Python/data_structures/stacks/postfix_evaluation.py

"""
Output:

Enter a Postfix Equation (space separated) = 5 6 9 * +
 Symbol  |    Action    | Stack
-----------------------------------
       5 | push(5)      | 5
       6 | push(6)      | 5,6
       9 | push(9)      | 5,6,9
         | pop(9)       | 5,6
         | pop(6)       | 5
       * | push(6*9)    | 5,54
         | pop(54)      | 5
         | pop(5)       |
       + | push(5+54)   | 59

	Result =  59
"""

import operator as op

def Solve(Postfix):
    Stack = []
    Div = lambda x, y: int(x/y)     # integer division operation
    Opr = {'^':op.pow, '*':op.mul, '/':Div, '+':op.add, '-':op.sub}     # operators & their respective operation

    # print table header
    print('Symbol'.center(8), 'Action'.center(12), 'Stack', sep = " | ")
    print('-'*(30+len(Postfix)))

    for x in Postfix:
        if( x.isdigit() ):          # if x in digit
            Stack.append(x)         # append x to stack
            print(x.rjust(8), ('push('+x+')').ljust(12), ','.join(Stack), sep = " | ")      # output in tabular format
        else:
            B = Stack.pop()             # pop stack
            print("".rjust(8), ('pop('+B+')').ljust(12), ','.join(Stack), sep = " | ")      # output in tabular format

            A = Stack.pop()             # pop stack
            print("".rjust(8), ('pop('+A+')').ljust(12), ','.join(Stack), sep = " | ")      # output in tabular format

            Stack.append( str(Opr[x](int(A), int(B))) )         # evaluate the 2 values poped from stack & push result to stack
            print(x.rjust(8), ('push('+A+x+B+')').ljust(12), ','.join(Stack), sep = " | ")      # output in tabular format

    return int(Stack[0])


if __name__ == "__main__":
    Postfix = input("\n\nEnter a Postfix Equation (space separated) = ").split(' ')
    print("\n\tResult = ", Solve(Postfix))