List Basics
===========
.. index:: ! list, ! element
The Python **list** data type is another example of an *ordered collection*.
Lists store data values, which are called **elements**. Just like with strings, each
element has its own index value. However, strings are ordered collections of *characters* and the elements of lists can be of any data type.
.. figure:: figures/list-diagram.png
:alt: A label, languages, pointing to an array that contains "Python" at index 0, "C#" at index 1, "Java" at index 2, and "JavaScript" at index 3.
This list contains four strings, and each value has its own index position.
Create a New List
-----------------
There are several ways to create a new list. The simplest is to enclose the
elements in square brackets ``[]``, with each element separated from the others
by commas.
.. admonition:: Example
.. sourcecode:: Python
:linenos:
numbers = [5, 15, 15, 0, 25]
strings = ['banana', 'broccoli', 'kale', 'applesauce']
mixed_data = ["Hello", 42, True, 3.14, [-3, 48.5]]
empty_list = []
.. index::
single: list; empty
#. Line 1 assigns a list of five integers to the variable ``numbers``.
#. Line 2 assigns a list of four strings.
#. The elements of a list don’t have to be the same data type! The list in line
3 contains a string, an integer, a boolean, a float, and another list.
#. Line 4 assigns a special list that contains no elements, called the
**empty list**.
.. admonition:: Note
A list within another list is said to be *nested*. We will explore this idea
later in the chapter.
Accessing Elements
------------------
With strings, we accessed individual characters by using square brackets. With lists, we use square brackets to access list elements. The
integer or expression inside the brackets gives the *index* for the element we
want.
Remember that index values start at 0.
Any integer (or an expression that returns a whole number) can be used as the
index. Negative index values identify elements from right-to-left, beginning
at ``-1`` for the last element in the list.
.. admonition:: Try It!
Use index values to print out different elements from a list.
#. Add at least three new elements to ``my_list``. Feel free to use values
of the same data type or different data types.
#. Note that line 4 prints the entire list (with brackets) to the console.
#. Change the index value in line 5 to print different elements from
``my_list``. Be sure to try both positive and negative integers.
#. Try using an expression (like ``len(my_list) - 2``) inside the brackets
in line 5.
#. What happens if you use an index value larger than the number of elements
in the list?
.. raw:: html
Check Your Understanding
------------------------
.. admonition:: Question
List elements must all be the same data type.
.. raw:: html
True
False
.. Answer = b
.. admonition:: Question
Identify the length of these two lists. (The answers list ``classes`` first,
then ``teachers``).
.. sourcecode:: Python
:linenos:
classes = ["Chemistry, US History, Intro To Coding"]
teachers = ["Cortez", "Holmes", "Bracey"]
.. raw:: html
1 and 3
3 and 1
3 and 3
1 and 1
.. Answer = a
.. admonition:: Question
Identify the output from the following statements:
.. sourcecode:: Python
:linenos:
a_list = ["Hello", 42, True, 3.14]
print(a_list[2])
.. raw:: html