Evaluate Postfix Expression Using Stack in DSA Python - Build from Scratch

Evaluate Postfix Expression Using Stack

📖 Scenario: You are building a simple calculator that reads a postfix expression and calculates the result. Postfix expressions have operators after their operands, like 23+ which means 2 + 3.Using a stack helps to solve this easily by pushing numbers and applying operators step-by-step.

🎯 Goal: Build a program that uses a stack to evaluate a postfix expression and prints the final result.

📋 What You'll Learn

Create a stack as a list

Use a loop to process each character in the postfix expression

Push numbers onto the stack

Pop two numbers when an operator is found and apply the operator

Push the result back to the stack

Print the final result from the stack

💡 Why This Matters

🌍 Real World

Postfix expressions are used in calculators and computer programs to simplify expression evaluation without parentheses.

💼 Career

Understanding stack-based expression evaluation is important for programming, compiler design, and technical interviews.

1

Create the postfix expression string

Create a variable called postfix_expr and set it to the string '231*+9-' which represents the postfix expression.

DSA Python

# Create the postfix expression string
# Your code here

Hint

Postfix expressions are strings like '231*+9-'. Just assign it to postfix_expr.

2

Create an empty stack list

Create an empty list called stack to use as the stack for evaluation.

DSA Python

postfix_expr = '231*+'
# Create an empty list called stack
# Your code here

Hint

Use stack = [] to create an empty list for the stack.

3

Process each character and evaluate using stack

Write a for loop using char to go through each character in postfix_expr. Inside the loop, if char is a digit, convert it to int and append it to stack. If char is an operator (+, -, *, /), pop the top two numbers from stack, apply the operator, and push the result back to stack. Use if and elif to check operators.

DSA Python

postfix_expr = '231*+9-'
stack = []
# Loop through each character in postfix_expr
# Your code here

Hint

Use char.isdigit() to check digits. Use stack.pop() to get last two numbers. Apply operator and push result back.

4

Print the final result from the stack

Print the last remaining value in stack which is the result of the postfix expression evaluation.

DSA Python

postfix_expr = '231*+9-'
stack = []
for char in postfix_expr:
    if char.isdigit():
        stack.append(int(char))
    elif char == '+':
        b = stack.pop()
        a = stack.pop()
        stack.append(a + b)
    elif char == '-':
        b = stack.pop()
        a = stack.pop()
        stack.append(a - b)
    elif char == '*':
        b = stack.pop()
        a = stack.pop()
        stack.append(a * b)
    elif char == '/':
        b = stack.pop()
        a = stack.pop()
        stack.append(a // b)
# Print the final result
# Your code here

Hint

The final result is the last item in the stack. Use print(stack[-1]).