CyberStars

Lists

So far, every variable held one thing: one name, one number, one boolean. But what if I want to store the names of all the characters from GTA Vice City?

I could do this

name1 = "Tommy Vercetti"
name2 = "Lance Vance"
name3 = "Cortez"
name4 = "Phil Cassidy"
name5 = "Sonny Forelli"

Ugly. And what if I wanted 100 names? 1000? No way. There has to be a better solution

There is. It’s called a list

names = ["Tommy Vercetti", "Lance Vance", "Cortez", "Phil Cassidy", "Sonny Forelli"]
print(names)

A list is a collection of values stored in a single variable. We use the square brackets [ ] and separate the elements with commas

If we run this, Python will display the whole list at once

['Tommy Vercetti', 'Lance Vance', 'Cortez', 'Phil Cassidy', 'Sonny Forelli']

We can also access a single element of the list, just like we did with strings: by index

names = ["Tommy Vercetti", "Lance Vance", "Cortez"]
print(names[0])    # Tommy Vercetti
print(names[1])    # Lance Vance
print(names[2])    # Cortez

Once again, counting starts from 0. The first element is names[0], NOT names[1]

What happens if we ask for an index that doesn’t exist?

names = ["Tommy Vercetti", "Lance Vance", "Cortez"]
print(names[10])

Run it. You’ll see Python complains with an IndexError, because there is no element at position 10. Always read your errors :)

How many elements does a list have? len() again

names = ["Tommy Vercetti", "Lance Vance", "Cortez"]
print(len(names))   # 3

Same len() that we used on strings. Python is smart about it

We can add new elements to a list with .append()

names = ["Tommy Vercetti", "Lance Vance"]
names.append("Cortez")
names.append("Phil Cassidy")
print(names)

Output

['Tommy Vercetti', 'Lance Vance', 'Cortez', 'Phil Cassidy']

.append() adds the new element at the end of the list. The list has changed, this is different from upper() and lower() on strings, which returned a new string. Lists are modified in place

We can also change a value at a given index

names = ["Tommy Vercetti", "Lance Vance", "Cortez"]
names[1] = "Lance Vance Dance"
print(names)

Output

['Tommy Vercetti', 'Lance Vance Dance', 'Cortez']

Lists can hold any kind of values, not just strings

ages = [42, 35, 60, 29]
prices = [3.14, 9.99, 12.50]
flags = [True, False, True]

We can even mix them, but in practice it’s rare and usually a sign that something is wrong with the design

Mission: Cargo Bay

The station tracks its cargo in a list. Create an empty list called cargo. Then read 4 items from input, one per line, and do the following, in order:

  1. Add each of the 4 items you read to the list
  2. Print the list
  3. Print the first item
  4. Print the last item
  5. Print how many items are loaded
  6. A leak empties the item at index 1, replace it with empty
  7. Print the list

Example

If the user types

oxygen
water
food
fuel

the program prints

['oxygen', 'water', 'food', 'fuel']
oxygen
fuel
4
['oxygen', 'empty', 'food', 'fuel']

Notice the last line: when you print a whole list, Python shows the brackets and quotes for you.