from pyliststack import Stack
import math

"""
    Evaulate an postfix expression.<p>
   
    Precondition: the postfix is a valid postfix expression.<br>
      Currently only limited forms of expressions are supported. In
      particular, the numbers have to be single digit, the operators
      are limited to +,-,*,/ only.<p>
   
    Postcondition: the value of the expression is returned.<p>
   
    @param postfix a postfix form of an expression.
   
    @return the integer value of the expression.
"""
def evaluate_postfix( expression ):
    
    exp_stack = Stack()
    
    result = 0
    for ch in expression:
        if is_op( ch ):
            right = exp_stack.pop()   # right operand is always on top of left!
            left  = exp_stack.pop()
            result = do_op( ch, left, right )
        else: # the 'ch' must be an operand
            result = ord( ch ) - ord( '0' )   # convert it to a digit
        exp_stack.push( result )

    return result

def is_op( op ):
    return op in '+-*/^'

def do_op( op, left, right ):
    r = 0
    if op == '+':
        r = left + right
    elif op == '-':
        r = left - right
    elif op == '*':
        r = left * right
    elif op == '/':
        r = left / right
    elif op == '^':
        r = math.pow( left, right )
    return r

def main():
    print( "Hit Enter to end this program." )
    postfix = input( "Enter a simple arithmetic expression in postfix form : " )
    while not postfix == "":
        result = evaluate_postFix( postfix )
        print( postfix + " ==> " + str( result ) )
        postfix = input( "Enter a simple arithmetic expression in postfix form : " )

def test_eval():
    expressions = ['23^1-', '23+4*', '56-2*', '34-2^', '89*2-', '82/5+']
    result = [7, 20, -2, 1, 70, 9]
    for i in range( len( result ) ):
        r = result[ i ]
        e = expressions[ i ]
        print( 'result should be ', r )
        print( e + ' ==> ' + str( evaluate_postfix( e ) ) )

#main()
test_eval()