CS 2120: Topic 7

Videos for this week:

A note on comments

• Helpful for others who are reading your code (they make it easier to read and understand)

• Helpful for you when you revisit your own code (i.e. when you return to a project a month later)

• You can add comments to your code in Python with #:

  # we can comment above
  my_car = 5 #or to the right
  

• Once Python sees # it ignores the rest of the current line

• Don’t just repeat what’s obvious from reading the code… provide a higher level description of what’s happening.

Python docstrings

• These are comments for functions (also other things like methods, classes, etc.)

• Used immediately after the function definition:

  def set_up_cities(names):
     """
     Set up a collection of cities (world) for our simulator.
     Each city is a 3 element list, and our world will be a list of cities.
  
     :param names: A list with the names of the cities in the world.
  
     :return: a list of cities
     """
  
     function code here
  

• Explain what the function is going to do, in plain English.

• Also explain every parameter and what the function returns

More on strings

• Question: How is a string different from the other data types we’ve seen (int, float, bool, etc.)?

• Answer: We can print individual characters of a string by indexing the string:

  >>> a='Boomboombobo'
  >>> print(a[0])
  B
  >>> print(a[1])
  o
  

• Note that in Python, the first index is 0, not 1

• What does print(a[0:4]) do?

  >>> print(a[0:4])
  Boom
  

• This process is called slicing.

More on loops

• while loops are great, but they rely on a boolean condition returning false (i.e. while condition is true, do something):

  while(mouse_not_yet_clicked = True):
          print("Please click the mouse")
  

• for loops allow for iteration through a specified range or over elements in a list (among others):

  # one way you may use a for loop
  for i in range(len(a)):
     print(a[i])
  

• for i from 0 until the length of the string a, print the ith element of a:

  # another way you may use a for loop
  for char in a:
     print(char)
  

• for each character in the string a, we run the indented code block.

• Yes, you could create a while loop which does exactly the same thing… the for loop is just cleaner here (less typing).

Mutability

• We can access an individual character in a string with an index (i.e. a[0])… can we set an individual character by index as well?

• Let’s try:

  >>> a[7]='x'
  Traceback (most recent call last):
  line 4, in <module>
  a[7] = 'x'
  TypeError: 'str' object does not support item assignment
  

• Variables of some types are mutable… i.e. they can be changed.

• Strings are immutable — you can’t change a string, you have to make a new one:

  >>> new_a = a[:7] + 'x' + a[8:]
  >>> print(new_a)
  Boombooxbobo
  

Python Lists

• You’ve seen these in Assignment 1, and maybe you made your own in Activity 2

• What is it? An ordered and changeable collection of values or elements

• Lists are like strings, except that the elements can be any type (not just characters):

>>> myList = []
>>> myList.append(5)
>>> print(myList)
[5]
>>> myList.append(10)
>>> print(myList)
[5,10]
>>> print(myList[0])
5

• We can add to the end of lists with append

• Like strings, we can access the elements of a list with myList[i], where i is the list’s index (starts from 0 for the first element)

For next class

• Read chapter 9 of the text

• Take a look at Assignment 2 (due October 30th)