.. _dictionary-iteration:
Iterating Through Dictionaries
==============================
Many times in this book, we looped through the characters in a string or the
elements in a list without using the index values.
.. admonition:: Example
.. sourcecode:: python
:linenos:
my_string = 'Rutabagas!'
numbers = [33, -25, 3.14, 86, 1168, 42, 6.022e23]
for character in my_string:
print(character)
total = 0
for number in numbers:
total += number*2
Each time the loop body repeats, the loop variable (``character`` or
``number``) gets assigned the next value in the collection.
We can do something similar with dictionaries.
Loop by Keys or Values
----------------------
To loop through a dictionary, we need to specify whether to assign the loop
variable the keys or the values from the collection. To do this, we use the
``keys()`` or ``values()`` method in the ``for`` statement:
.. sourcecode:: python
:linenos:
for key in dictionary_name.keys():
# Loop body...
for value in dictionary_name.values():
# Loop body...
In the first ``for`` loop, each time the code repeats, the loop variable
``key`` gets assigned one of the key names from the dictionary. In the second
loop, the variable ``value`` gets assigned a new value each iteration.
.. admonition:: Note
We do not have to use the name ``key`` or ``value`` for the loop variable,
but doing so helps keep our code clear.
.. admonition:: Try It!
Run the following program to see how iterating through a dictionary works.
.. raw:: html
Try adding or removing key/value pairs to change the output.
Note that on lines 21 and 26, we access each value in the dictionary by using
bracket notation and the loop variable ``key``. Each iteration, ``key``
represents a new label from ``comics``, so ``comics[key]`` accesses a different
value each time the loop repeats.
.. admonition:: Try It!
What happens if we forget to attach the ``keys()`` or ``values()`` method in
the ``for`` statement? Let's find out!
Run the following program to see how Python deals with a missing method.
.. raw:: html
Aha! By default, Python iterates through a dictionary by using the key names.
.. _key-value-iteration:
Loop by Key/Value Pairs
^^^^^^^^^^^^^^^^^^^^^^^
The ``items()`` method returns each key/value pair as a unit, and it allows us
to assign BOTH the key and value from the dictionary to separate variables. The
general syntax for this is:
.. sourcecode:: python
for (key, value) in dictionary_name.items():
In the ``for`` statement, we define a pair of variables ``(key, value)`` to
hold a key name and its linked value from the dictionary. Each iteration, these
two variables represent a new key/value pair from the collection.
.. admonition:: Example
Compare the following two loops, which do exactly the same thing:
.. sourcecode:: python
:linenos:
comics = {
'Gary Larson' : 'The Far Side',
'Terri Libenson' : 'Pajama Diaries',
'Hilary B. Price' : 'Rhymes with Orange',
'Jim Toomey' : "Sherman's Lagoon"
}
# Iterate by keys, and print out the dictionary key/value pairs:
for key in comics.keys():
print(key, comics[key])
# Iterate by key/value pairs:
for (key, value) in comics.items():
print(key, value)
By defining a pair of variables, we can access the values from the
dictionary without needing to use bracket notation. On line 14, the variable
``value`` replaces ``comics[key]`` in our code.
Sorting by Keys
---------------
Dictionaries are *unordered* collections, so they do NOT include any type of
sorting method. However, sometimes we might want to access or display the
key/value pairs in a particular order---like alphabetically by key name.
If we want to sort a dictionary, the short answer is...we can't. However, we
can use a work-around. We won't change the order of the key/value pairs in the
dictionary. Instead, we will use a list, which can be sorted.
.. admonition:: Try It!
Let's see how to print out the key/value pairs in a dictionary
alphabetically by key name.
#. Run the program as-is. Notice that the loop prints the key/value pairs in
the order they occur in the collection.
#. On line 13, add the statement ``keys_list = list(grocery_bill.keys())``.
On line 14, print ``keys_list`` and run the program to see the result.
- Line 13 does NOT change the dictionary. Instead, the ``list()``
function creates a new list that contains copies of all the key names
from ``grocery_bill``.
#. On line 14, replace the ``print`` statement with ``keys_list.sort()``,
which alphabetizes the strings in the list.
- Note that line 14 sorts the *list* and NOT the dictionary!
#. Change the ``for`` statement to loop through ``keys_list`` instead of
``grocery_bill.keys()``. Run the program again so see the alphabetical
result.
.. raw:: html
Note that we do not need to change the final ``print`` statement. It still
uses the loop variable ``key``, but in this case it takes elements from the
list we created in line 13 and sorted in line 14.
``grocery_bill[key]`` still refers to a value from the dictionary, but now
we access the values in a different (sorted) order!
If we print the ``grocery_bill`` dictionary at the end of the program, we
can prove that we did NOT alter the order of the key/value paris.
By adding copies of the key names to a separate list, we can sort the list to
get the order we want. Then we use the key names *from the sorted list* to
access the values in the dictionary.
Check Your Understanding
------------------------
.. admonition:: Question
Given the code:
.. sourcecode:: python
:linenos:
comics = {
'Georgia Dunn' : 'Breaking Cat News',
'Jan Eliot' : 'Stone Soup',
'Wiley Miller' : 'Non Sequitur',
'Bill Watterson' : 'Calvin and Hobbs'
}
for key in comics.keys():
print(key, comics[key])
What is the value of ``comics[key]`` the *third* time through the loop?
.. raw:: html