Exercise 40: Dictionaries, Oh Lovely Dictionaries

Now I have to hurt you with another container you can use, because once you learn this container a massive world of ultra-cool will be yours. It is the most useful container ever: the dictionary.

Python calls them "dicts", other languages call them, "hashes". I tend to use both names, but it doesn't matter. What does matter is what they do when compared to lists. You see, a list lets you do this:

>>> things = ['a', 'b', 'c', 'd']
>>> print things[1]
b
>>> things[1] = 'z'
>>> print things[1]
z
>>> print things
['a', 'z', 'c', 'd']
>>>

You can use numbers to "index" into a list, meaning you can use numbers to find out what's in lists. You should know this by now, but what a dict does is let you use anything, not just numbers. Yes, a dict associates one thing to another, no matter what it is. Take a look:

>>> stuff = {'name': 'Zed', 'age': 36, 'height': 6*12+2}
>>> print stuff['name']
Zed
>>> print stuff['age']
36
>>> print stuff['height']
74
>>> stuff['city'] = "San Francisco"
>>> print stuff['city']
San Francisco
>>>

You will see that instead of just numbers we're using strings to say what we want from the stuff dictionary. We can also put new things into the dictionary with strings. It doesn't have to be strings though, we can also do this:

>>> stuff[1] = "Wow"
>>> stuff[2] = "Neato"
>>> print stuff[1]
Wow
>>> print stuff[2]
Neato
>>> print stuff
{'city': 'San Francisco', 2: 'Neato',
    'name': 'Zed', 1: 'Wow', 'age': 36,
    'height': 74}
>>>

In this one I just used numbers. I could use anything. Well almost but just pretend you can use anything for now.

Of course, a dictionary that you can only put things in is pretty stupid, so here's how you delete things, with the del keyword:

>>> del stuff['city']
>>> del stuff[1]
>>> del stuff[2]
>>> stuff
{'name': 'Zed', 'age': 36, 'height': 74}
>>>

We'll now do an exercise that you must study very carefully. I want you to type this exercise in and try to understand what's going on. It is a very interesting exercise that will hopefully make a big light turn on in your head very soon.

cities = {'CA': 'San Francisco', 'MI': 'Detroit',
                     'FL': 'Jacksonville'}

cities['NY'] = 'New York'
cities['OR'] = 'Portland'

def find_city(themap, state):
    if state in themap:
        return themap[state]
    else:
        return "Not found."

# ok pay attention!
cities['_find'] = find_city

while True:
    print "State? (ENTER to quit)",
    state = raw_input("> ")

    if not state: break

    # this line is the most important ever! study!
    city_found = cities['_find'](cities, state)
    print city_found

Warning

Notice how I use themap instead of map? That's because Python has a function called map, so if you try to use that you can have problems later.

What You Should See

$ python ex40.py 
State? (ENTER to quit) > CA
San Francisco
State? (ENTER to quit) > FL
Jacksonville
State? (ENTER to quit) > O
Not found.
State? (ENTER to quit) > OR
Portland
State? (ENTER to quit) > VT
Not found.
State? (ENTER to quit) >

Extra Credit

1. Go find the Python documentation for dictionaries (a.k.a. dicts, dict) and try to do even more things to them.
2. Find out what you can't do with dictionaries. A big one is that they do not have order, so try playing with that.
3. Try doing a for-loop over them, and then try the items() function in a for-loop.