CS110 - Lecture 6

Review

global Statement
if, if-else, if-elif-else Statements
while Statement
Executing a block of code a fixed number of times
'Key' events and functions
Assignment 2

Lists

A List is a data structure that can holds a sequence of items
A list can hold any number of items, of any type
Syntactically, it is a comma-separated sequence surrounded by square brackets
```
myList = ["a string", 1.234, True]
```
Lists are mutable (they can be modified in code)
Use the len() function to determine the number of items in the list
```
>>> len( myList )
3
```

Lists can even contain other lists

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Lists

The elements of a list can be accessed using square brackets and an index number

Indexes are 0-based

>>> options = ["Create", "Read", "Update", "Delete"]
>>> options[2]
'Update'

>>> options[1] = "Select"
>>> options
['Create', 'Select', 'Update', 'Delete']

Lists

Recall how we use a while-loop to generate a series of numbers
```
i = 0
while i < 10:
    print(i)
    i = i + 1
```
How can we access all elements of a list?

Lists

How can we use a while-loop to access all elements of a list?

options = ["Create", "Read", "Update", "Delete"]

i = 0
while i < len(options):
    print( options[i] )
    i = i + 1

How can we do the same starting from the end of the list and proceeding to the beginning?

Lists

Lists provide a variety of functions to manipulate their members

Use the del keyword to delete a list element

>>> items = [0, 1, 2, 3, 4, 5]
>>> del items[2]
>>> items
[0, 1, 3, 4, 5]

Use the '+' operator to concatenate lists

>>> ["a", "b", "c"] + ["d", "e"]
['a', 'b', 'c', 'd', 'e']

Lists

Use the ':' operator to slice out subsets of a list

>>> items = [0, 1, 2, 3, 4, 5]

>>> items[1:3]  # elements from index 1 to 3,
[1, 2]          # including 1, excluding 3

>>> items[:3]   # elements up to but excluding 3
[0, 1, 2]

>>> items[1:]   # elements from index 1 to end
[1, 2, 3, 4, 5]

Lists

Use negative indexes to access list elements from the right (end)

>>> items = [0, 1, 2, 3, 4, 5]

>>> items[-1]       # Get the last element on the list   
5

>>> items[-2]       # Second-to-last element on the list 
4

>>> items[-3:]      # Third-to-last to the end 
[3, 4, 5]

List Methods

Access many list manipulation functions using dot-notation (just like modules)

L.append(x)	- add an item to the end of L
L.extend(M)	- append all items in M to the end of L
L.insert(i, x)	- insert x into L at position i
L.remove(x)	- remove the first occurance of x from L
L.pop(i)	- remove the item at position i and return it (or from end if no i)
L.index(x)	- find the index of the first item that matches x on L
L.count(x)	- return the number of times x occurs in L
L.sort()	- sort the items in L, in place
L.reverse()	- reverse the order of items in L, in place.

Sequence Types

A list is one of several sequence data types in Python

Another sequence type is a string, which implies...

>>> s = 'abcdefg'
>>> len(s)
6

>>> s[0]
'a'

>>> s[1]
'b'

>>> s[:3]
'abc'

>>> s[-3:]
'def'

Tuples

Similar to lists, only not mutable (cannot be changed programmatically)

Tuples use parentheses to construct

>>> consts = ('CENTER', 'CORNER', 'OTHER')
>>> consts[1]
'CORNER'

>>> consts[1:]
('CORNER', 'OTHER')

>>> del consts[0]
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: 'tuple' object doesn't support item deletion

More Iteration: for-in Statement

Use the for-in statement to loop over all elements of a sequence type (list, string, ...)

for variable in list :
    statement1
    statement2

>>> for op in ['tall', 'grande', 'venti']:
>>>    print( op + ' is an option' )

tall is an option
grande is an option
venti is an option

More Iteration: for-in Statement

The built-in range() function helps us with for-in statements

range(min, max)	- generates a list of numbers from min to max-1
range(max)	- generates a list of numbers from 0 to max-1
range(min, max, skip)	- generates a list of every skip number from min to max-1

>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> range(4, 10)
[4, 5, 6, 7, 8, 9]

>>> range(4, 10, 2)
[4, 6, 8]

More Iteration: for-in Statement

The built-in range() function helps us with for-in statements

sizes = ['tall', 'grande', 'venti']

for i in range( len(sizes) ):
    print( i, sizes[i] )

0 tall
1 grande
2 venti

Problem Solving with Loop Patterns

Generating series of numbers (series#.py)
Visiting each element in a sequence data type (seq#.py)
Making a matrix with lists of lists. Nested loops. (mat#.py)
Accumulating using a temporary variable (accum#.py)
Managing behavior with state variables (walker.py, lasso#.py, lineRotate2.py, twinkle.py)
Animation with a while-loop and delay() (randomEllipse2.py)
...
http://pythontutor.com ... Use it!