Lists

Lists are by far the most used data structure in Python, so we will spend a bit more time on them.

The list type in Python is used to store multiple items (objects) in a single variable.

We have already seen them in use, but here is a quick example:

car_list = ["Volvo", "Audi", "Honda", "Ford", "Mercedes"]

This is a list of strings which has 5 items. If you need convincing that this is a list type, then you can use type(car_list) to check the type.

Lists are hetrogeneous, which is a fancy way of saying we can store different data types. They can also store duplicates e.g.

car_list = ["Volvo", 1, False, "Ford", "Ford", False, 3.14]

This list has both different types and duplicates.

1. List Items

Each object in the list is called an item (element). We will use the previous example to demonstrate simple operations on lists.

If you want to follow along then type this into the Python interactive shell.

car_list = ["Volvo", "Audi", "Honda", "Ford", "Mercedes"]

1.1 Access List Items

You can access a list item by it's index. Python indexing starts at 0, so to get the first item of a list you would use.

For example we can print the first item using,

print(car_list[0]) # prints out Volvo

or the third item by using:

car_list[2] # prints out Honda

Notice how the car_list has 5 items. So it's indexing goes from 0-4.

If you try typing,

car_list[5] # IndexError

you will get an IndexError because your list index if not in the valid range 0-4.

1.1.1 Slicing

You can also access slices (portions) of a list. For example we can access items 1,2,3 using:

print(car_list[1:4]) # prints ["Volvo", "Audi", "Honda"]

1.1.2 Negative Indices

You can also access from the back of the list. To get the last item use -1.

print(car_list[-1]) # prints ["Mercedes"]

You can also get slices such as the last three items.

print(car_list[-4,-1]) # prints ["Honda", "Ford", "Mercedes"]

1.2 Changing List Items

You change a list item using indexing as follows:

car_list[2] = "Ferrari"

If we print out the list,

print(car_list) # prints ["Volvo", "Audi", "Ferrari", "Ford", "Mercedes"]

you will get ['Volvo', 'Audi', 'Ferrari', 'Ford', 'Mercedes'].

1.3 Adding List Items

You can add to a list by using the append() and insert() methods.

Methods are very much like functions, they take in arguments and return something. Hey, they even look like them!

The key difference is a method is attached to an object and you call the method using dot notation.

To add to the end of the list use append() method. Note that it is called using a .:

car_list.append("Skoda") # ["Volvo", "Audi", "Ferrari", "Ford", "Mercedes", "Skoda"]

Here the str "Skoda" is added to the end of the list.

To insert an item somewhere use the insert() method, you supply the index as the first parameter and the item as the second.

car_list.insert(2, "Porsche") # insert "Porsche" at index 2, i.e. 3rd item 
# ["Volvo", "Audi", "Porsche", "Ferrari", "Ford", "Mercedes", "Skoda"]

1.4 Extending and Joining Lists

To extend the list by multiple items you can use either the extend() method or the + operator.

list_one = ["a", "b" , "c"]
list_two = [1, 2, 3]
list_one.extend(list_two) # list_one now contains ["a", "b", "c", 1, 2, 3]

list_one = ["a", "b" , "c"]
list_two = [1, 2, 3]
list_three = list_one + list_two # list_three contains ["a", "b", "c", 1, 2, 3]

Note the difference, the extend() method extends the list it is operating on (e.g. list_one). The + operator creates a new list by joining the lists, we stored this in list_three.

We can use the + operator to join many lists.

list_one = ["a", "b" , "c"]
list_two = [1, 2, 3]
list_three = [True, False]
list_four = list_one + list_two + list_three # list4 contains ["a", "b", "c", 1, 2, 3, True, False]

1.5 Remove List Items

Again working on the car example:

car_list = ["Volvo", "Audi", "Honda", "Ford", "Mercedes"]

You can remove a list item using either remove() or pop() methods, or the del keyword.

remove() lets you remove an item by specifying the value of the item, .e.g.

car_list.remove("Audi") # removes the second item of the list 

If you try to remove an object that isn't in the list you will get a ValueError. For example

car_list.remove("Ferrari") # ValueError: list.remove(x): x not in list

Note that this returns None. This is demonstrated by this code snippet.

car_list = ["Volvo", "Audi", "Honda", "Ford", "Mercedes"]
item_removed = car_list.remove("Audi")
print(car_list) # prints ["Volvo", "Honda", "Ford", "Mercedes"]
print(item_removed) # prints None

Here we stored the return value of remove() in item_removed which was None.

pop() lets you remove an item by index, e.g.

car_list.pop(1) # removes the second item of the list

Note that this returns the item that is removed from the list. This is demonstrated by this code snippet.

car_list = ["Volvo", "Audi", "Honda", "Ford", "Mercedes"]
item_removed = car_list.pop(1)
print(car_list) # prints ["Volvo", "Honda", "Ford", "Mercedes"]
print(item_removed) # prints Audi

The del keyword also lets you remove by index, e.g.

del car_list[1] # removes the second item of the list

We can also remove slices.

car_list = ["Volvo", "Audi", "Honda", "Ford", "Mercedes"]
del car_list[1:3]
print(car_list) # prints ["Volvo", "Ford", "Mercedes"]

1.6 Note on In-place Methods

All the methods we have seen operating on a list are known as an in-place method as they update the list directly. We will discuss this more in the next lesson.

Also be careful with understanding what a method returns. If you tried the following,

print(car_list.append("Renault")) # prints None

it will update the list, but print the return value which is None.

2. List Methods

There are a number of built-in list methods you can use:

For example we can sort the list as follows:

car_list = ["Volvo", "Audi", "Honda", "Ford", "Mercedes"]

car_list.sort() # in-place operation, sorts car_list

print(car_list) # prints ['Audi', 'Ford', 'Honda', 'Mercedes', 'Volvo']

3. Looping Through a List

You can loop through a list using a for loop.

car_list = ["Volvo", "Audi", "Honda", "Ford", "Mercedes"]

for car in car_list:
    print(car)

# prints out...
# Volvo
# Audi
# Honda
# Ford
# Mercedes

We can also loop through a list using a while loop.

car_list = ["Volvo", "Audi", "Honda", "Ford", "Mercedes"]

i = 0
while i < len(car_list):
    print(car_list[i])
    i += 1

# prints out...
# Volvo
# Audi
# Honda
# Ford
# Mercedes

3.1. List of lists

We can also have lists of lists. These are very common and you will need to now how to iterate (loop) over them

Here is an example that just iterates over the list and prints out each of the sublists.

list_one = [[4,2,6], [3,8,3], [5,4,7]]
for sub_list in list_one:
    print(sub_list)

# prints out...
# [4,2,6]
# [3,8,3]
# [5,4,7]

As each of the sublists is a list, we can iterate over each of these.

list_one = [[4, 2, 6], [3, 8, 3], [5, 4, 7]]
for sub_list in list_one:
    for x in sub_list:
        print(x)
# prints out...
# 4
# 2
# 6
# 3
# 8
# 3
# 5
# 4
# 7

We can even write a simple program that gives us back a list of the total of each sublist. Here we create an empty list called sum_list and then we compute the total of each sublist and store it in the sum_list.

list_one = [[4, 2, 6], [3, 8, 3], [5, 4, 7]]

sum_list = []

for sub_list in list_one:
    total = 0
    for x in sub_list:  # iterate over the sub_list
        total += x  # keep adding items in the sub_list to total

    sum_list.append(total)  # append the result to sum_list

print(sum_list)  # prints [12,14,16]

3.2. Zipping Two Lists

Zipping lists together is a very useful thing to do when we want to combine lists together and loop over them.

The zip operation is demonstrated by the following code snippet. Note both must be the same length.

list_one = [1,2,3]
list_two = ["a","b","c"]

for x,y in zip(list_one, list_two):
    print(x,y)

# prints
# 1 a
# 2 b
# 3 c

=== TASK ===

Copy the following into a new Python file to get started.

def longest_word(string_list):
    pass # placeholder, delete and replace with your code
  
def num_characters(string_list):
    pass # placeholder, delete and replace with your code
  
if __name__ == "__main__":
    test_string = [["Hello", "World"], ["You", "say", "you", "want", "a", "revolution"], ["It's", "fine", "to", "celebrate", "success", "but", "it", "is", "more", "important", "to", "heed", "the", "lessons", "of", "failure"]]
    test_string2 = [["Hello", "World"], ["Apple", "Pears"]]
    print(longest_word(test_string)) # should print revolution
    print(num_characters(test_string)) # should print 105
    print(longest_word(test_string2)) # should print hello
    print(num_characters(test_string2)) # should print 20

The first function is called longest_word which takes in a list of a list of strings and returns the longest word in any of the lists. In the case of equal length words, return the first word you encounter. In the case of no words, e.g. an empty list [] return None.

For example:

longest_word([["Hello", "World"], ["You", "say", "you", "want", "a", "revolution"], ["It's", "fine", "to", "celebrate", "success", "but", "it", "is", "more", "important", "to", "heed", "the", "lessons", "of", "failure"]]) # return revolution

The second is called num_characters which takes in a list of a list of strings and returns the total number of characters in all of the strings in the list lists.

num_characters([["Hello", "World"], ["You", "say", "you", "want", "a", "revolution"], ["It's", "fine", "to", "celebrate", "success", "but", "it", "is", "more", "important", "to", "heed", "the", "lessons", "of", "failure"]]) # return 105

References

Python List Methods - W3schools