Cambridge-ICCS / ss23-git-3

Repository for the GitHub talk in ICCS Summer School '23.

Reverse Polish notation calculator in Python

Reverse Polish notation (RPN) is a postfix expression notation. Unlike the infix notation taught in schools, RPN defines an unambiguous evaluation strategy without brackets. This makes it especially simple to implement an RPN calculator -- all you need is a small amount of parsing code, and a stack (a first in, first out data structure).

Usage

From the rpn-calc/src directory:

python rpn_calc.py

Expressions are read from stdin. Input strings are broken into space-separated tokens.

• Binary operators: +, -, *, /
• Special functions: p (pops number on top of stack and prints)
• Numbers are parsed as floats.

At input end, the top of the stack is popped and printed.

Binary operators work by popping two arguments off the stack, running a calculation, and pushing the result. The top of the stack becomes the left operand, and the element one below the top becomes the right operand. See below for an example.

Example

The input string 1 2 + processed as follows:

• 1: push 1
• 2: push 2
• +: pop 2, pop 1, push 2 + 1

Here's an example shell session:

$ cd rpn-calc/src
$ python rpn_calc.py << EOM
1 2 +
EOM
3.0

Installation (including pytest for testing)

git clone https://github.com/Cambridge-ICCS/ss23-git-2.git
cd ss23-git
python -m venv venv
source venv/bin/activate
pip install pytest
cd rpn-calc/src