Introductory Programming
Fall 2004

For next Tuesday:

1) finish homework 8

2) prepare for an evaluation on dictionaries.


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 types"