Software Design
Spring 2007

For today you should have:

1) read Chapter 10

2) worked on Homework 4

3) prepared for a quiz


For Monday you should:

1) read Chapter 11

2) work on Homework 4


Lists
-----

Ways to create a list.

1) bracket operator:  [], [1], [1, 2, 3]

2) list function:  list('allen')

   this is why I don't recommend using list as a variable name

3) other functions that return lists: range(3)

4) slice operator:  t[1:3],  t[:]


Indexing, slicing and len() are the same as for strings.

The in operator works with lists.

So do + and *

The for loop works with lists:

for var in t:
    print var

In this context, var is called a "loop variable"

You can use any legal identifier for the loop variable.


List vocabulary
---------------

compound object:

mutable:

index:

element:

slice:


Lists are mutable
-----------------

You can use the [] operator on the left side of an assignment:

t = range(3)
t[1] = 17
print t

Now it really matters when two variables refer to the
same list.

Run:  python lumpy_test.py


Modify lumpy_test.py with this code:

t1 = range(3)
t2 = t1
t2[1] = 17
print t1

And then this:

t1 = range(3)
t2 = t1[:]
t2[1] = 17
print t1



the is operator
---------------

Try this:

t1 = range(3)
t2 = t1[:]
print t1 == t2
print t1 is t2

These lists are equal but not the same!

print id(t1), id(t2)


identity (shallow equality):    same object
equivalence (deep equality):    same value



lists as parameters
-------------------

When lists are passed as arguments, they are passed by reference,
which means that the parent function and the child function refer
to the same object.

Draw a stack diagram for this program:

def deleteHead(t):
    del t[0]

x = range(3)
deleteHead(x)
print x

x and t are aliases for the same list object.

changes made in deleteHead are reflected in __main__ (as intended)


Now draw a stack diagram for this one:

def badDeleteHead(t):
    t = t[1:]
    print t

x = range(3)
badDeleteHead(x)
print x