Lists and data abstraction

Why a list?

• A program often needs many values at once, like a class of scores.
• One variable per value would be messy.
• A list holds many values under one name — a useful data abstraction.

What is a data abstraction?

• An abstraction hides detail so you can think about the whole, not the parts.
• A list lets you treat 1 value or 1000 values the same way.
• You write the same code no matter how long the list is.

Make a list and read it

• Write items inside square brackets: [10, 20, 30].
• Get one item by its index (its position). Python starts at 0.
• scores[0] is the first item; scores[-1] is the last item.

scores = [10, 20, 30]
print(scores[0])
print(scores[-1])
print(len(scores))

Grow a list with append

• scores.append(x) adds x to the end of the list.
• The list can grow while the program runs.
• len(scores) tells you how many items it now has.

scores = [10, 20]
scores.append(30)
print(scores)
print(len(scores))

Traverse (visit every item)

• To traverse a list means to go through every item, one by one.
• for s in scores: gives you one item each pass — this is a for-each loop.
• It is the clean way to process the whole list.

scores = [10, 20, 30]
total = 0
for s in scores:
    total = total + s
print(total)

In AP CSP pseudocode

• A list is the exam's list too — but CB lists are 1-indexed.
• So aList[1] is the first element in CB, while Python's first is scores[0].
• FOR EACH x IN list matches Python's for x in list:. DISPLAY = print, ← = =.

aList ← [10, 20, 30]
DISPLAY aList[1]        ## the FIRST item (1-indexed!)

total ← 0
FOR EACH x IN aList
{
    total ← total + x
}
DISPLAY total

Common mistakes

• Index a list from 0; the last item is a[-1].
• A list groups related values under one name.

Now you try

• Build, grow, or traverse a list in each task.
• Press Check answer to test your code.