Introductory Programming
Fall 2004

Dictionaries
------------

Make an empty dictionary:

d = {}
print d
print type(d)


Add an entry:

d['a'] = 1
print d

The string 'a' is a key
The integer 1 is a value.

Dictionaries map keys onto values.

Add another entry:

d['b'] = 2
print d

Now we can use a key to look up the corresponding value.

print d['a']

Add another entry, and then loop through the dictionary:

d['c'] = 1
for key in d:
    print d

applying a for loop to a dictionary loops through the keys

for key in d
    print d[key]

This loops through the keys, looks up each one, and prints
the corresponding values.

Notice that the entries in a dictionary are not in any
particular order.


Dictionary methods
------------------

There are a number of methods you can invoke on dictionaries.

Methods are functions that you invoke on objects using dot notation.

print d.keys()     # prints a list of keys

print d.values()   # print a list of values

print d.items()    # print a list of tuples, where each tuple contains
                   # a key-value pair

Other dictionary methods and operators are described at:

http://www.python.org/doc/current/lib/typesmapping.html

Notice that a dictionary is a kind of "mapping type"


Homework 8 Solutions
--------------------

First challenge: modify the program to count the number of times
each word appears:

        for word in t:
            d[word] = d.get(word, 0) + 1

This is a standard idiom for updating a dictionary.

Second challenge: print the 10 most common words

def print_most_common(d):
# Print the ten most common words in the dictionary.
# Make a list of tuples, where each tuple contains
# a frequency and a word.  Sort them in decreasing
# order of frequency and print the first ten.
    t = []
    for key in d:
        pair = (d[key], key)
        t.append(pair)
    t.sort()
    t.reverse()
    for i in range(10):
        print t[i]

This code builds a list of tuples and sorts it.

Sometimes called the DSU pattern, for "decorate, sort, undecorate"