Lists Are Like Strings
======================
Besides being ordered collections, Python lists share other similarities with strings.
List Length
-----------
.. index::
single: list; length
The ``len()`` function also returns the length of a list (the number of elements in the list).
.. admonition:: Example
.. sourcecode:: Python
:linenos:
letters = ['a', 'b', 'c', 'x', 'y', 'z']
print("The list {0} has {1} elements.".format(letters, len(letters))
**Console Output**
::
The list ['a', 'b', 'c', 'x', 'y', 'z'] has 6 elements.
In line 3, ``len(letters)`` returns the number of items stored in the
``letters`` list.
Note that the statement ``print(letters[len(letters)])`` will throw an *index
out of range* error. Since index values start at 0, the last element in any
list will always have a value of ``len(list_name) - 1``.
.. _concatenating-lists:
Combining Lists
---------------
Just like strings, we can use the ``+`` and ``*`` operators for *concatenation*
and *repetition*. Concatenation combines different lists to create one new,
longer list. Repetition makes multiple copies of the same elements within a
single list.
.. admonition:: Try It!
Experiment with using different operators on lists. Take notes about the
results you see.
#. Does order matter? Print ``first_list + second_list`` vs.
``second_list + first_list``, and then write down your answer.
#. Does the ``+`` operator change the original list? To check, print
``first_list`` again after your code from step 1.
#. Try printing ``first_list + 13`` vs. ``first_list + [13]``. What happens?
#. Can we subtract two lists? Try printing ``first_list - second_list``.
#. What does the ``*`` operator do to a list? Try printing
``second_list * 3``.
#. Does the ``*`` operator change the original list? To check, print
``second_list`` again after your code from step 5.
.. raw:: html
<iframe height="500px" width="100%" src="https://repl.it/@launchcode/LCHS-List-Concatentation?lite=true" scrolling="no" frameborder="yes" allowtransparency="true" allowfullscreen="true"></iframe>
``in`` and ``not in``
---------------------
Just like strings, we can use the ``in`` and ``not in`` operators to check if a
specific value is present in a list. The operators return ``True`` or ``False``
depending on if the value matches an element.
.. _in-not-in-editor:
.. admonition:: Try It!
Run this program to see the results returned by the ``in`` and ``not in``
operators.
.. raw:: html
<iframe height="500px" width="100%" src="https://repl.it/@launchcode/LCHS-in-and-not-in-List-Example?lite=true" scrolling="no" frameborder="no" allowtransparency="true" allowfullscreen="true"></iframe>
Note that even though the substring ``'nana'`` is present in ``'banana'``,
the result of line 5 is still ``False``. In this case, the ``in`` operator
checks if the string ``'nana'`` is its own element in ``fruit``. To check if
``'nana'`` is a smaller piece of each element requires more code.
.. _slicing-lists:
List Slices
-----------
.. index::
single: list; slice
Just like strings, we can return a *slice* (several elements) from a list.
Taking a slice creates a new list, and the syntax should be familiar:
.. sourcecode:: Python
list_name[start_index : end_index]
The new list contains the elements from ``start_index`` up to but NOT including
``end_index``. If we leave out ``start_index``, the slice starts at the
beginning of the list. If we leave out ``end_index``, the slice continues to
the end of the list.
The index values in the new list begin at 0.
.. admonition:: Example
.. sourcecode:: Python
:linenos:
original_list = [2, 4, 6, 8, 10, 12, 14]
new_list = original_list[2:5]
print(new_list, 'vs.', original_list)
print(new_list[0])
print(original_list[:3])
print(original_list[3:])
**Console Output**
::
[6, 8, 10] vs. [2, 4, 6, 8, 10, 12, 14]
6
[2, 4, 6]
[8, 10, 12, 14]
Try It!
^^^^^^^
In the :ref:`(in/not in) editor above <in-not-in-editor>`, add slices to check
only a portion of the ``fruit`` list (e.g. ``print("apple" in fruit[2:4])``).
Check Your Understanding
------------------------
.. admonition:: Question
What is printed by the following code?
.. sourcecode:: Python
:linenos:
a_list = [4, 2, 8, 6, 5, 4]
print(a_list[3])
.. raw:: html
<ol type="a">
<li><input type="radio" name="Q1" autocomplete="off" onclick="evaluateMC(name, false)"> 2</li>
<li><input type="radio" name="Q1" autocomplete="off" onclick="evaluateMC(name, false)"> 8</li>
<li><input type="radio" name="Q1" autocomplete="off" onclick="evaluateMC(name, true)"> 6</li>
<li><input type="radio" name="Q1" autocomplete="off" onclick="evaluateMC(name, false)"> 5</li>
</ol>
<p id="Q1"></p>
.. Answer = c
.. admonition:: Question
Given ``num_list = [8, 6, 7, 5, 3, 0, 9]``, what does ``num_list[2:5]``
return?
.. raw:: html
<ol type="a">
<li><input type="radio" name="Q2" autocomplete="off" onclick="evaluateMC(name, true)"> [7, 5, 3]</li>
<li><input type="radio" name="Q2" autocomplete="off" onclick="evaluateMC(name, false)"> [7, 5, 3, 0]</li>
<li><input type="radio" name="Q2" autocomplete="off" onclick="evaluateMC(name, false)"> [6, 7, 5]</li>
<li><input type="radio" name="Q2" autocomplete="off" onclick="evaluateMC(name, false)"> [6, 7, 5, 3]</li>
</ol>
<p id="Q2"></p>
.. Answer = a
.. admonition:: Question
What is printed by the following code?
.. sourcecode:: Python
:linenos:
a_list = [4, 2, 8]
print(a_list * 2)
.. raw:: html
<ol type="a">
<li><input type="radio" name="Q3" autocomplete="off" onclick="evaluateMC(name, false)"> [4, 4, 2, 2, 8, 8]</li>
<li><input type="radio" name="Q3" autocomplete="off" onclick="evaluateMC(name, true)"> [4, 2, 8, 4, 2, 8]</li>
<li><input type="radio" name="Q3" autocomplete="off" onclick="evaluateMC(name, false)"> [8, 4, 16]</li>
<li><input type="radio" name="Q3" autocomplete="off" onclick="evaluateMC(name, false)"> The code throws an error.</li>
</ol>
<p id="Q3"></p>
.. Answer = b
.. admonition:: Question
Given ``fruit = ["apple", "orange", "banana", "cherry", "tomato", "bell pepper"]``,
which of the following statements return ``True``? Select ALL that apply.
.. raw:: html
<ol type="a">
<li><span id = "a" onclick="highlight('a', false)">apples in fruit</span></li>
<li><span id = "b" onclick="highlight('b', false)">pepper in fruit</span></li>
<li><span id = "c" onclick="highlight('c', true)">banana in fruit[:3]</span></li>
<li><span id = "d" onclick="highlight('d', false)">tomato in fruit[1:4]</span></li>
<li><span id = "e" onclick="highlight('e', true)">broccoli not in fruit</span></li>
<li><span id = "f" onclick="highlight('f', true)">orange not in fruit[2:]</span></li>
</ol>
.. Answers = c, e, f