Reasoning about Code

Think about the following pieces of code and diagnose them on paper rather than simply typing them into the interpreter.

Dictionaries

A dictionary is an unordered collection of values each with distinct keys. The key-value pair stored within the map is called an item.

• The key is a distinct immutable object that represents the item as a whole.
• The value is any object.

Dictionaries might be used to associate student IDs with complete student records, or words to their definitions (like a real dictionary). What other examples of maps can you think of?

Files of Words

Look at these blocks of code that use a dictionary to organize information about words in a file. The first was discussed in class. For the rest, explain both the type and purpose of the key and its associated value — what questions could you answer with the words organized in this way?

Dictionaries in Python

In Python a dictionary associates any object with any other, like a list associates a number, or index, with the value it stores. For example, the code below

d = { 'a':1, 'b':2, 'c':3 }
print len(d), d

will generate the output

3, { 'a':1, 'b':2, 'c':3 }

Additionally items can be modified using the standard bracket operators for indexing, [],

d['a'] = d['b'] * 2
d['b'] += 1
d['z'] = 12
print d

will generate the output

{ 'a':4, 'b':3, 'c':3, 'z':12 }