What, How and Why

What is a list?

A list is just a collection of things. And it can be a collection of anything. You can have numbers, you can have text, you can have objects, you can have a mix of things, anything at all.

This is useful for not needing to write a million variables for similar things. 💯🦍

Part 1 - Creating Lists

Creating a list is very simple. All we need to do is use square brackets [ and ].

Task 1.1a

Use square brackets and create a list containing animals, then print it.

dyr = ["katt", "hund", "mus", "fisk", "fugl"]
print(dyr)

animal1 = "cat"
animal2 = "dog"
animal3 = "mouse"
animal4 = "fish"
animal5 = "bird"
animal6 = "elephant"
animal7 = "kangaroo"
animal8 = "koala"
animal9 = "parrot"
animal10 = "tiger"

animals = ["cat", "dog", "mouse", "fish", "bird", "elephant", "kangaroo", "koala", "parrot", "tiger"]
print(animals)

Task 1.1b

Add some more animals to the list using additional items, what happens now?

Task 1.2

Lists don’t have to contain only text, try creating a list with numbers.

tall = [47, 137, 214, 1337]
print(tall)

Task 1.3

Now try to create a list that contains a mix of text and numbers to see what happens:

liste = ["cat", 1337, 214, "dog", "fish", 137]

print(liste)

Part 2 - Retrieving from Lists

In Part 1 we saw that we can use print() to print out the entire list. But what if we only want to retrieve something from the list? Then we can use [] to retrieve an “index”.

Task 2.1

Look at the list below:

dyr = ["katt", "hund", "kenguru", "fisk", "mus"] # animals

What happens when you write, meaning we want to retrieve what is at index number 1 in the list:

# Skriver ut det andre elementet i listen 'dyr'
print(dyr[1])

Why is "hund" printed, and not "katt"?

print(dyr[0])

Task 2.2

Dette er en oppgave hvor du skal implementere en funksjon som beregner gjennomsnittlig alder av en liste med personer. Hver person er representert som et dictionary med nøkkelen “alder” som inneholder personens alder i år.

Input: En liste med dictionaries, hvor hvert dictionary representerer en person og har en “alder”-nøkkel.

Output: Gjennomsnittlig alder av personene i listen. Hvis listen er tom, skal funksjonen returnere 0.

Eksempel:

personer = [
    {"navn": "Alice", "alder": 30},
    {"navn": "Bob", "alder": 25},
    {"navn": "Charlie", "alder": 35}
]

gjennomsnittlig_alder = beregn_gjennomsnittlig_alder(personer)
print(gjennomsnittlig_alder)  # Output: 30.0

Krav:

• Funksjonen skal håndtere tomme lister.
• Funksjonen skal håndtere lister med dictionaries som ikke inneholder “alder”-nøkkelen (i slike tilfeller skal personen ignoreres).
• Funksjonen skal returnere en float.

This is a task where you are to implement a function that calculates the average age of a list of people. Each person is represented as a dictionary with the key “age” containing the person’s age in years.

Input: A list of dictionaries, where each dictionary represents a person and has an “age” key.

Output: The average age of the people in the list. If the list is empty, the function should return 0.

Example:

people = [
    {"name": "Alice", "age": 30},
    {"name": "Bob", "age": 25},
    {"name": "Charlie", "age": 35}
]

average_age = calculate_average_age(people)
print(average_age)  # Output: 30.0

Requirements:

• The function should handle empty lists.
• The function should handle lists with dictionaries that do not contain the “age” key (in such cases, the person should be ignored).
• The function should return a float.

dyr = ["katt", "hund", "kenguru", "fisk", "mus", "gorilla", "hest", "ku", "elefant"]

From the example list above, try to fetch and print() the following:

• "kenguru"
• "fisk"
• "elefant"
• "hest"

Task 2.3

What happens if you try to write:

# Skriver ut det tiende elementet i listen 'dyr'
print(dyr[9])

Task 2.4 - Retrieve and Store

Try to retrieve a value from the list in Task 2.2, and then store the value in a variable.

animals = ["cat", "dog", "kangaroo", "fish", "mouse", "gorilla", "horse", "cow", "elephant"]

animal = animals[3]

Task 2.5a

Fetch two different animals and store them in variables, for example animal1 and animal2.

dyr = ["katt", "hund", "kenguru", "fisk", "mus", "gorilla", "hest", "ku", "elefant"]

dyret1 = dyr[3]
dyret2 = dyr[5]

Task 2.5b

Use an if and else check to see if the animals are the same or not.

dyr = ["katt", "hund", "kenguru", "fisk", "mus", "gorilla", "hest", "ku", "elefant"]

dyret1 = dyr[3]
dyret2 = dyr[5]

if dyret1 == dyret2:
    print("Dyrene er like!")
else:
    print("Dyrene er ikke like... :(")

What happens if you try to retrieve the same index on both animals now?

Part 3 - Changing the Value of Something in a List

Task 3.1a

Try to write this code:

tall = [1, 5, 7, 11, 13, 17, 19, 23]
print(tall[3])

tall[3] = 137
print(tall[3])

Here we see that we can change the value in a list, values in lists largely function like a variable.

Task 3.1b

Try to change another index with a value.

Task 3.1c

Try printing the entire list in both print() statements, is it clearer then?

tall = [1, 5, 7, 11, 13, 17, 19, 23]
print(tall)

tall[3] = 137
print(tall)

[1, 5, 7, 11, 13, 17, 19, 23]
[1, 5, 7, 137, 13, 17, 19, 23]

Part 4 - Adding Items to a List

A very useful concept with lists is that it is possible to add things to lists. Here we will learn two things:

1. Adding things to lists
2. Using functions on lists

The second probably doesn’t tell you much, but we will look at what this means with an example! 👍

Task 4.1 - The append function

Look at this list:

dyr = ["katt", "hund", "kenguru"] # animals
print(dyr)

Use the append function to add a new animal to this list, then print the list again.

animals.append("the animal")

dyr = ["katt", "hund", "kenguru"]
print(dyr)

dyr.append("elefant")
print(dyr)

Task 4.2

Then try to add a few more things to the list with multiple lines using append.

Task 4.3

Ask the user for an animal to add using input, then add the animal to the list.

animals = ["cat", "dog", "kangaroo"]
print(animals)

animal = input("Enter a new animal: ")

animals.append(animal)
print(animals)

Part 5 - Deleting Items from a List

I hate cats! I want to erase cats from the world! (🐱 -> 💀)

We will now look at two ways to delete things from lists. And that is by using two new functions:

• remove
• pop

The remove function

The remove function is a function that can be used to delete things from a list. It takes as input a value we want to delete.

Task 5.1 - Use remove

Look at this list:

dyr = ["katt", "hund", "kenguru"] # animals
print(dyr)

Use the remove function to delete "cat" from this list.

Hint: it uses exactly the same spelling as append but where we write remove instead.

dyr = ["katt", "hund", "kenguru"]
print(dyr)

dyr.remove("katt")
print(dyr)

Task 5.2

Use remove to delete some other things from the list.

Task 5.3

Use append to add something to a list, then use remove to delete it from the list again.

dyr = ["katt", "hund", "kenguru"]
print(dyr)

dyr.append("elefant")
print(dyr)

dyr.remove("elefant")
print(dyr)

pop function

This function unfortunately doesn’t play pop music (💃🕺), but it can “pop” something off a list. It’s probably best we look at an example.

pop function is another function that can be used to delete things from a list. Instead of taking the value of what we want to delete, it takes an index for deletion.

Task 5.3 - Use pop

Start with this code:

dyr = ["katt", "hund", "kenguru", "elefant"] # animals
print(dyr)

Use pop to delete "cat" from the list.

animals.pop(index)

animals = ["cat", "dog", "kangaroo", "elephant"]
print(animals)

animals.pop(0)
print(animals)

Task 5.4 - Use pop again

After deleting "cat", try deleting "dog" from the list. What do you need to do?

animals = ["cat", "dog", "kangaroo", "elephant"]
print(animals)

animals.pop(0) # deletes cat
animals.pop(0) # deletes dog
print(animals)

Part 6 - Sorting a List

The last thing we will look at in Python 2 is sorting lists. This is a very important function.

Here the sort function is used!

Task 6.1 - Use sort

Start with this code:

dyr = ["katt", "hund", "kenguru", "elefant", "koala", "fugl", "papegøye", "mus"] # animals
print(dyr)

Use the sort function to sort the animals, then print the list to see the difference. Here, functions are used in a similar way, but you don’t need to take any arguments in. If you are unsure what this means, look at the answer below.

animals = ["cat", "dog", "kangaroo", "elephant", "koala", "bird", "parrot", "mouse"]
print(animals)

animals.sort()
print(animals)

Task 6.2 - Sorting Numbers

You can sort things other than text as well, try sorting a list of numbers. Here is an example list (feel free to use your own):

tall = [1, 5, 92, 12, 137, 49, 47, 137, 22, 0, 214, -3, -56, 23]
# Dette er en liste med tall. # This is a list of numbers.

tall = [1, 5, 92, 12, 137, 49, 47, 137, 22, 0, 214, -3, -56, 23]

tall.sort()
# Prints out [-56, -3, 0, 1, 5, 12, 22, 23, 47, 49, 92, 137, 137, 214]
print(tall)

Extra - Other list functions that may be useful

If you want more information about these, you can go here: W3schools Python Lists

dyr = ["katt", "hund", "kenguru", "elefant"]
print(dyr)

dyr.clear()
print(dyr) # the list is empty

dyr = ["katt", "hund", "kenguru", "elefant"]
print(dyr)

andre_dyr = dyr.clone()
print(andre_dyr) # the list is the same as dyr

dyr = ["katt", "hund", "kenguru", "elefant", "katt", "mus", "elefant", "katt"]

print(dyr.count("hund"))    # 1 dog in the list
print(dyr.count("elefant")) # 2 elephants in the list
print(dyr.count("katt"))    # 3 cats in the list

dyr = ["katt", "hund", "kenguru", "elefant"]

print(dyr.index("kenguru")) # prints 2, since that's where it is.

dyr = ["katt", "hund", "kenguru", "elefant"]
print(dyr)

dyr.reverse()

print(dyr) # prints out ["elefant", "kenguru", "hund", "katt"]