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).

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, as in the following illustration:

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

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