Nov 15
for the good of the order
Questions about anything?
- Assignments through the end of the term are posted.
- I'm going to go light on homework to give you time to work on projects - please do use that time.
old business : projects ...
What are you thinking of doing ?
new business : chap 12 material
data structures
We've talked about lists (and the similar tuples) and dictionaries.
These really become powerful when you start embedding them :
- build up tree-like data structures.
- often mixed types of content
- often embedded within class or classes
Look at some examples :
- list of lists :
- matrix
- board for game
- list of students, list of grades for each student
- list of dicts
- list of students, name/phone/address by key for each
- dict of lists
- hand of cards; keys by suits, each a list of cards
- dict of dicts
- keys north,south,east,west; dict scene description for each
Specific example: the ATM machine (in the Department of Reduncy of Reduncies)
objects might be something like
GUI - all interaction with user
Data - file input/output
Person - name, password
Account - each pot of money, e.g. checking, savings
Transaction - deposit or withdrawal or transfer with time
The Data object could put the information could be stored
in a single python data structure,
stored as a string in a file with pprint
read in as a string and then eval() to put it into a variable,
which could then be converted to, say, a list of Persons,
each with a list of history Transactions and several Accounts.
The storage file might look like this :
[{'name': 'John Smith', 'password':'...',
'accounts': [
{'name':'checking', 'balance': 120.23 # or store as pennies},
{'name':'savings', 'balance':4000.00
],
'transactions': [
{'date':..., 'amount':..., 'from':..., 'to':...},
{...},
]
},
{ # similar for another person }
]
The the top level of the program might be something like
storage = Data('stored_data.txt')
people = storage.readPeople()
gui = GUI(people)
while True:
person = gui.login() # ask for name/pass; return user if match
while person.transaction():
storage.save() # update disk file
where several of these objects contain the others, and would call methods on some of the others to get the work done.
I'm imagining that Data contains a list of people, and so can use that to get a string to send to the file.
Likewise, each person contains the gui, so person.transaction() would
- use gui to ask about what to do,
- create a Transaction object and append it
- update the person's balances, and
- return False if user is done.
start recursion (depending on time)
After you finish reading chapter 12 ...
... then read chapter 13 : algorithms & recursion! (Cool stuff ...)
"""
an example of counting using recursion.
$ python count_recursion.py
How many times? 5
1
2
3
4
5
Done.
Exercise: write this without
using any of the normal python
looping constructs, "for", "while",
those guys.
"""
count = input("How many times? ")
def loop(i):
""" print one line of the count """
print i
if i < count:
loop(i + 1)
loop(1)
Look at this with the "visualize python" site (see the resources page) to see the allocated memory for each recursive call.
Try with count=2000 to see Python's recursive depth limit. (Python isn't very good at recursion ...)
recursive data structures
If time allows : talk about recursive object structures, i.e. a Person object that contains a list of children ... which are also Person objects.
This is powerful way to represent graphs, and allow recursive functions to traverse the graph. We explore these sorts of things in the "Algorithms" course.
This is tricky stuff - we'll see if we can make sense of what's going on.
