Watch: Building a Reverse Polish Notation (RPN) Evaluator in Python

Image credit: Author


  1. Reverse Polish Notation (RPN)
  2. Example RPN expressions
  3. Source code
  4. Code walk-through

Reverse Polish Notation (RPN)

Reverse Polish notation (also known as postfix notation) is one of multiple notations for representing mathematical expressions. Most of us are familiar with infix notation, but there are also the less popular prefix and postfix notations.

The most obvious difference among these lie in the position of operators in the expression.

  • Infix: operators are “in” between the operands
  • Prefix: operators are before (pre) the operands
  • Postfix: operators are after (post) the operands

Other differences include the significance of space as a separator (prefix and postfix are space sensitive while infix is not) and the way order of operations is expressed. The order of operation is inherent in the expression structure in prefix and postfix notations while the infix notation requires the use of parentheses to make the order of operations explicit.

# Ambiguity in order of operations for infix notation
1 + 2 + 3
(1 + 2) + 3
1 + (2 + 3)
# No ambiguity in prefix and postfix (using postfix as an example)
# The structure tells us that 1 and 2 are added first, then 3
1 2 + 3 +

Example RPN expressions

# Example 11 10 30 + *
1 40 *
# Example 21 2 + 3 +
3 3 +
# Example 31 2 - 2 +
-1 2 +
# Example 41 2 /

Source code

Python script for evaluating expressions in Reverse Polish Notation

Code walk-through

Watch the video here

Thank you for watching. Please LIKE and SUBSCRIBE to show your support and watch more videos like this one.

Videos you should watch next

More Computing resources

Watch videos covering a variety of topics in Computing at




Software Engineer — Data & Machine Learning

Love podcasts or audiobooks? Learn on the go with our new app.

Recommended from Medium

How long does it take to learn Python?

Career Trends in Web Development

Create a local YUM repository from an ISO file — CentOS

Upcoming Features of iOS 15 to Look Out for in 2021

7 Steps to Software Development Success

How to Validate an Email Address in C/C++

Production grade Azure Logic App deployment using ARM templates: A comprehensive guide

How to Convert a Video File to GIF in Go

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store
Onel Harrison

Onel Harrison

Software Engineer — Data & Machine Learning

More from Medium


Programming Methods: Recursion

Sudoku Solver with Python : a methodical approach for algorithm optimization[part 3]

Sudoku Solution via iterations

Training vs Validation vs Testing in Machine Learning

Training vs Testing vs Validation- Whats the difference?