### CS30 - Spring 2016 - Class 5

#### Example code in this lecture

while.py
scores-lists.py

#### Lecture notes

- 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