# Lists and Tuples¶

In this notebook, you will learn to store more than one valuable in a single variable. This by itself is one of the most powerful ideas in programming, and it introduces a number of other central concepts such as loops. If this section ends up making sense to you, you will be able to start writing some interesting programs, and you can be more confident that you will be able to develop overall competence as a programmer.

# Introducing Lists¶

## Example¶

A list is a collection of items, that is stored in a variable. The items should be related in some way, but there are no restrictions on what can be stored in a list. Here is a simple example of a list, and how we can quickly access each item in the list.

In [3]:
students = ['bernice', 'aaron', 'cody']

for student in students:
print("Hello, " + student.title() + "!")

Hello, Bernice!
Hello, Aaron!
Hello, Cody!


## Naming and defining a list¶

Since lists are collection of objects, it is good practice to give them a plural name. If each item in your list is a car, call the list 'cars'. If each item is a dog, call your list 'dogs'. This gives you a straightforward way to refer to the entire list ('dogs'), and to a single item in the list ('dog').

In Python, square brackets designate a list. To define a list, you give the name of the list, the equals sign, and the values you want to include in your list within square brackets.

In [ ]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']


## Accessing one item in a list¶

Items in a list are identified by their position in the list, starting with zero. This will almost certainly trip you up at some point. Programmers even joke about how often we all make "off-by-one" errors, so don't feel bad when you make this kind of error.

To access the first element in a list, you give the name of the list, followed by a zero in parentheses.

In [18]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

dog = dogs[0]
print(dog.title())

Border Collie


The number in parentheses is called the index of the item. Because lists start at zero, the index of an item is always one less than its position in the list. So to get the second item in the list, we need to use an index of 1.

In [7]:
###highlight=[4]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

dog = dogs[1]
print(dog.title())

Australian Cattle Dog


### Accessing the last items in a list¶

You can probably see that to get the last item in this list, we would use an index of 2. This works, but it would only work because our list has exactly three items. To get the last item in a list, no matter how long the list is, you can use an index of -1.

In [8]:
###highlight=[4]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

dog = dogs[-1]
print(dog.title())

Labrador Retriever


This syntax also works for the second to last item, the third to last, and so forth.

In [32]:
###highlight=[4]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

dog = dogs[-2]
print(dog.title())

Australian Cattle Dog


You can't use a negative number larger than the length of the list, however.

In [33]:
###highlight=[4]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

dog = dogs[-4]
print(dog.title())

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
1 dogs = ['border collie', 'australian cattle dog', 'labrador retriever']
2
----> 3 dog = dogs[-4]
4 print(dog.title())

IndexError: list index out of range

## Exercises¶

#### First List¶

• Store the values 'python', 'c', and 'java' in a list. Print each of these values out, using their position in the list.

#### First Neat List¶

• Store the values 'python', 'c', and 'java' in a list. Print a statement about each of these values, using their position in the list.
• Your statement could simply be, 'A nice programming language is value.'

• Think of something you can store in a list. Make a list with three or four items, and then print a message that includes at least one item from your list. Your sentence could be as simple as, "One item in my list is a __."

# Lists and Looping¶

## Accessing all elements in a list¶

This is one of the most important concepts related to lists. You can have a list with a million items in it, and in three lines of code you can write a sentence for each of those million items. If you want to understand lists, and become a competent programmer, make sure you take the time to understand this section.

We use a loop to access all the elements in a list. A loop is a block of code that repeats itself until it runs out of items to work with, or until a certain condition is met. In this case, our loop will run once for every item in our list. With a list that is three items long, our loop will run three times.

Let's take a look at how we access all the items in a list, and then try to understand how it works.

In [15]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

for dog in dogs:
print(dog)

border collie
australian cattle dog


We have already seen how to create a list, so we are really just trying to understand how the last two lines work. These last two lines make up a loop, and the language here can help us see what is happening:

for dog in dogs:


• The keyword "for" tells Python to get ready to use a loop.
• The variable "dog", with no "s" on it, is a temporary placeholder variable. This is the variable that Python will place each item in the list into, one at a time.
• The first time through the loop, the value of "dog" will be 'border collie'.
• The second time through the loop, the value of "dog" will be 'australian cattle dog'.
• The third time through, "dog" will be 'labrador retriever'.
• After this, there are no more items in the list, and the loop will end.

The site pythontutor.com allows you to run Python code one line at a time. As you run the code, there is also a visualization on the screen that shows you how the variable "dog" holds different values as the loop progresses. There is also an arrow that moves around your code, showing you how some lines are run just once, while other lines are run multiple tiimes. If you would like to see this in action, click the Forward button and watch the visualization, and the output as it is printed to the screen. Tools like this are incredibly valuable for seeing what Python is doing with your code.

### Doing more with each item¶

We can do whatever we want with the value of "dog" inside the loop. In this case, we just print the name of the dog.

print(dog)



We are not limited to just printing the word dog. We can do whatever we want with this value, and this action will be carried out for every item in the list. Let's say something about each dog in our list.

In [16]:
###highlight=[5]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

for dog in dogs:
print('I like ' + dog + 's.')

I like border collies.
I like australian cattle dogs.


Visualize this on pythontutor.

### Inside and outside the loop¶

Python uses indentation to decide what is inside the loop and what is outside the loop. Code that is inside the loop will be run for every item in the list. Code that is not indented, which comes after the loop, will be run once just like regular code.

In [18]:
###highlight=[6,7,8]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

for dog in dogs:
print('I like ' + dog + 's.')
print('No, I really really like ' + dog +'s!\n')

print("\nThat's just how I feel about dogs.")

I like border collies.
No, I really really like border collies!

I like australian cattle dogs.
No, I really really like australian cattle dogs!

No, I really really like labrador retrievers!

That's just how I feel about dogs.


Notice that the last line only runs once, after the loop is completed. Also notice the use of newlines ("\n") to make the output easier to read. Run this code on pythontutor.

## Enumerating a list¶

When you are looping through a list, you may want to know the index of the current item. You could always use the list.index(value) syntax, but there is a simpler way. The enumerate() function tracks the index of each item for you, as it loops through the list:

In [4]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

print("Results for the dog show are as follows:\n")
for index, dog in enumerate(dogs):
place = str(index)
print("Place: " + place + " Dog: " + dog.title())

Results for the dog show are as follows:

Place: 0 Dog: Border Collie
Place: 1 Dog: Australian Cattle Dog


To enumerate a list, you need to add an index variable to hold the current index. So instead of

for dog in dogs:



You have

for index, dog in enumerate(dogs)



The value in the variable index is always an integer. If you want to print it in a string, you have to turn the integer into a string:

str(index)



The index always starts at 0, so in this example the value of place should actually be the current index, plus one:

In [5]:
###highlight=[6]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

print("Results for the dog show are as follows:\n")
for index, dog in enumerate(dogs):
place = str(index + 1)
print("Place: " + place + " Dog: " + dog.title())

Results for the dog show are as follows:

Place: 1 Dog: Border Collie
Place: 2 Dog: Australian Cattle Dog


### A common looping error¶

One common looping error occurs when instead of using the single variable dog inside the loop, we accidentally use the variable that holds the entire list:

In [19]:
###highlight=[5]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

for dog in dogs:
print(dogs)

['border collie', 'australian cattle dog', 'labrador retriever']
['border collie', 'australian cattle dog', 'labrador retriever']
['border collie', 'australian cattle dog', 'labrador retriever']


In this example, instead of printing each dog in the list, we print the entire list every time we go through the loop. Python puts each individual item in the list into the variable dog, but we never use that variable. Sometimes you will just get an error if you try to do this:

In [20]:
###highlight=[5]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

for dog in dogs:
print('I like ' + dogs + 's.')

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-20-8e7acc74d7a9> in <module>()
2
3 for dog in dogs:
----> 4     print('I like ' + dogs + 's.')

TypeError: Can't convert 'list' object to str implicitly

## Exercises¶

#### First List - Loop¶

• Repeat First List, but this time use a loop to print out each value in the list.

#### First Neat List - Loop¶

• Repeat First Neat List, but this time use a loop to print out your statements. Make sure you are writing the same sentence for all values in your list. Loops are not effective when you are trying to generate different output for each value in your list.

#### Your First List - Loop¶

• Repeat Your First List, but this time use a loop to print out your message for each item in your list. Again, if you came up with different messages for each value in your list, decide on one message to repeat for each value in your list.

# Common List Operations¶

## Modifying elements in a list¶

You can change the value of any element in a list if you know the position of that item.

In [2]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

dogs[0] = 'australian shepherd'
print(dogs)

['australian shepherd', 'australian cattle dog', 'labrador retriever']


## Finding an element in a list¶

If you want to find out the position of an element in a list, you can use the index() function.

In [11]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

print(dogs.index('australian cattle dog'))

1


This method returns a ValueError if the requested item is not in the list.

In [13]:
###highlight=[4]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

print(dogs.index('poodle'))

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-13-a9e05e37e8df> in <module>()
1 dogs = ['border collie', 'australian cattle dog', 'labrador retriever']
2
----> 3 print(dogs.index('poodle'))

ValueError: 'poodle' is not in list

## Testing whether an item is in a list¶

You can test whether an item is in a list using the "in" keyword. This will become more useful after learning how to use if-else statements.

In [12]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']

print('australian cattle dog' in dogs)
print('poodle' in dogs)

True
False


## Adding items to a list¶

### Appending items to the end of a list¶

We can add an item to a list using the append() method. This method adds the new item to the end of the list.

In [22]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']
dogs.append('poodle')

for dog in dogs:
print(dog.title() + "s are cool.")

Border Collies are cool.
Australian Cattle Dogs are cool.
Poodles are cool.


### Inserting items into a list¶

We can also insert items anywhere we want in a list, using the insert() function. We specify the position we want the item to have, and everything from that point on is shifted one position to the right. In other words, the index of every item after the new item is increased by one.

In [24]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']
dogs.insert(1, 'poodle')

print(dogs)

['border collie', 'poodle', 'australian cattle dog', 'labrador retriever']


Note that you have to give the position of the new item first, and then the value of the new item. If you do it in the reverse order, you will get an error.

## Creating an empty list¶

Now that we know how to add items to a list after it is created, we can use lists more dynamically. We are no longer stuck defining our entire list at once.

A common approach with lists is to define an empty list, and then let your program add items to the list as necessary. This approach works, for example, when starting to build an interactive web site. Your list of users might start out empty, and then as people register for the site it will grow. This is a simplified approach to how web sites actually work, but the idea is realistic.

Here is a brief example of how to start with an empty list, start to fill it up, and work with the items in the list. The only new thing here is the way we define an empty list, which is just an empty set of square brackets.

In [25]:
# Create an empty list to hold our users.

# Greet all of our users.
print("Welcome, " + username.title() + '!')

Welcome, Bernice!
Welcome, Cody!
Welcome, Aaron!


If we don't change the order in our list, we can use the list to figure out who our oldest and newest users are.

In [2]:
###highlight=[10,11,12]
# Create an empty list to hold our users.

# Greet all of our users.
print("Welcome, " + username.title() + '!')

# Recognize our first user, and welcome our newest user.
print("\nThank you for being our very first user, " + usernames[0].title() + '!')
print("And a warm welcome to our newest user, " + usernames[-1].title() + '!')

Welcome, Bernice!
Welcome, Cody!
Welcome, Aaron!

Thank you for being our very first user, Bernice!
And a warm welcome to our newest user, Aaron!


Note that the code welcoming our newest user will always work, because we have used the index -1. If we had used the index 2 we would always get the third user, even as our list of users grows and grows.

## Sorting a List¶

We can sort a list alphabetically, in either order.

In [16]:
students = ['bernice', 'aaron', 'cody']

# Put students in alphabetical order.
students.sort()

# Display the list in its current order.
print("Our students are currently in alphabetical order.")
for student in students:
print(student.title())

#Put students in reverse alphabetical order.
students.sort(reverse=True)

# Display the list in its current order.
print("\nOur students are now in reverse alphabetical order.")
for student in students:
print(student.title())

Our students are currently in alphabetical order.
Aaron
Bernice
Cody

Our students are now in reverse alphabetical order.
Cody
Bernice
Aaron


### sorted() vs. sort()¶

Whenever you consider sorting a list, keep in mind that you can not recover the original order. If you want to display a list in sorted order, but preserve the original order, you can use the sorted() function. The sorted() function also accepts the optional reverse=True argument.

In [24]:
students = ['bernice', 'aaron', 'cody']

# Display students in alphabetical order, but keep the original order.
print("Here is the list in alphabetical order:")
for student in sorted(students):
print(student.title())

# Display students in reverse alphabetical order, but keep the original order.
print("\nHere is the list in reverse alphabetical order:")
for student in sorted(students, reverse=True):
print(student.title())

print("\nHere is the list in its original order:")
# Show that the list is still in its original order.
for student in students:
print(student.title())

Here is the list in alphabetical order:
Aaron
Bernice
Cody

Here is the list in reverse alphabetical order:
Cody
Bernice
Aaron

Here is the list in its original order:
Bernice
Aaron
Cody


### Reversing a list¶

We have seen three possible orders for a list:

• The original order in which the list was created
• Alphabetical order
• Reverse alphabetical order

There is one more order we can use, and that is the reverse of the original order of the list. The reverse() function gives us this order.

In [17]:
students = ['bernice', 'aaron', 'cody']
students.reverse()

print(students)

['cody', 'aaron', 'bernice']


Note that reverse is permanent, although you could follow up with another call to reverse() and get back the original order of the list.

### Sorting a numerical list¶

All of the sorting functions work for numerical lists as well.

In [29]:
numbers = [1, 3, 4, 2]

# sort() puts numbers in increasing order.
numbers.sort()
print(numbers)

# sort(reverse=True) puts numbers in decreasing order.
numbers.sort(reverse=True)
print(numbers)

[1, 2, 3, 4]
[4, 3, 2, 1]

In [30]:
numbers = [1, 3, 4, 2]

# sorted() preserves the original order of the list:
print(sorted(numbers))
print(numbers)

[1, 2, 3, 4]
[1, 3, 4, 2]

In [31]:
numbers = [1, 3, 4, 2]

# The reverse() function also works for numerical lists.
numbers.reverse()
print(numbers)

[2, 4, 3, 1]


## Finding the length of a list¶

You can find the length of a list using the len() function.

In [38]:
usernames = ['bernice', 'cody', 'aaron']

print(user_count)

3


There are many situations where you might want to know how many items in a list. If you have a list that stores your users, you can find the length of your list at any time, and know how many users you have.

In [42]:
# Create an empty list to hold our users.

# Add some users, and report on how many users we have.

print("We have " + str(user_count) + " user!")

print("We have " + str(user_count) + " users!")

We have 1 user!
We have 3 users!


On a technical note, the len() function returns an integer, which can't be printed directly with strings. We use the str() function to turn the integer into a string so that it prints nicely:

In [43]:
usernames = ['bernice', 'cody', 'aaron']

print("This will cause an error: " + user_count)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-43-92e732ef190e> in <module>()
3
----> 4 print("This will cause an error: " + user_count)

TypeError: Can't convert 'int' object to str implicitly
In [44]:
###highlight=[5]

print("This will work: " + str(user_count))

This will work: 3


## Exercises¶

#### Working List¶

• Make a list that includes four careers, such as 'programmer' and 'truck driver'.
• Use the list.index() function to find the index of one career in your list.
• Use the in function to show that this career is in your list.
• Use the append() function to add a new career to your list.
• Use the insert() function to add a new career at the beginning of the list.
• Use a loop to show all the careers in your list.

#### Starting From Empty¶

• Create the list you ended up with in Working List, but this time start your file with an empty list and fill it up using append() statements.
• Print a statement that tells us what the first career you thought of was.
• Print a statement that tells us what the last career you thought of was.

#### Ordered Working List¶

• You are going to print out the list in a number of different orders.
• Each time you print the list, use a for loop rather than printing the raw list.
• Print a message each time telling us what order we should see the list in.
• Print the list in its original order.
• Print the list in alphabetical order.
• Print the list in its original order.
• Print the list in reverse alphabetical order.
• Print the list in its original order.
• Print the list in the reverse order from what it started.
• Print the list in its original order
• Permanently sort the list in alphabetical order, and then print it out.
• Permanently sort the list in reverse alphabetical order, and then print it out.

#### Ordered Numbers¶

• Make a list of 5 numbers, in a random order.
• You are going to print out the list in a number of different orders.
• Each time you print the list, use a for loop rather than printing the raw list.
• Print a message each time telling us what order we should see the list in.
• Print the numbers in the original order.
• Print the numbers in increasing order.
• Print the numbers in the original order.
• Print the numbers in decreasing order.
• Print the numbers in their original order.
• Print the numbers in the reverse order from how they started.
• Print the numbers in the original order.
• Permanently sort the numbers in increasing order, and then print them out.
• Permanently sort the numbers in descreasing order, and then print them out.

#### List Lengths¶

• Copy two or three of the lists you made from the previous exercises, or make up two or three new lists.
• Print out a series of statements that tell us how long each list is.

# Removing Items from a List¶

Hopefully you can see by now that lists are a dynamic structure. We can define an empty list and then fill it up as information comes into our program. To become really dynamic, we need some ways to remove items from a list when we no longer need them. You can remove items from a list through their position, or through their value.

## Removing items by position¶

If you know the position of an item in a list, you can remove that item using the del command. To use this approach, give the command del and the name of your list, with the index of the item you want to move in square brackets:

In [30]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']
# Remove the first dog from the list.
del dogs[0]

print(dogs)

['australian cattle dog', 'labrador retriever']


## Removing items by value¶

You can also remove an item from a list if you know its value. To do this, we use the remove() function. Give the name of the list, followed by the word remove with the value of the item you want to remove in parentheses. Python looks through your list, finds the first item with this value, and removes it.

In [31]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']
# Remove australian cattle dog from the list.
dogs.remove('australian cattle dog')

print(dogs)

['border collie', 'labrador retriever']


Be careful to note, however, that only the first item with this value is removed. If you have multiple items with the same value, you will have some items with this value left in your list.

In [32]:
letters = ['a', 'b', 'c', 'a', 'b', 'c']
# Remove the letter a from the list.
letters.remove('a')

print(letters)

['b', 'c', 'a', 'b', 'c']


## Popping items from a list¶

There is a cool concept in programming called "popping" items from a collection. Every programming language has some sort of data structure similar to Python's lists. All of these structures can be used as queues, and there are various ways of processing the items in a queue.

One simple approach is to start with an empty list, and then add items to that list. When you want to work with the items in the list, you always take the last item from the list, do something with it, and then remove that item. The pop() function makes this easy. It removes the last item from the list, and gives it to us so we can work with it. This is easier to show with an example:

In [35]:
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']
last_dog = dogs.pop()

print(last_dog)
print(dogs)

labrador retriever
['border collie', 'australian cattle dog']


This is an example of a first-in, last-out approach. The first item in the list would be the last item processed if you kept using this approach. We will see a full implementation of this approach later on, when we learn about while loops.

You can actually pop any item you want from a list, by giving the index of the item you want to pop. So we could do a first-in, first-out approach by popping the first iem in the list:

In [36]:
###highlight=[3]
dogs = ['border collie', 'australian cattle dog', 'labrador retriever']
first_dog = dogs.pop(0)

print(first_dog)
print(dogs)

border collie


## Exercises¶

#### Famous People¶

• Make a list that includes the names of four famous people.
• Remove each person from the list, one at a time, using each of the four methods we have just seen:
• Pop the last item from the list, and pop any item except the last item.
• Remove one item by its position, and one item by its value.
• Print out a message that there are no famous people left in your list, and print your list to prove that it is empty.

# Want to see what functions are?¶

At this point, you might have noticed we have a fair bit of repetetive code in some of our examples. This repetition will disappear once we learn how to use functions. If this repetition is bothering you already, you might want to go look at Introducing Functions before you do any more exercises in this section.

# Slicing a List¶

Since a list is a collection of items, we should be able to get any subset of those items. For example, if we want to get just the first three items from the list, we should be able to do so easily. The same should be true for any three items in the middle of the list, or the last three items, or any x items from anywhere in the list. These subsets of a list are called slices.

To get a subset of a list, we give the position of the first item we want, and the position of the first item we do not want to include in the subset. So the slice list[0:3] will return a list containing items 0, 1, and 2, but not item 3. Here is how you get a batch containing the first three items.

In [5]:
usernames = ['bernice', 'cody', 'aaron', 'ever', 'dalia']

# Grab the first three users in the list.

for user in first_batch:
print(user.title())

Bernice
Cody
Aaron


If you want to grab everything up to a certain position in the list, you can also leave the first index blank:

In [8]:
###highlight=[5]
usernames = ['bernice', 'cody', 'aaron', 'ever', 'dalia']

# Grab the first three users in the list.

for user in first_batch:
print(user.title())

Bernice
Cody
Aaron


When we grab a slice from a list, the original list is not affected:

In [6]:
###highlight=[7,8,9]
usernames = ['bernice', 'cody', 'aaron', 'ever', 'dalia']

# Grab the first three users in the list.

# The original list is unaffected.
print(user.title())

Bernice
Cody
Aaron
Ever
Dalia


We can get any segment of a list we want, using the slice method:

In [7]:
usernames = ['bernice', 'cody', 'aaron', 'ever', 'dalia']

# Grab a batch from the middle of the list.

for user in middle_batch:
print(user.title())

Cody
Aaron
Ever


To get all items from one position in the list to the end of the list, we can leave off the second index:

In [9]:
usernames = ['bernice', 'cody', 'aaron', 'ever', 'dalia']

# Grab all users from the third to the end.

for user in end_batch:
print(user.title())

Aaron
Ever
Dalia


### Copying a list¶

You can use the slice notation to make a copy of a list, by leaving out both the starting and the ending index. This causes the slice to consist of everything from the first item to the last, which is the entire list.

In [15]:
usernames = ['bernice', 'cody', 'aaron', 'ever', 'dalia']

# Make a copy of the list.

# Remove the first two users from the copied list.
print("\nTwo users removed from copied list:\n\t", copied_usernames)

# The original list is unaffected.

The full copied list:
['bernice', 'cody', 'aaron', 'ever', 'dalia']

Two users removed from copied list:
['aaron', 'ever', 'dalia']

The original list:
['bernice', 'cody', 'aaron', 'ever', 'dalia']


## Exercises¶

#### Alphabet Slices¶

• Store the first ten letters of the alphabet in a list.
• Use a slice to print out the first three letters of the alphabet.
• Use a slice to print out any three letters from the middle of your list.
• Use a slice to print out the letters from any point in the middle of your list, to the end.

#### Protected List¶

• Your goal in this exercise is to prove that copying a list protects the original list.
• Make a list with three people's names in it.
• Use a slice to make a copy of the entire list.
• Add at least two new names to the new copy of the list.
• Make a loop that prints out all of the names in the original list, along with a message that this is the original list.
• Make a loop that prints out all of the names in the copied list, along with a message that this is the copied list.

# Numerical Lists¶

There is nothing special about lists of numbers, but there are some functions you can use to make working with numerical lists more efficient. Let's make a list of the first ten numbers, and start working with it to see how we can use numbers in a list.

In [23]:
# Print out the first ten numbers.
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

for number in numbers:
print(number)

1
2
3
4
5
6
7
8
9
10


## The range() function¶

This works, but it is not very efficient if we want to work with a large set of numbers. The range() function helps us generate long lists of numbers. Here are two ways to do the same thing, using the range function.

In [24]:
# Print the first ten numbers.
for number in range(1,11):
print(number)

1
2
3
4
5
6
7
8
9
10


The range function takes in a starting number, and an end number. You get all integers, up to but not including the end number. You can also add a step value, which tells the range function how big of a step to take between numbers:

In [25]:
# Print the first ten odd numbers.
for number in range(1,21,2):
print(number)

1
3
5
7
9
11
13
15
17
19


If we want to store these numbers in a list, we can use the list() function. This function takes in a range, and turns it into a list:

In [26]:
# Create a list of the first ten numbers.
numbers = list(range(1,11))
print(numbers)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]


This is incredibly powerful; we can now create a list of the first million numbers, just as easily as we made a list of the first ten numbers. It doesn't really make sense to print the million numbers here, but we can show that the list really does have one million items in it, and we can print the last ten items to show that the list is correct.

In [31]:
# Store the first million numbers in a list.
numbers = list(range(1,1000001))

# Show the length of the list:
print("The list 'numbers' has " + str(len(numbers)) + " numbers in it.")

# Show the last ten numbers:
print("\nThe last ten numbers in the list are:")
for number in numbers[-10:]:
print(number)

The list 'numbers' has 1000000 numbers in it.

The last ten numbers in the list are:
999991
999992
999993
999994
999995
999996
999997
999998
999999
1000000


There are two things here that might be a little unclear. The expression

str(len(numbers))



takes the length of the numbers list, and turns it into a string that can be printed.

The expression

numbers[-10:]



gives us a slice of the list. The index -1 is the last item in the list, and the index -10 is the item ten places from the end of the list. So the slice numbers[-10:] gives us everything from that item to the end of the list.

## The min(), max(), and sum() functions¶

There are three functions you can easily use with numerical lists. As you might expect, the min() function returns the smallest number in the list, the max() function returns the largest number in the list, and the sum() function returns the total of all numbers in the list.

In [39]:
ages = [23, 16, 14, 28, 19, 11, 38]

youngest = min(ages)
oldest = max(ages)
total_years = sum(ages)

print("Our youngest reader is " + str(youngest) + " years old.")
print("Our oldest reader is " + str(oldest) + " years old.")
print("Together, we have " + str(total_years) + " years worth of life experience.")

Our youngest reader is 11 years old.
Our oldest reader is 38 years old.
Together, we have 149 years worth of life experience.


## Exercises¶

#### First Twenty¶

• Use the range() function to store the first twenty numbers (1-20) in a list, and print them out.

#### Larger Sets¶

• Take the first_twenty.py program you just wrote. Change your end number to a much larger number. How long does it take your computer to print out the first million numbers? (Most people will never see a million numbers scroll before their eyes. You can now see this!)

#### Five Wallets¶

• Imagine five wallets with different amounts of cash in them. Store these five values in a list, and print out the following sentences:
• "The fattest wallet has $value in it." • "The skinniest wallet has$ value in it."
• "All together, these wallets have \$ value in them."

# List Comprehensions¶

I thought carefully before including this section. If you are brand new to programming, list comprehensions may look confusing at first. They are a shorthand way of creating and working with lists. It is good to be aware of list comprehensions, because you will see them in other people's code, and they are really useful when you understand how to use them. That said, if they don't make sense to you yet, don't worry about using them right away. When you have worked with enough lists, you will want to use comprehensions. For now, it is good enough to know they exist, and to recognize them when you see them. If you like them, go ahead and start trying to use them now.

## Numerical Comprehensions¶

Let's consider how we might make a list of the first ten square numbers. We could do it like this:

In [42]:
# Store the first ten square numbers in a list.
# Make an empty list that will hold our square numbers.
squares = []

# Go through the first ten numbers, square them, and add them to our list.
for number in range(1,11):
new_square = number**2
squares.append(new_square)

# Show that our list is correct.
for square in squares:
print(square)

1
4
9
16
25
36
49
64
81
100


This should make sense at this point. If it doesn't, go over the code with these thoughts in mind:

• We make an empty list called squares that will hold the values we are interested in.
• Using the range() function, we start a loop that will go through the numbers 1-10.
• Each time we pass through the loop, we find the square of the current number by raising it to the second power.
• We add this new value to our list squares.
• We go through our newly-defined list and print out each square.

Now let's make this code more efficient. We don't really need to store the new square in its own variable new_square; we can just add it directly to the list of squares. The line

new_square = number**2



is taken out, and the next line takes care of the squaring:

In [43]:
###highlight=[8]
# Store the first ten square numbers in a list.
# Make an empty list that will hold our square numbers.
squares = []

# Go through the first ten numbers, square them, and add them to our list.
for number in range(1,11):
squares.append(number**2)

# Show that our list is correct.
for square in squares:
print(square)

1
4
9
16
25
36
49
64
81
100


List comprehensions allow us to collapse the first three lines of code into one line. Here's what it looks like:

In [44]:
###highlight=[2,3]
# Store the first ten square numbers in a list.
squares = [number**2 for number in range(1,11)]

# Show that our list is correct.
for square in squares:
print(square)

1
4
9
16
25
36
49
64
81
100


It should be pretty clear that this code is more efficient than our previous approach, but it may not be clear what is happening. Let's take a look at everything that is happening in that first line:

We define a list called squares.

Look at the second part of what's in square brackets:

for number in range(1,11)



This sets up a loop that goes through the numbers 1-10, storing each value in the variable number. Now we can see what happens to each number in the loop:

number**2



Each number is raised to the second power, and this is the value that is stored in the list we defined. We might read this line in the following way:

squares = [raise number to the second power, for each number in the range 1-10]

### Another example¶

It is probably helpful to see a few more examples of how comprehensions can be used. Let's try to make the first ten even numbers, the longer way:

In [45]:
# Make an empty list that will hold the even numbers.
evens = []

# Loop through the numbers 1-10, double each one, and add it to our list.
for number in range(1,11):
evens.append(number*2)

# Show that our list is correct:
for even in evens:
print(even)

2
4
6
8
10
12
14
16
18
20


Here's how we might think of doing the same thing, using a list comprehension:

evens = [multiply each number by 2, for each number in the range 1-10]

Here is the same line in code:

In [46]:
###highlight=[2,3]
# Make a list of the first ten even numbers.
evens = [number*2 for number in range(1,11)]

for even in evens:
print(even)

2
4
6
8
10
12
14
16
18
20


## Non-numerical comprehensions¶

We can use comprehensions with non-numerical lists as well. In this case, we will create an initial list, and then use a comprehension to make a second list from the first one. Here is a simple example, without using comprehensions:

In [47]:
# Consider some students.
students = ['bernice', 'aaron', 'cody']

# Let's turn them into great students.
great_students = []
for student in students:
great_students.append(student.title() + " the great!")

# Let's greet each great student.
for great_student in great_students:
print("Hello, " + great_student)

Hello, Bernice the great!
Hello, Aaron the great!
Hello, Cody the great!


To use a comprehension in this code, we want to write something like this:

great_students = [add 'the great' to each student, for each student in the list of students]

Here's what it looks like:

In [48]:
###highlight=[5,6]
# Consider some students.
students = ['bernice', 'aaron', 'cody']

# Let's turn them into great students.
great_students = [student.title() + " the great!" for student in students]

# Let's greet each great student.
for great_student in great_students:
print("Hello, " + great_student)

Hello, Bernice the great!
Hello, Aaron the great!
Hello, Cody the great!


## Exercises¶

If these examples are making sense, go ahead and try to do the following exercises using comprehensions. If not, try the exercises without comprehensions. You may figure out how to use comprehensions after you have solved each exercise the longer way.

#### Multiples of Ten¶

• Make a list of the first ten multiples of ten (10, 20, 30... 90, 100). There are a number of ways to do this, but try to do it using a list comprehension. Print out your list.

#### Cubes¶

• We saw how to make a list of the first ten squares. Make a list of the first ten cubes (1, 8, 27... 1000) using a list comprehension, and print them out.

#### Awesomeness¶

• Store five names in a list. Make a second list that adds the phrase "is awesome!" to each name, using a list comprehension. Print out the awesome version of the names.

#### Working Backwards¶

• Write out the following code without using a list comprehension:

plus_thirteen = [number + 13 for number in range(1,11)]

# Strings as Lists¶

Now that you have some familiarity with lists, we can take a second look at strings. A string is really a list of characters, so many of the concepts from working with lists behave the same with strings.

## Strings as a list of characters¶

We can loop through a string using a for loop, just like we loop through a list:

In [1]:
message = "Hello!"

for letter in message:
print(letter)

H
e
l
l
o
!


We can create a list from a string. The list will have one element for each character in the string:

In [3]:
message = "Hello world!"

message_list = list(message)
print(message_list)

['H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '!']


## Slicing strings¶

We can access any character in a string by its position, just as we access individual items in a list:

In [5]:
message = "Hello World!"
first_char = message[0]
last_char = message[-1]

print(first_char, last_char)

('H', '!')


We can extend this to take slices of a string:

In [7]:
message = "Hello World!"
first_three = message[:3]
last_three = message[-3:]

print(first_three, last_three)

('Hel', 'ld!')


## Finding substrings¶

Now that you have seen what indexes mean for strings, we can search for substrings. A substring is a series of characters that appears in a string.

You can use the in keyword to find out whether a particular substring appears in a string:

In [12]:
message = "I like cats and dogs."
dog_present = 'dog' in message
print(dog_present)

True


If you want to know where a substring appears in a string, you can use the find() method. The find() method tells you the index at which the substring begins.

In [13]:
message = "I like cats and dogs."
dog_index = message.find('dog')
print(dog_index)

16


Note, however, that this function only returns the index of the first appearance of the substring you are looking for. If the substring appears more than once, you will miss the other substrings.

In [19]:
###highlight=[2]
message = "I like cats and dogs, but I'd much rather own a dog."
dog_index = message.find('dog')
print(dog_index)

16


If you want to find the last appearance of a substring, you can use the rfind() function:

In [22]:
###highlight=[3,4]
message = "I like cats and dogs, but I'd much rather own a dog."
last_dog_index = message.rfind('dog')
print(last_dog_index)

48


## Replacing substrings¶

You can use the replace() function to replace any substring with another substring. To use the replace() function, give the substring you want to replace, and then the substring you want to replace it with. You also need to store the new string, either in the same string variable or in a new variable.

In [24]:
message = "I like cats and dogs, but I'd much rather own a dog."
message = message.replace('dog', 'snake')
print(message)

I like cats and snakes, but I'd much rather own a snake.


## Counting substrings¶

If you want to know how many times a substring appears within a string, you can use the count() method.

In [25]:
message = "I like cats and dogs, but I'd much rather own a dog."
number_dogs = message.count('dog')
print(number_dogs)

2


## Splitting strings¶

Strings can be split into a set of substrings when they are separated by a repeated character. If a string consists of a simple sentence, the string can be split based on spaces. The split() function returns a list of substrings. The split() function takes one argument, the character that separates the parts of the string.

In [26]:
message = "I like cats and dogs, but I'd much rather own a dog."
words = message.split(' ')
print(words)

['I', 'like', 'cats', 'and', 'dogs,', 'but', "I'd", 'much', 'rather', 'own', 'a', 'dog.']


Notice that the punctuation is left in the substrings.

It is more common to split strings that are really lists, separated by something like a comma. The split() function gives you an easy way to turn comma-separated strings, which you can't do much with in Python, into lists. Once you have your data in a list, you can work with it in much more powerful ways.

In [27]:
animals = "dog, cat, tiger, mouse, liger, bear"

# Rewrite the string as a list, and store it in the same variable
animals = animals.split(',')
print(animals)

['dog', ' cat', ' tiger', ' mouse', ' liger', ' bear']


Notice that in this case, the spaces are also ignored. It is a good idea to test the output of the split() function and make sure it is doing what you want with the data you are interested in.

One use of this is to work with spreadsheet data in your Python programs. Most spreadsheet applications allow you to dump your data into a comma-separated text file. You can read this file into your Python program, or even copy and paste from the text file into your program file, and then turn the data into a list. You can then process your spreadsheet data using a for loop.

## Other string methods¶

There are a number of other string methods that we won't go into right here, but you might want to take a look at them. Most of these methods should make sense to you at this point. You might not have use for any of them right now, but it is good to know what you can do with strings. This way you will have a sense of how to solve certain problems, even if it means referring back to the list of methods to remind yourself how to write the correct syntax when you need it.

## Exercises¶

#### Listing a Sentence¶

• Store a single sentence in a variable. Use a for loop to print each character from your sentence on a separate line.

#### Sentence List¶

• Store a single sentence in a variable. Create a list from your sentence. Print your raw list (don't use a loop, just print the list).

#### Sentence Slices¶

• Store a sentence in a variable. Using slices, print out the first five characters, any five consecutive characters from the middle of the sentence, and the last five characters of the sentence.

#### Finding Python¶

• Store a sentence in a variable, making sure you use the word Python at least twice in the sentence.
• Use the in keyword to prove that the word Python is actually in the sentence.
• Use the find() function to show where the word Python first appears in the sentence.
• Use the rfind() function to show the last place Python appears in the sentence.
• Use the count() function to show how many times the word Python appears in your sentence.
• Use the split() function to break your sentence into a list of words. Print the raw list, and use a loop to print each word on its own line.
• Use the replace() function to change Python to Ruby in your sentence.

## Challenges¶

#### Counting DNA Nucleotides¶

• Project Rosalind is a problem set based on biotechnology concepts. It is meant to show how programming skills can help solve problems in genetics and biology.
• If you have understood this section on strings, you have enough information to solve the first problem in Project Rosalind, Counting DNA Nucleotides. Give the sample problem a try.
• If you get the sample problem correct, log in and try the full version of the problem!

#### Transcribing DNA into RNA¶

• You also have enough information to try the second problem, Transcribing DNA into RNA. Solve the sample problem.
• If you solved the sample problem, log in and try the full version!

#### Complementing a Strand of DNA¶

• You guessed it, you can now try the third problem as well: Complementing a Strand of DNA. Try the sample problem, and then try the full version if you are successful.

# Tuples¶

Tuples are basically lists that can never be changed. Lists are quite dynamic; they can grow as you append and insert items, and they can shrink as you remove items. You can modify any element you want to in a list. Sometimes we like this behavior, but other times we may want to ensure that no user or no part of a program can change a list. That's what tuples are for.

Technically, lists are mutable objects and tuples are immutable objects. Mutable objects can change (think of mutations), and immutable objects can not change.

## Defining tuples, and accessing elements¶

You define a tuple just like you define a list, except you use parentheses instead of square brackets. Once you have a tuple, you can access individual elements just like you can with a list, and you can loop through the tuple with a for loop:

In [36]:
colors = ('red', 'green', 'blue')
print("The first color is: " + colors[0])

print("\nThe available colors are:")
for color in colors:
print("- " + color)

The first color is: red

The available colors are:
- red
- green
- blue


If you try to add something to a tuple, you will get an error:

In [37]:
colors = ('red', 'green', 'blue')
colors.append('purple')

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-37-ed1dbff53ab2> in <module>()
1 colors = ('red', 'green', 'blue')
----> 2 colors.append('purple')

AttributeError: 'tuple' object has no attribute 'append'

The same kind of thing happens when you try to remove something from a tuple, or modify one of its elements. Once you define a tuple, you can be confident that its values will not change.

## Using tuples to make strings¶

We have seen that it is pretty useful to be able to mix raw English strings with values that are stored in variables, as in the following:

In [41]:
animal = 'dog'
print("I have a " + animal + ".")

I have a dog.


This was especially useful when we had a series of similar statements to make:

In [42]:
animals = ['dog', 'cat', 'bear']
for animal in animals:
print("I have a " + animal + ".")

I have a dog.
I have a cat.
I have a bear.


I like this approach of using the plus sign to build strings because it is fairly intuitive. We can see that we are adding several smaller strings together to make one longer string. This is intuitive, but it is a lot of typing. There is a shorter way to do this, using placeholders.

Python ignores most of the characters we put inside of strings. There are a few characters that Python pays attention to, as we saw with strings such as "\t" and "\n". Python also pays attention to "%s" and "%d". These are placeholders. When Python sees the "%s" placeholder, it looks ahead and pulls in the first argument after the % sign:

In [43]:
animal = 'dog'
print("I have a %s." % animal)

I have a dog.


This is a much cleaner way of generating strings that include values. We compose our sentence all in one string, and then tell Python what values to pull into the string, in the appropriate places.

This is called string formatting, and it looks the same when you use a list:

In [44]:
animals = ['dog', 'cat', 'bear']
for animal in animals:
print("I have a %s." % animal)

I have a dog.
I have a cat.
I have a bear.


If you have more than one value to put into the string you are composing, you have to pack the values into a tuple:

In [46]:
animals = ['dog', 'cat', 'bear']
print("I have a %s, a %s, and a %s." % (animals[0], animals[1], animals[2]))

I have a dog, a cat, and a bear.


### String formatting with numbers¶

If you recall, printing a number with a string can cause an error:

In [47]:
number = 23
print("My favorite number is " + number + ".")

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-47-1ed2c5bb2bba> in <module>()
1 number = 23
----> 2 print("My favorite number is " + number + ".")

TypeError: cannot concatenate 'str' and 'int' objects

Python knows that you could be talking about the value 23, or the characters '23'. So it throws an error, forcing us to clarify that we want Python to treat the number as a string. We do this by casting the number into a string using the str() function:

In [48]:
###highlight=[3]
number = 23
print("My favorite number is " + str(number) + ".")

My favorite number is 23.


The format string "%d" takes care of this for us. Watch how clean this code is:

In [49]:
###highlight=[3]
number = 23
print("My favorite number is %d." % number)

My favorite number is 23.


If you want to use a series of numbers, you pack them into a tuple just like we saw with strings:

In [50]:
numbers = [7, 23, 42]
print("My favorite numbers are %d, %d, and %d." % (numbers[0], numbers[1], numbers[2]))

My favorite numbers are 7, 23, and 42.


Just for clarification, look at how much longer the code is if you use concatenation instead of string formatting:

In [51]:
###highlight=[3]
numbers = [7, 23, 42]
print("My favorite numbers are " + str(numbers[0]) + ", " + str(numbers[1]) + ", and " + str(numbers[2]) + ".")

My favorite numbers are 7, 23, and 42.


You can mix string and numerical placeholders in any order you want.

In [53]:
names = ['eric', 'ever']
numbers = [23, 2]
print("%s's favorite number is %d, and %s's favorite number is %d." % (names[0].title(), numbers[0], names[1].title(), numbers[1]))

Eric's favorite number is 23, and Ever's favorite number is 2.


There are more sophisticated ways to do string formatting in Python 3, but we will save that for later because it's a bit less intuitive than this approach. For now, you can use whichever approach consistently gets you the output that you want to see.

## Exercises¶

#### Gymnast Scores¶

• A gymnast can earn a score between 1 and 10 from each judge; nothing lower, nothing higher. All scores are integer values; there are no decimal scores from a single judge.
• Store the possible scores a gymnast can earn from one judge in a tuple.
• Print out the sentence, "The lowest possible score is ___, and the highest possible score is ___." Use the values from your tuple.
• Print out a series of sentences, "A judge can give a gymnast ___ points."
• Don't worry if your first sentence reads "A judge can give a gymnast 1 points."
• However, you get 1000 bonus internet points if you can use a for loop, and have correct grammar. hint

#### Revision with Tuples¶

• Choose a program you have already written that uses string concatenation.
• Save the program with the same filename, but add _tuple.py to the end. For example, gymnast_scores.py becomes gymnast_scores_tuple.py.
• Rewrite your string sections using %s and %d instead of concatenation.
• Repeat this with two other programs you have already written.

# Coding Style: PEP 8¶

You are now starting to write Python programs that have a little substance. Your programs are growing a little longer, and there is a little more structure to your programs. This is a really good time to consider your overall style in writing code.

## Why do we need style conventions?¶

The people who originally developed Python made some of their decisions based on the realization that code is read much more often than it is written. The original developers paid as much attention to making the language easy to read, as well as easy to write. Python has gained a lot of respect as a programming language because of how readable the code is. You have seen that Python uses indentation to show which lines in a program are grouped together. This makes the structure of your code visible to anyone who reads it. There are, however, some styling decisions we get to make as programmers that can make our programs more readable for ourselves, and for others.

#### Yourself, 6 months from now¶

• You know what you are thinking when you write code for the first time. But how easily will you recall what you were thinking when you come back to that code tomorrow, next week, or six months from now? We want our code to be as easy to read as possible six months from now, so we can jump back into our projects when we want to.

#### Other programmers you might want to collaborate with¶

• Every significant project is the result of collaboration these days. If you stay in programming, you will work with others in jobs and in open source projects. If you write readable code with good commments, people will be happy to work with you in any setting.

#### Potential employers¶

• Most people who hire programmers will ask to see some code you have written, and they will probably ask you to write some code during your interview. If you are in the habit of writing code that is easy to read, you will do well in these situations.

## What is a PEP?¶

A PEP is a Python Enhancement Proposal. When people want to suggest changes to the actual Python language, someone drafts a Python Enhancement Proposal. One of the earliest PEPs was a collection of guidelines for writing code that is easy to read. It was PEP 8, the Style Guide for Python Code. There is a lot in there that won't make sense to you for some time yet, but there are some suggestions that you should be aware of from the beginning. Starting with good style habits now will help you write clean code from the beginning, which will help you make sense of your code as well.

## Basic Python style guidelines¶

#### Indentation¶

• Use 4 spaces for indentation. This is enough space to give your code some visual structure, while leaving room for multiple indentation levels. There are configuration settings in most editors to automatically convert tabs to 4 spaces, and it is a good idea to check this setting. On Geany, this is under Edit>Preferences>Editor>Indentation; set Width to 4, and Type to Spaces.

#### Line Length¶

• Use up to 79 characters per line of code, and 72 characters for comments. This is a style guideline that some people adhere to and others completely ignore. This used to relate to a limit on the display size of most monitors. Now almost every monitor is capable of showing much more than 80 characters per line. But we often work in terminals, which are not always high-resolution. We also like to have multiple code files open, next to each other. It turns out this is still a useful guideline to follow in most cases. There is a secondary guideline of sticking to 99 characters per line, if you want longer lines.

Many editors have a setting that shows a vertical line that helps you keep your lines to a certain length. In Geany, you can find this setting under Edit>Preferences>Editor>Display. Make sure "Long Line Marker" is enabled, and set "Column" to 79.

#### Blank Lines¶

• Use single blank lines to break up your code into meaningful blocks. You have seen this in many examples so far. You can use two blank lines in longer programs, but don't get excessive with blank lines.

• Use a single space after the pound sign at the beginning of a line. If you are writing more than one paragraph, use an empty line with a pound sign between paragraphs.

#### Naming Variables¶

• Name variables and program files using only lowercase letters, underscores, and numbers. Python won't complain or throw errors if you use capitalization, but you will mislead other programmers if you use capital letters in variables at this point.

That's all for now. We will go over more style guidelines as we introduce more complicated programming structures. If you follow these guidelines for now, you will be well on your way to writing readable code that professionals will respect.

## Exercises¶

#### Skim PEP 8¶

• If you haven't done so already, skim PEP 8 - Style Guide for Python Code. As you continue to learn Python, go back and look at this every once in a while. I can't stress enough that many good programmers will take you much more seriously from the start if you are following community-wide conventions as you write your code.

#### Implement PEP 8¶

• Take three of your longest programs, and add the extension _pep8.py to the filename of each program. Revise your code so that it meets the styling conventions listed above.

# Overall Challenges¶

#### Programming Words¶

• Make a list of the most important words you have learned in programming so far. You should have terms such as list,
• Make a corresponding list of definitions. Fill your list with 'definition'.
• Use a for loop to print out each word and its corresponding definition.
• Maintain this program until you get to the section on Python's Dictionaries.

# Hints¶

These are placed at the bottom, so you can have a chance to solve exercises without seeing any hints.

#### Gymnast Scores¶

• Hint: Use a slice.