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The List Data Structure
Introduction to Programming - Python
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Variables vs. Lists
n So far we have been working with variables, which can be
thought of as “buckets” that hold a particular piece of data
n Variables can only hold one piece of data at a time. Example
n x = 5
n y = 5.0
n z = ‘hello’
n q = True
n However, there are times when we need to keep track of
multiple pieces of data at the same time, and a single
variable is limited to holding just one thing at a time
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Lists
n Lists are considered a “sequence” object. Sequence objects
have the ability to hold multiple pieces of data at the same
time.
n We can use a single sequence variable to hold any number of
values.
n In most programming languages we call these “arrays.” In
Python we call these “lists.”
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Lists vs. Variables
List Variable
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Variables vs. Lists
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Variables vs. Lists
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Lists in Python
n You can create a list in Python by using bracket notation.
Example:
my_list = [1, 2, 3]
n The above code will create a new list in Python that holds
three integers – 1, 2 and 3 – in that order.
n Think of a list as a “book” that holds a series of sheets of
paper (variables)
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Lists in Python
n Lists can contain any data type that we have covered so far.
Example:
my_list = [‘Craig’, ‘John’, ‘Chris’]
n Lists can also mix data types. Example:
my_list = [‘Craig’, 5.0, True, 67]
n You can print the value of a list using the print() function.
Example:
print (my_list)
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List Repetition
n You can use the repetition operation (“*”) to ask Python to
repeat a list, much like how you would repeat a string.
Example:
my_list = [1, 2, 3] * 3!
print (my_list)!
!
>> [1, 2, 3, 1, 2, 3, 1, 2, 3]
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List Concatenation
n You can use the concatenation operation (“+”) to ask Python
to combine lists, much like how you would combine strings.
Example:
my_list = [1, 2, 3] + [99, 100, 101]!
print (my_list)!
!
>> [1, 2, 3, 99, 100, 101]
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Indexing List Elements
n In a book you can reference a page by its page number
n In a list you can reference an element by its index number
n Indexes start at the number zero.
n Example:
my_list = [‘Craig’, ‘John’, ‘Chris’]!
print (my_list[0])!
!
>> Craig
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Invalid indexes
n You will raise an exception if you attempt to access an
element outside the range of a list. For example:
my_list = ['Craig', 'John', 'Chris’]!
!
print (my_list[4]) # Index doesn’t exist!
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Changing the value of an item in a
list
n Lists are “mutable,” which means that they can be changed
once they have been created (unlike strings)
n Example:
my_list = [1, 2, 3]!
print (my_list)!
>> [1,2,3]!
!
my_list[0] = 99!
print (my_list)!
>> [99,2,3]!
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List Mechanics
n List variables are considered “references”
n This means that they “reference” or “point” to a specific region
of your computer’s memory. This behavior can cause some
interesting side effects. For example, the following two list
variables refer to the same list in memory.
mylist1 = [1,2,3]!
mylist2 = mylist1!
!
print (mylist1)!
print (mylist2)!
!
>> [1,2,3]!
>> [1,2,3]
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List Mechanics
n This means that you can change one of the lists and the
change will be reflected in the other.
mylist1 = [1,2,3]!
mylist2 = mylist1!
!
mylist1[0] = 999!
!
print (mylist1)!
print (mylist2)!
!
>> [999,2,3]!
>> [999,2,3]
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Copying a List
n Python will only create new lists when you use [] syntax to define
a list for the first time
n You can take advantage of this behavior to create true copies of
your list objects. For example:
!
mylist1 = [1,2,3]!
mylist2 = [] + mylist1!
!
mylist1[0] = 999!
!
print (mylist1)!
print (mylist2)!
!
>> [999,2,3]!
>> [1,2,3]
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Creating Lists
n You can create an empty list with no elements using the
following syntax:
mylist = []!
n Sometimes you want to create a list that contains a certain
number of “pre-set” elements. For example, to create a list
with 10 elements that are all set to zero you could do the
following:
mylist = [0] * 10 !
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Creating Lists
n You can also create lists using the range() function. For
example, to create a list of all even numbers between 0 and
100 you can do the following:
even_numbers = list(range(0,100,2))
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Iterating over a list
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Using a “for” loop to iterate
through a List
n You can also use a for loop to iterate through a list. When you
do this the target variable of your loop assumes each value of
each element of the list in order. Example:
my_list = [1,2,3]!
!
for number in my_list:!
print (number)!
!
>> 1!
>> 2!
>> 3
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Programming Challenge: Count
the A’s
n Given the following list:
grades = [90,100,70,45,76,84,93,21,36,99,100]
n Write a program that counts the # of A’s (scores between 90
and 100)
n Extension: Count the # of B’s, C’s, D’s and F’s
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Drawbacks to using “for” loops to
iterate through a List
n A for loop is a convenient way to sequentially iterate through a list.
n The target variable in a for loop assumes the value of the current item in
the list as you iterate.
n However, the target variable isn’t very helpful if you want to change the
value of an item in a list since it is just a copy of the data that exists in the
list. For example:
mylist = [1,2,3]!
!
for n in mylist:!
n = n * 5!
!
print (mylist)!
!
>> [1,2,3]
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Changing List Items
n In order to change a list item you need to know the index of
the item you wish to change. For example:
mylist = [1,2,3]!
!
mylist[0] = 999!
!
print (mylist)!
!
>> [999, 2, 3]
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Iterating over a list using index
values
n There are two main techniques for iterating over a list using
index values. They include:
n Setting up a counter variable outside the list and continually
updating the variable as you move to the next position in the list
n Using the range() function to create a custom range that
represents the size of your list
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Using a counter variable and a for
loop to iterate over a list
n If you set up an accumulator variable outside of your loop you
can use it to keep track of where you are in a list. For
example:
mylist = [1,2,3]!
counter = 0!
!
for num in mylist:!
mylist[counter] = mylist[counter] * 2!
counter += 1!
!
print (mylist)!
!
>> [2,4,6]
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Using the range() function to
iterate over a list
n You can also use the range() function to construct a custom
range that represents all of the indexes in a list. Example:
mylist = [1,2,3]!
!
for counter in range(0,len(mylist)):!
mylist[counter] = mylist[counter] * 2!
!
print (mylist)!
!
>> [2,4,6]
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Programming Challenge
n Given the following list of prices, write a program that
modifies the list to include 7% sales tax
prices = [1.99, 2.99, 3.99, 4.99, 5.99, 6.99]
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Working with Lists
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Creating empty lists for processing
n Since you cannot access an element outside of the range of a
list it is sometimes necessary to set up a correctly sized list
before you begin working with it.
n Example:
# create a list of 7 zeros!
daily_sales = [0] * 7
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Programming Challenge
n Write a program that asks the
user for daily sales figures for
a full week (Sunday –
Saturday)
n Store these values in a list and
print them out at the end of the
end of your program
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Slicing Lists
n Sometimes you need to extract multiple items from a list.
n Python contains some built in functions that make it easy for
you to “slice” out a portion of a list. Example:
list_1 = ['zero', 'one', 'two', 'three', 'four', 'five’]!
list_2 = list_1[1:3]!
!
print (list_1)!
print (list_2)!
!
>> ['zero', 'one', 'two', 'three', 'four', 'five’]!
>> ['one', 'two']
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Slicing Lists
n To slice a list you use a series of “slice indexes” to tell Python
which elements you want to extract. Example:
!
new_list = old_list[start:end]
n Python will copy out the elements from the list on the right
side of the assignment operator based on the start and end
indexes provided.
n Note that indexes work just like the range() function – you
will grab items up until the end index, but you will not grab
the end index itself
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Slicing Lists
n If you omit the start_index in a slice operation, Python will
start at the first element of the list
n If you omit the end_index in a slice operation, Python will go
until the last element of the list
n If you supply a third index, Python will assume you want to
use a step value. This works the same as the step value you
would pass to the range() function
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Programming Challenge
n Given the following list:
my_list = ['a', 'b', 'c', 'd', 'e', 'f', 'g’]
Write a program that does the following:
n Extract the first 3 elements of the list into a new list
n Extract the characters b, c, and d into a new list
n Extract the last 4 characters into a new list
n Write a program that creates a list of all #’s between 1 and
100. Then create a list of all even numbers using your first list
as input.
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Finding items in a list
n You can easily find a particular item in a list by using the “in”
operator. Here’s an example:
my_list = ['pie', 'cake', 'pizza’]!
if 'cake' in my_list:!
print ("I found cake!”)!
else:!
print ("No cake found.")
n The “in” operator lets you search for any item in a list. It will
return a Boolean value that indicates whether the item exists
somewhere in the list.
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Programming Challenge
n Given the following lists, write a program that lets the user
type in a product code. If the product name exists in our
inventory you should print out that it is in our inventory.
Otherwise you should print out that the product is not found.
products = [‘X125’, ‘X127’, ‘Z121’, ‘Z991’]
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Programming Challenge
n Given these two lists:
a = [1,2,3,4,5]!
b = [2,3,10,11,12,1]
n Write a program that finds all elements that exist in both lists
(i.e. the integer 2 exists in both lists). Store your result in a
list and print it out to the user.
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Adding items to a list
n You have already seen a few ways in which you can add items to
lists:
n Repeat the list using the “*” operator
n Concatenate the list using the “+” operator
n Another way to add items to a list is to use the “append” method.
n The append method is a function that is built into the list
datatype. It allows you to add items to the end of a list.
Example:
mylist = [‘Christine’, ‘Jasmine’, ‘Renee’]!
mylist.append(‘Kate’)!
print (mylist)!
!
>> [‘Christine’, ‘Jasmine’, ‘Renee’, ‘Kate’]
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Programming Challenge
n Write a program that continually prompts a user to enter in a
series of first names.
n Store these first names in a list.
n Print them out at the end of your program
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Sorting list items
n You can have Python sort items in a list using the sort()
method. Here’s an example:
my_list = ['pie', 'cake', 'pizza’]!
my_list.append('apple’)!
print (my_list)!
!
my_list.sort()!
print (my_list)!
!
>> ['pie', 'cake', 'pizza', 'apple’]!
>> ['apple', 'cake', 'pie', 'pizza']
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Programming Challenge
n Take the name program you wrote earlier and update it to
print out the names entered in alphabetical order.
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Finding the position of an item in a
list
n You can use the “index” method to ask Python to tell you the index of an
item in a list.
n The “index” method takes one argument – an element to search for –
and returns an integer value of where that element can be found.
n Caution: The index method will throw an exception if it cannot find the
item in the list.
n Here’s an example:
my_list = ['pizza', 'pie', 'cake’]!
if 'pie' in my_list:!
location = my_list.index('pie’)!
print ('pie is at postion #', location)!
else:!
print ('not found!')
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Programming Challenge
n Given that the following lists match up with one another (i.e.
the product at the first position of the products list matches
the price at the first position in the prices list), write a
product price lookup program.
products = ['peanut butter', 'jelly', 'bread’]!
prices = [3.99, 2.99, 1.99]
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Getting the largest and smallest
values in a list
n Python has two built in functions that let you get the highest
and lowest values in a list. They are called “min” and “max” –
here’s an example:
prices = [3.99, 2.99, 1.99]!
biggest = max(prices)!
smallest = min(prices)!
print (smallest, 'up to', biggest)
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Removing items from a list
n You can remove an item from a list by using the “remove”
method. Here’s an example:
prices = [3.99, 2.99, 1.99]!
prices.remove(2.99)!
print (prices)
n Note that you will raise an exception if you try and remove
something that is not in the list. Make sure to test to see if it is
in the list first using the “in” operator, or use a try / except
block to catch any errors you might raise.
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Removing items from a list
n You can also remove an item from a list based on its index
position. We can do this using the 'del' keyword, like this:
prices = [3.99, 2.99, 1.99]!
del prices[0] # remove whatever is at slot 0!
print (prices)
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Reversing a list
n You can use the reverse method on a list to reverse its
elements.
n This will not sort the list in reverse order – it will simply
shuffle the elements of a list such that the first element
becomes the last element, the second element becomes the
second to last element, etc.
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Programming Problems
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Programming Challenge
n Write a weight loss program that
prompts the user to enter in 7
days of weight values
n At the end of the program, print
out the following:
n Weight on the first day
n Weight on the last day
n Change from the first day to
the last day
n Average weight for the period
n Highest weight for the period
n Lowest weight for the period
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Programming Challenge
n Ask the user for 5 colors
n Don't allow duplicate values
n Sort the colors in both
ascending and descending
order
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Programming Challenge
n Write a program that
generates a random 5 digit
lottery number
n Numbers must be between 1
and 60
n No duplicate numbers are
permitted
n Sort the numbers in ascending
order and print them out to the
user
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Programming Challenge
n Roulette is a game where a ball is
rolled around a circular track –
eventually the ball will come to rest
in a slot that is labeled with the
numbers 0 through 36
n Gamblers can bet on an individual
number – if they are successful they
win a large prize (36 : 1)
n Write a program that asks the user
for a number between 0 and 36
n Then spin the roulette wheel 1000
times. Find out how many times the
user’s # came up.
n Also tell the user the most frequent
number and the least frequent
number that came up
