TGGS Computer Science

Spring 1 - Lesson 5 - Lists and For Loops

🐍 Activity 1 – Understanding Lists

🔎 Understanding Lists 1: What is a List?

A Python list is like a variable but it can contain more than one item

dogs = ["pug", "cockerpoo", "labradoodle", "dachsund", "jack russell"]

🔎 Understanding Lists 2: Index Numbers

We can access any item in a Python list via its index number

dogs = ["pug", "cockerpoo", "labradoodle", "dachsund", "jack russell"]

print(dogs[1]) # This outputs cockerpoo
print(dogs[3]) # This outputs dachsund

🔎 Understanding Lists 3: Curious properties

A curious property of a Python list is that:

  • The index number of the first item is 0
  • The index number of the last item is 4
# index   0         1             2            3              4
dogs = ["pug", "cockerpoo", "labradoodle", "dachsund", "jack russell"]

print(dogs[0]) # This outputs pug
print(dogs[4]) # This outputs jack russell

🐍 Activity 1 - Coding Challenges

Open the file called Activity 1.py

  • Read the instruction for each challenge
  • Then complete it
📖 Activity 2 – Recapping For Loops

🔎 Recap - How a for loop works (1)

  • You now know a for loop is controlled by a special variable
  • This special variable is called the loop counter variable
  • We will generally use the letter i for this variable; however it can be anything
   ⬇️
for i in range(1,11):

🔎 Recap - How a for loop works (2)

  • We use the range() function to decide how many times a for loop will iterate (repeat)
  • This loop begins with i being set to 0
    •               ⬇️                              
for i in range(0, 4):
  • i will increase by 1, every iteration of the loop
  • This will continue to occur until i is equal to 4
  • At which point, the loop will end
    • i = 0 
 🔁
  i = 1
   🔁 
    i = 2
     🔁
      i = 3
       🔁
        i = 4 ... END!

🔎 Recap - How a for loop works (3)

  • If we place lines code inside the for loop
  • They will be executed each iteration
    • for i in range(0, 4):
    print("Penguins")
    print("--------")
  • This will output as
    • Penguins
--------
Penguins
--------
Penguins
--------
Penguins
--------

🔎 Recap - How a for loop works (4)

  • We can also output the value of i in each iteration
    • for i in range(0, 4):
    print(f"i is currently {i}")
    print("i will now increase by 1")

print("i reached 5 - end of loop")
  • This will output as
    • i is currently 0
i will now increase by 1
i is currently 1
i will now increase by 1
i is currently 2
i will now increase by 1
i is currently 3
i will now increase by 1
i reached 5 - end of loop

📝 Activity - Answer Questions about For Loops

  • Open the file called (2) Activity 2.md
  • Complete the questions
🐍 Activity 3 – Use a for loop to iterate through a list

🔎🐍 (1) Remember how to output each item in a list

You have been given a list of European Capitals

capitals = ["Paris", "Berlin", "Madrid", "Lisbon", "Brussells", "Amsterdam"]
  • Output the first three items as part of a sentence
    • Paris is a European Capital
Berlin is a European Capital
Madrid is a European Capital
Click to see how the code should look 
print(f"{capitals[0]} is a European Capital")
print(f"{capitals[1]} is a European Capital")
print(f"{capitals[2]} is a European Capital")

🔎🐍 (2) Comment out the code you have just written

  • You will now learn how to do this in an easier way (not needing to write each output separately)
  • Firstly you must comment-out each of your print functions
  • This will ensure they are still there but won't execute when you want to run further code
Click to see how the code should look

Literally just put a hash tag at the start of each line

#print(f"{capitals[0]} is a European Capital")
#print(f"{capitals[1]} is a European Capital")
#print(f"{capitals[2]} is a European Capital")

🔎🐍 (3) Create a for loop to display each capital

  • Create a for loop with a range 0 - 5
  • Place the code to print only one city within it
    • Click to see how the code should look 
    for i in range(0, 5):
    print(f"{capitals[0]} is a European Capital")
  • Run the code to see what happens
  • It's not quite where we want it...

🔎🐍 (4) Changing the number in the square brackets

The loop repeatedly outputs Paris!

This is because we have just told it to output capitals[0] for each iteration

  • Change the number 0 for the loop counter variable
  • As the loop counter variable changes after each iteration, it will output a different city
    • Click to see how the code should look 
    for i in range(0, 5):
    print(f"{capitals[i]} is a European Capital")
🐍 Activity 4 – Use a for loop to combine values in two lists

🔎🐍 (1) Create a for loop

You have been given:

  • A list of European Capitals
  • A list of countries that match each capital
    • capitals= ["Paris", "Dublin", "London", "Madrid", "Lisbon", "Berlin"]
countries = ["France", "Ireland", "England", "Spain", "Portugal", "Germany"]
  • Use a for loop to output each city and its country e.g.
    • Paris is the capital of France
Dublin is the capital of Ireland
London is the capital of England
etc...
Click to see how the code should look 
for i in range(0, 6):
    print(f"{capitals[i]} is the capital of {countries[i]}")