Lists

Many values, one name

Predict what this prints.

prices = [250, 40, 1200]
print(prices[0], prices[-1])

250 1200

Square brackets with values separated by commas make a listlistOne value that holds several values in order, written with square brackets: [250, 40, 1200]. Each value in it is an item.. prices is one name, and it refers to one list. The list holds three items, in the order you wrote them.

Positions work exactly like the characters of a string in lesson 1.3: the first item is at index 0, and -1 is the last. len() tells you how many items there are.

prices = [250, 40, 1200]
print(len(prices))
print(prices[1])
print(prices[-2])
print(prices)

Add a fourth price to the list and run it again. A list can hold text too, like ["teff", "onions", "coffee"].

Three items have positions 0, 1, and 2. There is no position 3:

Part of a list

A sliceslicePart of a list (or string), written list[start:stop]. It starts at start and stops before stop. gives you a new list with some of the items: prices[start:stop]. Like range() in lesson 3.2, the stop is not included.

prices = [250, 40, 1200, 85, 30]
print(prices[1:3])
print(prices[:2])
print(prices[3:])

Leave out the start and the slice begins at 0. Leave out the stop and it runs to the end. prices[1:3] has two items, positions 1 and 2.

Lists can change

A string can't be changed in place. A list can: give one of its positions a new value.

prices = [250, 40, 1200]
prices[1] = 45
print(prices)

Read prices[1] = 45 as "position 1 of prices gets 45". It's still the same list; position 1 now refers to a different value. Try the same on a string and Python stops you:

That difference, lists can change and strings can't, looks small now. In lesson 4.8 it turns out to explain some of the most confusing bugs in Python.

Going through a list

for works on a list just as it works on text (lesson 3.2): each round, the loop variable refers to the next item. That makes the plans from Module 3 easy to use. Here is a running total over a list. Step through it:

1490

And here is best so far. With a list you can start from the first item, instead of a number below every possible score:

scores = [67, 82, 45]
highest = scores[0]
for score in scores:
    if score > highest:
        highest = score
print(highest)

These two plans are so common that Python has them built in. sum() is the running total, max() is the best so far, and min() is the lowest so far:

prices = [250, 40, 1200]
scores = [67, 82, 45]
print(sum(prices))
print(max(scores))
print(min(scores))

When all you need is the number, use the built-in. Write the loop when you need something extra, like printing each price on the way. Add a price to prices and run it again: nothing else has to change, however long the list gets.