.. _random-numbers:

Random Numbers
==============


.. image:: dice_1.svg
    :width: 20%
    :class: right-image

Now if our games do the same thing each time the game runs, that's
not very interesting. At the end of this chapter we'll introduce
**random numbers**. That is, we can have the computer roll the
dice to change up the game each time we run it.

.. image:: dice_2.svg
    :width: 20%
    :class: right-image

Random numbers are heavily used in computer science for programs that involve
games or simulations.

The ``randrange`` Function
^^^^^^^^^^^^^^^^^^^^^^^^^^

By default, Python does not know how to make random numbers. It is necessary
to have Python import a code library that can create random numbers. So to use
random numbers, the first thing that should appear at the top of the program
is an import statement:

.. code-block:: python

    import random

Just like with pygame, it is important not to create a file with the same name
as what is being imported. Creating a file called random.py will cause Python
to start importing that file instead of the system library that creates random
numbers.

After this, random numbers can be created with the ``randrange`` function.
For example, this code creates random numbers from 0 to 49. By default the
lower bound is 0.

.. code-block:: python
    :caption: Random number from 0 to 49

    my_number = random.randrange(50)

The next code example generates random numbers from 100 to 200. Just like the
range function the second parameter specifies an upper-bound that is not
inclusive. Therefore if you want random numbers up to and including 200,
specify 201.

.. code-block:: python
    :caption: Random number from 100 to 200

    my_number = random.randrange(100, 201)

.. _random_chance:

Random Chance
^^^^^^^^^^^^^

Often in games there is a random chance of something happening. How do we program this? It isn't too hard.
Here's an example where we have a 1 in 5 chance of meeting a dragon:

.. code-block:: python
    :caption: Random Chance of Something Happening
    :linenos:

    import random


    # The line below will "roll the dice" 20 times.
    # Don't copy this 'for' loop into your program.
    # It is just here so we can try this example over and over.
    for i in range(20):

        # The line below will roll a random number 0-4.
        # If we roll a '0' then print that we encountered a dragon.
        if random.randrange(5) == 0:
            print("DRAGON!!!")
        else:
            print("No dragon.")

This code repeats twenty times. Inside the loop, we "roll the dice" and get a number between 0 and 4. If we
roll a 0, then we encounter a dragon. Otherwise we don't.

The ``random`` Function
^^^^^^^^^^^^^^^^^^^^^^^

All of the prior code generates integer numbers. If a floating point number
is desired, a programmer may use the ``random`` function.

The code below generates a random number from 0 to 1 such as
0.4355991106620656.

.. code-block:: python
    :caption: Random floating point number from 0 to 1

    my_number = random.random()

With some simple math, this number can be adjusted. For example, the code below generates a random floating point number between 10 and 15:

Random floating point number between 10 and 15

.. code-block:: python

    my_number = random.random() * 5 + 10

.. _number-guessing-game-code:

Number Guessing Game Example
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Here is the code for the number guessing game at the start of the chapter.

.. literalinclude:: number_guessing_game.py
    :emphasize-lines: 11-13, 22-24
    :language: python
    :linenos:


Mudball Example
^^^^^^^^^^^^^^^

This is a fun text-only game that two players can play. It uses a few concepts
we haven't covered yet.

.. literalinclude:: mudball.py
    :language: python
    :linenos:

Review
------

Online Coding Problems
^^^^^^^^^^^^^^^^^^^^^^

Practice on-line by completing the first set of coding problems available here:

https://repl.it/community/classrooms/174286

All problems beginning with ``04`` can be done with the knowledge from this
chapter.