Lesson 4

Last week the computer handed you numbers — range(1, 11) gave you 1, 2, 3, and your loops turned them into triangles. Today the computer hands you something you actually care about: a list of things. Names of players. And by the end, the program will let you add and remove them while it runs. But first, the homework.

One name, or many

You know how to store a name — it's a string, one value in memory:

name = "scott"

But a team isn't one name, it's several. A list holds many values, wrapped in square brackets and separated by commas:

names = ["scott", "rish", "tom", "jerry"]

One variable, names, now remembers all four. Lists are special — they're the first value we've met that holds other values inside it.

Looping with a counter

We want to print every other name — the ones at position 0, 2, 4... Here's the loop:

names = ["scott", "rish", "tom", "jerry"]

i = 0
for name in names:
    if i % 2 == 0:
        print(names[i])
    i = i + 1

scott
tom

Three things are happening, and you already know all three:

names[i] reaches into the list and pulls out the item at position i. Like range, positions start at 0, so names[0] is "scott", names[1] is "rish". That position number is called the index.
i % 2 == 0 is the even test from Lesson 2, back again — this time asking about the index instead of a number the user typed.
i = i + 1 is new-looking but simple. Read the right side first (just like = always works): take the current i, add one, and store it back into i. So i climbs 0, 1, 2, 3 as the loop runs — a hand-cranked counter that keeps pace with the loop.

So the loop walks the list, and i walks the positions alongside it. When i is even (0, then 2), we print — scott, then tom.

Growing the list from the keyboard

A list isn't frozen. We can add to it with append — and we can ask the user what to add using input, exactly like Lesson 1:

new_player = input("your name: ")
print(new_player)

names.append(new_player)
print(names)

your name: john
john
['scott', 'rish', 'tom', 'jerry', 'john']

names.append(new_player) sticks the new name onto the end of the list. Notice the last print(names) shows the whole list, brackets and all — that's Python showing you a list as a value. The team grew from four to five, and we didn't type the fifth name — the user did, while the program was running.

Removing from the list

If a list can grow, it can shrink. remove takes out a name by its value:

player_to_remove = input("player to remove: ")

names.remove(player_to_remove)
print(names)

player to remove: tom
['scott', 'rish', 'jerry', 'john']

append adds to the end; remove deletes whichever item equals what you gave it. Together they're how a program keeps a list up to date instead of being stuck with whatever you typed in the code.

A list of lists

Here's where "a list holds other values" gets powerful. Those values can themselves be lists — and suddenly you have a table:

people = [
    ["name", "address"],
    ["scott", "1 apple ave"],
    ["rishi", "2 google drive"]
]

Read it as rows. people[0] is the whole first row, ["name", "address"] — the headings. people[1] is ["scott", "10 alabama ave"]. And because each row is itself a list, you can index twice: people[1][0] is "scott" (row 1, column 0), and people[1][1] is "10 alabama ave". The first bracket picks the row, the second picks the column — just like a spreadsheet.

Reaching in safely

Let the user pick which name to see, by index:

names = ["scott", "rish", "tom", "jerry", "john"]

name_index = int(input("give me the index of the person's name "))
names_length = len(names)

if name_index < names_length:
    print(names[name_index])

Two familiar tools do the work. int(input(...)) reads a number from the keyboard — input always gives a string, so we convert, exactly like Lesson 1's calculator. And len(names) tells us how many items are in the list: here, 5.

Why the if? Because the list has 5 items, the valid indexes are 0, 1, 2, 3, 4 — the last index is one less than the length. Ask for names[5] and Python stops you:

>>> names[5]
IndexError: list index out of range

The error is a hint, not a punishment: you reached past the end. The line if name_index < names_length is a guard — it only prints when the index is in range, so a careless 9 produces nothing instead of a crash. Same < you've used since Lesson 2, now protecting your program.

Watch the numbers change

Same skill as always: when a list confuses you, print the index next to the item it unlocks.

names = ["scott", "rish", "tom", "jerry", "john"]

for i in range(len(names)):
    print(i, names[i])

0 scott
1 rish
2 tom
3 jerry
4 john

There they are — every index and the name it points to. range(len(names)) gives 0, 1, 2, 3, 4, one short of the length, exactly as range has behaved all along. Master this little table and lists hold no more mystery.