Calculator

Whiteboard

Harder

Challenge

Medium

Concepts

Stacks

Download

calculator.zip

Solution

Calculator: Solution

In this exercise, you’ll build a “polish notation calculator”.

Polish notation is a different way to write an artithmetic expression. For example, instead of writing 1 + 2 * 3, as we would in normal (“infix”) style, we could write it with the operators to the left of their arguments. This expression would become + 1 * 2 3. You can read a polish notation expression backwards to see exactly what it does — in this case, multiply 2 times 3, and add that result to 1.

Let’s try this out:

>>> calc("+ 1 2")  # 1 + 2
3

>>> calc("* 2 + 1 2")  # 2 * (1 + 2)
6

>>> calc("+ 9 * 2 3")   # 9 + (2 * 3)
15

Let’s make sure we have non-commutative operators (subtraction and division) working:

>>> calc("- 1 2")  # 1 - 2
-1

>>> calc("- 9 * 2 3")  # 9 - (2 * 3)
3

>>> calc("/ 6 - 4 2")  # 6 / (4 - 2)
3

We’ve given you a file, calculator.py, with a stub function:

def calc(s):
    """Evaluate expression."""

Implement this.

Hint