1. Python – May 18 Quiz Relatives of the list: Tuple Dictionary Set
We’ll do more with dictionaries later

2. Lab notes
Interactive run – hard to tell when program’s output begins and ends – what can you do?
Review Battleship
How would you change to handle real game?

3. Sorting
Often we want to sort data in a list, but our list does not contain atoms like single numbers.
Need to tell Python how to compare elements
Analogous to comparators in Java.
Steps:
Create function, taking 2 arbitrary elements from your list. Return positive / negative / zero.
Call: list.sort(comparatorFunction)
See example handout (compare.py)

4. Tuple Similar to list: use ( ) instead of [ ]
Good for identifying a point in some space, like an ordered pair, triple
Often an anonymous object
Immutable – not meant to be updated, just throw away
We just have count & index functions
Syntax is straightforward: multiple assignment
Can re-use variables later
(a, b, c) = (10, 8, 2)

5. Dictionary A nice array
Index can be anything that Python can easily evaluate, such as a single int, float or string.
Typical procedure:
Initialize as { }
Add element by assignment, e.g. d[“USA”] = 308
Can traverse dictionary elegantly
for i in d:
print i, d[ i ]

6. Notes
In a dictionary, like a set, the order of the data is irrelevant. The “key” is already an index.
Example:
{ “Finland”: 5, “India”: 1150, “USA”, 308, “France” : 61 }
Don’t rely on Finland being at the “beginning” of the dictionary. The value 5 is obtained by d[“Finland”], not d[0] !
Python can quickly find your data in a dictionary 

7. Illustration “Kevin” in d  returns True “James” in d  returns False
d[“Tina”] = 3
d[“Kevin”] = 2
“Kevin” in d  returns True
“James” in d  returns False
d [“James”] gives a KeyError, so when in doubt, check to see that key actually exists!

8. Applications
Good for sparse array: only store the values you actually use.
Here, the value at each key can be a list:
f [1920] = [90, 230]
f [1920][0] = 90
1920
1951
1960
1986
2000
2010 90
150
650
6656
18768
37728
230
700
750
3176
5144
9572

9. Applications (2) Excellent for maintaining data
E.g. Stock portfolio: reading a list of buy/sell transactions. At any point in time we may want to know total holdings
Can remove an element from a dictionary using del:
del portfolio[“IBM”]
del enemy[“Soviet Union”]

10. Set
Essentially, a set is a list in which the elements do not repeat. (See chapter 7)
E.g. Useful when you need a lot of boolean values
Can convert a list to a set by using set( ).
s = set([1, 3, 5, 7, 9])
If you call set( ) on a string, you get a set of its characters!
Operations
in not in
& | ^
Note that you can’t use ~