Question

Implement in Python using stack operations.

Postfix Calculator

Post fix calculator

• We use a stack
• When an operand is read, push it on statck
• When an operator is read i.e +, *. /, -
– Pop two from the top of the stack and apply the operator and push the result on stack
if there is one value instead of two display an error message

• Keep repeating until an equal sign, = is read; pop from the top of stack and stop.

Postfix Expression          Result

4 5 7 2 + - * = -16 ===> (5 - (7 + 2 ) )* 4 = -16

3 4 + 2  * 7 / = 2

5 7 + 6 2 -  * = 48

4 2 3 5 1 - + * + = 18 ===> (5-1 + 3 ) * 2 + 4 = 18

List as Stack

"""

File: pyStackPostfix.py

Author: JD

"""

# 5 7 + 6 2 -  * = 48

print("Postfix Calculator\n")

stack = []              # Empty stack

y = int(0)

z = int(0)

w = int(0)

while True:

   x = input("Enter a postfix expression one by one:")

   if x >= '0' and x<= '9':

      x = int(x)

      stack.append(x)      # Push on top of stack

   elif x == '+':          # Got an operator

      if len(stack) >= 2:

         y = stack.pop()   # Pop from top of stack

         z = stack.pop()

         w = y + z

         stack.append(w)   # Push result back

      else:

         print("Stack error") # Not enough operhand A + B

         break

   elif x == '-':

      if len(stack) >= 2:

         y = stack.pop()

         z = stack.pop()

         w = y - z

         stack.append(w)  

      else:

         print("Stack error")

         break

   elif x == '*':

      if len(stack) >= 2:

         y = stack.pop()

         z = stack.pop()

         w = y * z

         stack.append(w)  

      else:

         print("Stack error")

         break

   elif x == '/':

      if len(stack) >= 2:

         y = stack.pop()

         z = stack.pop()

         w = y / z

         stack.append(w)  

      else:

         print("Stack error")

         break

   elif x == '=':             # Equal operator

      if len(stack) == 1:

         z = stack.pop()      # Pop the result

         print (z)

         break

      else:

         print("Stack error")

         break

IN PYTON

Answer 1

You just need to change the order of opertors in operations, i.e use z - y instead of y - z. Also you can input full experssion in one line.

"""
File: pyStackPostfix.py
Author: JD
"""
# 5 7 + 6 2 -  * = 48

print("Postfix Calculator\n")

stack = []              # Empty stack
y = int(0)
z = int(0)
w = int(0)

# expression list
exp = input("Enter a postfix expression:").split()

for x in exp:
  if x >= '0' and x<= '9':
      x = int(x)
      stack.append(x)      # Push on top of stack

  elif x == '+':          # Got an operator
      if len(stack) >= 2:
        y = stack.pop()   # Pop from top of stack
        z = stack.pop()
        w = z + y
        stack.append(w)   # Push result back
      else:
        print("Stack error") # Not enough operhand A + B
        break

  elif x == '-':
      if len(stack) >= 2:
        y = stack.pop()
        z = stack.pop()
        w = z - y
        stack.append(w)  
      else:
        print("Stack error")
        break

  elif x == '*':
      if len(stack) >= 2:
        y = stack.pop()
        z = stack.pop()
        w = z * y
        stack.append(w)  
      else:
        print("Stack error")
        break

  elif x == '/':
      if len(stack) >= 2:
        y = stack.pop()
        z = stack.pop()
        w = z / y
        stack.append(w)  
      else:
        print("Stack error")
        break

  elif x == '=':             # Equal operator
      if len(stack) == 1:
        z = stack.pop()      # Pop the result
        print(z)
        break

      else:
        print("Stack error")
        break