Basic Math
----------

Let's start by using Python like a calculator.   Python has an interactive mode called the `REPL <https://docs.python.org/3/tutorial/interpreter.html#interactive-mode>`_ (Read Evaluate Print Loop).

You can invoke the REPL from the command line by simply typing:

.. code ::

 > python
 Python 3.9.5 (tags/v3.9.5:0a7dcbd, May  3 2021, 17:27:52) [MSC v.1928 64 bit (AMD64)] on win32
 Type "help", "copyright", "credits" or "license" for more information.

Now whatever you type after the new prompt (>>>) will be executed interactivately by the Python interpreter.   We will use this triple bracket prompt (>>>) in the code examples in the book to indicate that the example was executed in the REPL environment.   A single bracket (>) indicates that we executed the command from the command line. 

Type in 2 + 2 and enter::

 >>> 2 + 2
 4

More examples:

 >>> 999 * 999
 998001
 >>> 1234 - 4321
 -3087
 >>> 1 / 3
 0.3333333333333333

For exponentiation use \*\* like this::

 >>> 2 ** 10
 1024
 >>> 2 ** (-3)
 0.125

For `floor division <https://docs.python.org/3/glossary.html#term-floor-division>`_ (drops the fractional part):

.. code ::
>>> 20 // 3
6
>>> 1 // 3
0

An interesting feature of Python is that integer operations have arbitrary precision::

 >>> 2 ** 200
 1606938044258990275541962092341162602522202993782792835301376


If you divide by zero, you will get an error that looks like this:

.. code ::
>>> 10 / 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

For `modular arthmetic <https://en.wikipedia.org/wiki/Modular_arithmetic>`_, use the `%` symbol:

.. code ::
>>> 27 % 12
3
>>> 11 % 2
1
>>> 91 % 13
0