Hello World! An Invitation to Computer Science

Lecture Slides

Lists in Python
Monday April 14, 2008

Key ideas from last time

• strings: sequences of characters
• characters (for most part) correspond to keyboard symbols
• operations on strings:
    len, chr, ord
    random access via subscript as in s[i]
    slices as in s[2:4]
    concatenation as in 'hello'+'nwheels'
• traversal via for loop

Lists: like strings, only more general

• len operation
• slices
• concatenation
• random access via subscript
• traversal via for loop
• lists can be:
  

    []                      # empty
    [3, "hello", False]     # heterogeneous
    [[1,"hi"], [2, "bye"]]  # nested
  

• example: sum over list

    sum = 0
    for x in my_numbers:
        sum += x
  

Membership: in and not in

a form of sequential search is primitive in Python:

  >>> my_list = [3, 1, 99, 8]
  >>> 99 in my_list
  True
  >>> 5 in my_list
  False
  >>> 5 not in my_list
  True

Ranges are lists

  >>> range(10)
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
  >>> range(1,5)
  [1, 2, 3, 4]
  >>> range(20, 1, -5)
  [20, 15, 10, 5]
  >>> range(3, 10, 2)
  [3, 5, 7, 9]

Mutability

• mutable object may change as side effect
• one of more subtler subjects in programming
• variable with value of list points
    to where list is stored in memory

>>> a = 'hello'
>>> a
'hello'
>>> b = a
>>> b
'hello'
>>> b = 'bye'
>>> b
'bye'
>>> a
'hello'

>>> my_list = [2, 9, 1]
>>> my_list
[2, 9, 1]
>>> your_list = my_list
>>> your_list
[2, 9, 1]
>>> your_list[2] = 63
>>> your_list
[2, 9, 63]
>>> my_list
[2, 9, 63]

Selection sort revisited

def find_largest(a_list, n):
  max_pos = 0
  i = 1
  while i < n:
      if a_list[i] > a_list[max_pos]:
          max_pos = i
      i += 1
  return max_pos


def exchange(a_list, i, j):
    temp = a_list[i]
    a_list[i] = a_list[j]
    a_list[j] = temp


def move_largest(a_list, n):
    max_pos = find_largest(a_list, n)
    if max_pos != n-1:
        exchange(a_list, max_pos, n-1)


def selection_sort(a_list):
    n = len(a_list)
    while n > 1:
        move_largest(a_list, n)
        n -= 1


>>> my_list = [17, 5, 94, 3, 1, 6]
>>> selection_sort(my_list)
>>> my_list
[1, 3, 5, 6, 17, 94]