CS30 - Spring 2016 - Class 5

Example code in this lecture

   while.py
   scores-lists.py

Lecture notes

  • admin
       - assignment 2 out

  • isprime
       - recall that last time we wrote a function called isprime (see while.py code)
          - take as input a number and returns a boolean (True or False) whether or not the number is a prime

          >>> isprime(5)
          True
          >>> isprime(6)
          False
          >>> isprime(100)
          False
          >>> isprime(101)
          True

       - how could we use this to print out the first 10 (100, 1000, etc) prime numbers?
          - like to do some sort of loop
          - will a for loop work?
             - we don't know when we're going to stop
             - we'd like to keep a count of how many we've seen and only stop when we've reached the number we want

  • while loop
       - another way to do repetition

       while <bool expression>:
          statement1
          statement2
          ...

       statement3

       as long as the <bool expression> evaluates to True, it continues to repeat the statements, when it becomes False, it then continues on and executes statement3, etc.

       - specifically:
          evaluates the boolean expression
             - if it's False
                - it "exits" the loop and goes on to statement3 and continues there
             - if it's True
                - executes statement1, statement2, ... (all statements inside the "block" of the loop, just like a for loop)
             - go back to beginning and repeat
       
       - how could we use a while loop for our prime numbers problem?
          - keep a count of how many primes we've found (initially starts at 0)
          - start count from 1 and work our way up
          - check each number
          - if it's prime
             - print it out
             - increment the counter of how many primes we've found
          - keep repeating this as long as (while) the number of primes we've printed is less than the number we want

  • can you emulate a for loop with a while loop?
       - yes!

       for i in range(10):
          ...

       is equivalent to writing:

       i = 0

       while i < 10:
          ...
          i = i + 1

  • look at firstprimes function in while.py code
       - current += 1 every time through the loop we increment the number we're examining
       - if that current number happens to be prime, we increment count
       - the loop continues "while" count < num, that is as long as the number we've found is less than the number we're looking for

  • infinite loops
       - what would the following code do?

       while True:
          print "hello"

       - will never stop
          - in this case you should see some output
          - sometimes, it will look like the program just froze if you're not actually printing anything out
       - you can stop this by selecting "reset shell"
       - be careful about these with your program. They're called an infinite loop.
       - if you think you might have an infinite loop
          - put in some print statements to debug
          - think about when the boolean expression will become False and make sure that is going to happen in your loop

  • run scores-lists.py code
       - First, prompts the user to enter a list of scores one at a time
          - how is this done?
             - while loop
             - what is the exit condition?
                - checks to see if the line is empty

                   while line != ""
                
       - then, calculate various statistics based on what was entered
       - how are we calculating these statistics?
          - average?
             - could keep track of the sum and the number of things entered
             - divide at the end
          - max?
             - keep track of the largest seen so far
             - each time a new one is entered, see if it's larger, if so, update the largest
          - min?
             - same thing
          - median?
             - the challenge with median is that we can't calculate it until we have all of the scores
             - need to sort them and then find the middle score

       - why can't we do this using int/float variables?
          - we don't know how many scores are going to be entered
          - even if we did, if we had 100 students in the class, we'd need 100 variables!

  • lists
       - lists are a data structure in Python
          - what is a data structure?
             - a way of storing and organizing data

       - lists allow us to store multiple values with a single variable

  • creating lists: we can create a new list using square brackets
       >>> [7, 4, 3, 6, 1, 2]
       [7, 4, 3, 6, 1, 2]
       >>> 10 # not a list
       10
       >>> [10]
       [10]
       >>> l = [7, 4, 3, 6, 1, 2]
       >>> l
       [7, 4, 3, 6, 1, 2]
       >>> type(l)
       <type 'list'>
          
       lists are a type and represent a value, just like floats, ints, bools and strings. We can assign them to variables, print them, etc.

       - what do you think [] represents?
          - empty list
          >>> []
          []
       
  • accessing lists
       - we can get at particular values in the list by using the [] to "index" into the list
          >>> l = [7, 4, 3, 6, 1, 2]
          >>> l[3]
          6

          notice that indexing starts counting at 0, not at 1!

          >>> l[0]
          7

       - What do you think l[20] will give us?
          >>> l[20]
          Traceback (most recent call last):
           File "<string>", line 1, in <fragment>
          IndexError: list index out of range

          we can only index from 0 up to the length of the list minus 1

       - What do you think l[-1] will give us?
          >>> l[-1]
          2

          if the index is negative it counts back from the end of the list

       - notice that the type thing in the list is as you'd expect:      
          >>> type(l[3])
          <type 'int'>

  • storing other things in lists
       - draw the list representation
       - a list is a contiguous set of spaces in memory
       - we can store anything in each of these spaces

          >>> ["this", "is", "a", "list", "of", "strings"]
          ['this', 'is', 'a', 'list', 'of', 'strings']
          >>> list_of_strings = ["this", "is", "a", "list", "of", "strings"]
          >>> list_of_strings[0]
          'this'
          >>> list_of_strings[1:4]
          ['is', 'a', 'list']
          >>> [1, 5.0, "my string"]
          [1, 5.0, 'my string']
          >>> l = [1, 5.0, "my string"]
          >>> type(l[0])
          <type 'int'>
          >>> type(l[1])
          <type 'float'>
          >>> type(l[2])
          <type 'str'>

       - In general, it's a good idea to have lists be homogeneous, i.e. be of the same type

  • slicing
       - sometimes we want more than just one item from the list (this is called "slicing")
       - We can specify a range in the square brackets, [], using the colon (:)

          >>> l = ["this", "is", "a", "list", "of", "strings"]
          >>> l[0:3]
          ['this', 'is', 'a']
          >>> l[1:5]
          ['is', 'a', 'list', 'of']
          >>> l[1:1]
          []
          >>> l[-3:-1]
          ['list', 'of']

          - generates a *new* list
          - that includes the items from the list starting at the first number and up to, but not including, the second number

  • range function
       - we've used the range function in loops to iterate over numbers, e.g.

          for i in range(10):
             # do something

       - what does the range function actually do?
          >>> range(10)
          [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
          >>> range(15,20)
          [15, 16, 17, 18, 19]
          >>> range(-5, 0)
          [-5, -4, -3, -2, -1]

  • looping over lists
       - all of our for loops so far have actually just been looping over lists

          for i in range(10):
             # do something

          is really the same as:
       
          some_list = range(10)

          for i in some_list:
             # do something

       - the way to read this list is:
          - for each element in the list, do something
          - for each iteration of the loop, i (or whatever variable you put there) will get the next value in the list

       >>> my_list = [4, 1, 8, 10, 11]
       >>> for i in my_list:
       ...    print i
       ...
       4
       1
       8
       10
       11

  • back to our stats program... how could we write average given what we know so far, that is a function that takes a list as a parameter and calculates the average?
       - look at the inelegant_average function in scores-lists.py code
          - loop over each of the elements in the list
          - accumulate a sum
          - accumulate a count
          - divide the sum by the count
       - look at the average function in scores-lists.py code

  • built-in functions over lists: there are also some built-in functions that take a list as a parameter
       - we can get the length of a list
          >>> len(l)
          3
          >>> len([1, 2, 3, 4, 5])
          5
          >>> len([])
          0
       - max
          >>> l = [5, 3, 2, 1, 10]
          >>> max(l)
          10

       - min
          >>> min(l)
          1
       - sum
          >>> sum(l)
          21

  • lists are objects and therefore have methods. Any guesses?
       - append: add a value on to the end of the list
          >>> my_list = [15, 2, 1, 20, 5]
          >>> my_list.append(100)
          >>> my_list
          [15, 2, 1, 20, 5, 100]

          - notice that append does NOT return a new list, it modifies the existing list!

       - try some out on your own:
          http://docs.python.org/tutorial/datastructures.html
       
          - pop: remove a value off of the end of the list and return it
             >>> my_list.pop()
             100
             >>> my_list
             [15, 2, 1, 20, 5]
          
             - notice that it both modifies the list and returns a value
             - if you want to use this value, you need to store it!
                >>> x = my_list.pop()
                >>> x
                5
             - pop also has another version where you can specify the index
       
                >>> my_list = [15, 2, 1, 20, 5]
                >>> my_list.pop(2)
                1
                >>> my_list
                [15, 2, 20, 5]
          - insert: inserts a value at a particular index
             >>> my_list = [15, 2, 1, 20, 5]
             >>> my_list.insert(2, 100)
             >>> my_list
             [15, 2, 100, 1, 20, 5]

             - again, lists are mutable, so insert does not return a new list, but modifies the underlying one
          - sort
             >>> my_list = [15, 2, 1, 20, 5]
             >>> my_list.sort()
             >>> my_list
             [1, 2, 5, 15, 20]
             >>> my_list = ["these", "are", "some", "words", "to", "sort"]
             >>> ["these", "are", "some", "words", "to", "sort"].sort()
             >>> my_list = ["these", "are", "some", "words", "to", "sort"]
             >>> my_list.sort()
             >>> my_list
             ['are', 'some', 'sort', 'these', 'to', 'words']

  • back to our grades program: look at scores-lists.py code
       - there is a function called get_scores. That gets the scores and returns them as a list. How?
          - starts with an empty list
          - uses append to add them on to the end of the list
          - returns the list when the loop finishes
       - average function
          - has a single parameter, but this parameter will represent a list
          - inelegant_average
             - calculates the sum and divides by the number of entries
                - uses a for loop to iterate over the values
                - often, we'll use something besides "i" as a variable name that makes our program more readable
          - is there a better way to do this?
             - look at fancy_average
                - us the sum function over lists
       - median function
          - sorts the values
             - notice again that sort does NOT return a value, but sorts the list that it is called on
          - returns the middle entry

  • lists are mutable
       - what does that mean?
          - we can change (or mutate) the values in a list
       
       - notice that many of the methods that we call on lists change the list itself

       - we can mutate lists with methods, but we can also change particular indices
       
          >>> my_list = [15, 2, 1, 20, 5]
          >>> my_list
          [15, 2, 1, 20, 5]
          >>> my_list[2] = 100
          >>> my_list
          [15, 2, 100, 20, 5]

  • sequences
       - lists are part of a general category of data structures called sequences that represent a sequence of things
       - *all* sequences support a number of shared behavior
          - the ability to index using []
          - the ability to slice using [:]
          - a number of built-in functions:
             - len
             - max
             - min
          - the ability to iterate over in with a for loop
       - We've actually seen one other sequence?
          - strings!


  • strings as sequences
       - notice that we can do all the sequence-like things with strings
          >>> s = "banana"
          >>> s[4]
          'n'
          >>> s[2:5]
          'nan'
          >>> len(s)
          6
          >>> for letter in s:
          ... print letter

          b
          a
          n
          a
          n
          a
       - strings, however, are immutable
          >>> s[4] = "c"
          Traceback (most recent call last):
           File "<string>", line 1, in <fragment>
          TypeError: 'str' object does not support item assignment

          - no matter how hard you try, you cannot mutate a string
    •