverse-polish-notation-calculator

Your job is to create a calculator which evaluates expressions in Reverse Polish notation.
For example expression 5 1 2 + 4 * + 3 - (which is equivalent to 5 + ((1 + 2) * 4) - 3 in normal notation)
should evaluate to 14.
For your convenience, the input is formatted such that a space is provided between every token.
Empty expression should evaluate to 0.
Valid operations are +, -, *, /.
You may assume that there won't be exceptional situations (like stack underflow or division by zero).
"""


def numeric(text):
    if text.isdigit():
        return int(text)
    return float(text)


def operation(op, a, b):
    if op == '+':
        return a + b
    elif op == '-':
        return a - b
    elif op == '*':
        return a * b
    elif op == '/':
        return a / b


def calc(expr):
    """
    >>> calc("")
    0
    >>> calc("3")
    3
    >>> calc("3.5")
    3.5
    >>> calc("1 3 +")
    4
    >>> calc("1 3 *")
    3
    >>> calc("1 3 -")
    -2
    >>> calc("4 2 /")
    2
    >>> calc("5 1 2 + 4 * + 3 -")
    14
    """
    if expr == '':
        return 0

    tokens = expr.split()
    # return solve(tokens[::-1])

    stack = []
    for token in tokens:
        if token in '+-*/':
            b = stack.pop()
            a = stack.pop()
            result = operation(token, a, b)
            stack.append(result)
        else:
            stack.append(numeric(token))

    return stack.pop()


def solve(tokens):
    """Try recursion approach. Not finised"""
    if not tokens:
        return 0

    op = tokens.pop(0)
    if op not in '+-*/':
        return numeric(op)
    
    b = tokens.pop(0)
    if b in '+-*/':
        tokens.append(b)
        b = solve(tokens)

    a = tokens.pop(0)
    if a in '+-*/':
        tokens.append(a)
        a = solve(tokens)

    a = numeric(a)
    b = numeric(b)
    return operation(op, a, b)