Chapter 8: Lists

8.1 Storing Lots of Variables

Imagine that we were writing a program where we wanted to store a large amount of data. For example, imagine we were writing a program to figure calculate your grade in a class. We might want to store all of our grades into the program. Let’s say we have 10 quiz grades in the class. We might write code like this:

quiz1 = 88
quiz2 = 94
quiz3 = 76
quiz4 = 100
quiz5 = 92
quiz6 = 89
quiz7 = 95
quiz8 = 85
quiz9 = 79
quiz10 = 99

Then we might want to add up all of these grades to figure out our average:

total = quiz1 + quiz2 + quiz3 + quiz4 + quiz5 + quiz6 + quiz7 + quiz8 + quiz9 + quiz10
average = total / 10

This hopefully seems a little bit tedious and repetitive. We have to make 10 different variables and do the same thing with all of them. Here, it’s not too bad with 10. But imagine if we had even more numbers we wanted to keep track. For instance, imagine you’re teaching a course and wanted to store all 10 quiz grades from 25 students. That would be a lot of variables!

There is a better way of doing this which is to use a list. A list is a collection of multiple pieces of data that are stored together in one variable.

8.2 Creating Lists

To create a list, we can put the values in the list between square brackets, separated by commas. For example, we can store our 10 quiz grades in a list like this:

quizzes = [88, 94, 76, 100, 92, 89, 95, 85, 79, 99]

We could also create a list of strings:

names = ["Bob", "Alice", "Joe", "Mary"]

Each of these lists stores all of the values inside them. Lists provide a convenient way to store a bunch of things together with one name to access them.

8.3 Accessing List Elements

Once we have created a list, we can access each thing in the list. To do this, we can use the position of each element we want to access. Like strings, the first thing in the list is at position 0. The second element is at position 1, and so on. Starting at position 1 is a common mistake in programming.

In order to access an element, we use the name of the list, then the position inside of brackets. So to access the first quiz grade in the above list, we would use:

print(quizzes[0])

To access the last name above, we can say:

print(names[3])

When we use the position number to access the things inside of a list, we say that we are indexing the list.

If we use an index that is too big for our list, Python will give us an error message:

>>> print(names[4])
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in 
    print(names[4])
IndexError: list index out of range

8.4 Example: Dates

Let’s say we want to write a program that converts a numerical date into one using words for the month. For example, we can put in 3 for the month and 25 for the day and it will print out “March 25”. We could do it with if statements like this:

if month == 1:
    print("January", day)
elif month == 2:
    print("February", day)
elif month == 3:
    print("March", day)
# and so on...

But this can actually be done with less code (and more efficiently) using lists. We can make a list of all the month names. We could then index the list with the month number. We will have to subtract 1 from the index because month numbers start at 1, but list numbers start at 0.

# get our input
month = int(input("What is the month? "))
day = int(input("What is the day? "))

# make a list of all the names of months
names = ["January", "February", "March", "April", "May", "June", "July",
         "August", "September", "October", "November", "December"]

# get the name of this month by indexing
monthName = names[month - 1]

# print our output
print("It is", monthName, day)

This program works by taking in the month as a number. It then subtracts one from this number and uses it to index the list. So if the month number is 5, it subtracts 1 to get 4. It then uses 4 as the index to get the name “May” out of the list.

Below is an example of this program running:

What is the month? 6
What is the day? 22
It is June 22

8.5 Looping Through a List

One super common thing to do with a list is to loop through everything in the list and do something with each thing in it. For example, we might loop through a list of our quiz scores to add them up, or loop through a list of names searching for one name in particular.

We can do this by using a for loop in Python. We have seen for loops for looping through strings and sequences of numbers using range(). They also work for lists.

For example, if we want to print all of our quiz grades, we could write code like this:

quizzes = [88, 94, 76, 100, 92, 89, 95, 85, 79, 99]

for quiz in quizzes:
    print(quiz)

The for loop assigns each thing in the list to quiz, one by one, and executes the lines under the for loop on it. In this case, it will print all of the quiz scores to the screen one by one. By changing the code in the loop, we can do different things with each item.

To add all of the quiz scores together, we can do the following:

# our list of quiz scores
quizzes = [88, 94, 76, 100, 92, 89, 95, 85, 79, 99]

# figure out the total score
total = 0
for quiz in quizzes:
    total = total + quiz

# find the average and print it
average = total / len(quizzes)
print("Your average score is", average)

The total variable here is worth talking about a little bit. We set it to 0 before the loop and then also set it inside the loop. What we set it to there is total + quiz. So the first thing it’s equal to is 0, then we set it to that 0 plus the first quiz value, so it becomes 88. Then the next time through the loop, we set it to the 88 it is now plus the 94, resulting in total storing 182. This kind of “accumulation” loop is common in coding.

This is much less code than having to add all 10 variables by typing them all out! We also can add more quiz scores to the list without having to redo the code to find the average. This example also shows using the len function to find the length of a list — this works the same way it does for strings.

8.6 Example: Smart Guess the Number

When we first looked at using loops, we saw an example of a guess the number program that started at 1, and then went on guessing up to 10. To refresh your memory, this code looked like this:

# guess 1 the first time
guess = 1
answer = input("Did you guess " + str(guess) + "? ")

# keep guessing the next number until they get it
while answer != "yes":
    guess = guess + 1
    answer = input("Did you guess " + str(guess) + "? ")

print("Got it!")

This program does in fact eventually guess the number the user is thinking of, but it doesn’t guess them in a very human-like manner. Only a robot would guess in order like that. Another way is to guess the numbers randomly, instead of in order:

import random

# guess a random number
guess = random.randint(1, 10)
answer = input("Did you guess " + str(guess) + "? ")

# keep guessing in order until we get it
while answer != "yes":
    guess = random.randint(1, 10)
    answer = input("Did you guess " + str(guess) + "? ")

print("Got it!")

Now the program no longer guesses the numbers in order, but it still doesn’t guess them very well. Now each guess is random with no memory of what the previous guesses were. It could even guess the same number twice in a row. What we’d like is for the number to guess the numbers from 1 to 10 in random order without repeating itself.

We can do this using a list of numbers to guess and then “shuffling” the list. The idea here is that we will make a list to store all of the numbers 1 through 10. Then we call the random.shuffle function which takes a list and shuffles it randomly.

We then loop through this shuffled list and guess the numbers in it one by one. Now the program will guess them in a random way, but it won’t ever guess the same number twice.

The code to do this is below:

import random

guesses = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
random.shuffle(guesses)

for number in guesses:
     if input("Did you guess " + str(number) + "? ") == "yes":
          print("Got it!")
          break

There’s something else new in this program which is the break command. This exits out of the loop immediately when we run it. By doing the break when we get the number right, we cause the loop to exit early, when it might not have gone through all of the numbers yet. Without this, the program would keep asking us until it went through all 10 numbers.

An example run of this program is below:

Did you guess 6? no
Did you guess 8? no
Did you guess 4? no
Did you guess 2? no
Did you guess 5? no
Did you guess 1? no
Did you guess 7? yes
Got it!

8.7 Splitting Input

One very helpful thing we can do with strings is to call their split method, which splits the string into parts by some separator (which we get to choose). This can allow us to go through each word of a sentence, for example.

The following program does just that. It asks the user to input a sentence. It then calls the split method to split it based on spaces. This gives us back a list variable (which we call words). We can then do what we want to with those individual words. Here we just print them out.

# read in a whole sentence
sentence = input("Enter a sentence: ")

# split it into words (separated by spaces)
words = sentence.split(" ")

# loop through each one
for word in words:
    print(word)

Below is an example run of this program:

Enter a sentence: the quick brown fox jumps over the lazy dog
the
quick
brown
fox
jumps
over
the
lazy
dog

We can use this to improve our quiz grade averaging program. In this program we “hard-coded” our quiz scores into the program with the following line of code:

quizzes = [88, 94, 76, 100, 92, 89, 95, 85, 79, 99]

Let’s say that our program was so super helpful that we wanted to share it with our friends. We would not necessarily want them to have to edit our code to put in their own grades. So instead we should make it so the program asks you for the quiz grades.

There are a couple of ways to do this, but one is to make the program read them in on one line and then split them up. We can use any kind of separator we want to separate the numbers. Let’s say we want to use a comma.

The following program does this by asking the user to enter their quiz grades, splitting them into individual things in a list, and then looping through them.

# read in a whole sentence
quizzes = input("Enter quiz grades separated by commas: ")

# split it into parts
quizzes = quizzes.split(",")

# loop through each one getting total
total = 0
for quiz in quizzes:
    total = total + int(quiz)

# print average
print("Average is", total / len(quizzes))

One wrinkle here is that our list is actually storing strings, because that’s what the split method returns to us. So when we do the adding we have got to convert the numbers to int first.

Below is an example of this program being run:

Enter quiz grades separated by commas: 92,78,88,70,100,94
Average is 87.0

Now we are reading in the list from the user, and looping over it to calculate the average.

8.8 Adding to a List

So far we have looked at making lists all in one go, either by getting the list contents from split, or by listing the things inside brackets, like this:

names = ["Bob", "Alice", "Joe", "Mary"]

But what if we want to add an item to a list that already exists? This can be done with the append list method. For instance, this code will add two new names to the list:

names.append("Joe")
names.append("Beatrice")

This allows us build a list as we go, rather than create the whole thing at once. As we will see, there are lots of cases where being able to add to a list is handy.

The append method adds items on to the end of the list. If we wanted to add an item somewhere else, we can use the insert method instead. This method takes two parameters. The first is the index we want to insert at. The second parameter is the item we would like to insert.

The following code starts by making an empty list, and then inserting some names into it at different positions:

names = []
names.insert(0, "Bob")
names.insert(0, "Alice")
names.insert(1, "James")
print(names)

This example will print ['Alice', 'James', 'Bob'].

When “Bob” is inserted at position 0, it’s the only one, so the list is ["Bob"]. Then, when “Alice” is inserted at location 0, the list becomes ["Alice", "Bob"]. Finally, when “James” is inserted at location 1, he is inserted between Alice and Bob to make the list ["Alice", "James", "Bob"].

If we care about the order of the list, insert lets us choose where to put new items.

8.9 Reading in a List

We’ve seen one way to read in a list in Python, using the split method. Here we ask the user to enter all the values on one line, with some separator like a space or comma. This works well sometimes, but there are some downsides. One is that it reads it all in as strings, and another is that it might be inconvenient if there’s a lot of items to read.

Another way to input a list from the user is to read in each value separately and then add them to the list one by one. With this approach, we can read them in as numbers.

The following program attempts to do this:

numbers = []
while True:
    item = int(input("Enter a number: "))
    numbers.append(item)
    print(numbers)

The only problem with this code is that it is an infinite loop. We need to have some way of knowing when to stop!

There are two ways that this could be done:

1. Ask the user up front how many items they would like to enter, and then loop that many times. The following example does this:

quizzes = []
count = int(input("How many quizzes are there? "))

for i in range(count):
    item = int(input("Enter a quiz grade: "))
    quizzes.append(item)

2. Have a certain value reserved to mean “I’m done now”. A value used in this manner is called a sentinel value. If we are entering numbers that should always be positive, like quiz grades, then we can use -1 as the sentinel.

   An example doing it this way is below:

quizzes = []
item = int(input("Enter a quiz grade: "))

while item >= 0:
    quizzes.append(item)
    item = int(input("Enter a quiz grade: "))
    
print(quizzes)

Note in this example that we need to read in a quiz grade twice. The first time is done before the loop to make sure that the item variable is defined before we test it in our while condition. Then we read it again inside the loop to make sure it can happen for every quiz the user wants to enter.

8.10 Comprehension Questions

1. What is the main purpose of using lists in Python?
2. How can you access the third element in a list named items?
3. If you have a list pets equal to ["dog", "cat", "bird"], how do you add “fish” to the end of the list?
4. What will len(pets) return after executing the append operation in the previous question?
5. Is it more common to loop over a list with a for loop or a while loop?
6. What happens if you try to access an index from a list that does not exist? Like it we try to read pets[99] in the list above?

8.11 Programming Exercises

1. Write a program to read in a list of numbers and print one randomly selected element.
2. Write a program to read in a list of strings and then one string to search for. Then loop through the list and tell the user if the string they are looking for is in the list or not.
3. Write a program which reads in a list of numbers from the user and then prints all of those numbers which are greater than 50.
4. Write a program which reads in a list of strings from the user. Your program should then print out the list of strings backwards. There are two main ways to do this. You could put the strings into the list backwards right when you read them in. This will build the list backwards, and then you can print it out forwards. Or you could put the strings into the list in order, then loop through them backwards when you print them. You can use a for loop with range to go through the indexes in reverse order.

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Licensed under a Creative Commons Attribution 4.0 International License.