Lists: twelve names, counted from zero

Now the names of the twelve apostles are these; The first, Simon, who is called Peter, and Andrew his brother; James the son of Zebedee, and John his brother; Philip, and Bartholomew; Thomas, and Matthew the publican; James the son of Alphaeus, and Lebbaeus, whose surname was Thaddaeus; Simon the Canaanite, and Judas Iscariot, who also betrayed him. — Matthew 10:2–4 (KJV)

Matthew doesn’t just say “twelve apostles”. He names them, in order, starting with “the first, Simon”. When you need to keep many values together, in order, programming has a tool for exactly that: the list.

Making a list

apostles = ["Simon", "Andrew", "James",
            "John", "Philip", "Bartholomew",
            "Thomas", "Matthew", "James",
            "Thaddaeus", "Simon", "Judas"]

• Square brackets [ ] hold the list; commas separate the items.
• The order you write them in is the order they stay in.
• A list can hold the same value twice: there are two Jameses and two Simons, and that’s fine.
• The line breaks are only for reading; Python treats it as one list.

Indexes start at zero

Every item has a position called its index, and Python starts counting at 0:

print(apostles[0])     # Simon
print(apostles[3])     # John
print(apostles[-1])    # Judas   (negative indexes count from the end)

Why zero? The index means “how many steps from the start”. Simon is zero steps from the start.

len: how many items

print(len(apostles))   # 12

Twelve items means the indexes go from 0 to 11. Asking for apostles[12] is an error:

IndexError: list index out of range

Replacing an item

And they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles. — Acts 1:26 (KJV)

After Judas, the apostles chose Matthias to take the empty place. In code, you put a new value at an index with =:

apostles[11] = "Matthias"
print(apostles[11])    # Matthias
print(len(apostles))   # still 12

It’s the same = from the variables lesson: point this place at a new value.

Going through a list

A for loop can walk through a list item by item:

for name in apostles:
    print(name)

The same idea in JavaScript

const apostles = ["Simon", "Andrew", "James", "John" /* … */];
console.log(apostles[0]);       // Simon
console.log(apostles.length);   // the length
apostles[11] = "Matthias";

It’s everywhere

Common mistakes

• Starting at 1. The first item is [0]; the last is [len(list) - 1].
• Going one past the end: apostles[12] on a 12-item list → IndexError.
• Forgetting the quotes around text items: [Simon, Andrew] looks for variables called Simon and Andrew.
• Missing commas between items.

Try it

1. Print the fifth apostle. (Careful: what index is the fifth?)

2. Find where Thomas is with apostles.index("Thomas"). Is it what you expected?

3. Make an empty list songs = [], add three songs with songs.append(...), then print len(songs) and the last song with songs[-1].

   Answer to 1 and 2

   The fifth apostle is at index 4: apostles[4] is "Philip". apostles.index("Thomas") is 6.

Reflect

Twelve ordinary names, written down one by one. Matthew even lists himself: “Matthew the publican”, the tax collector. Every name in that list was called on purpose — and when a place was empty, it was filled.