lesson 32 Lists & Tuples · beginner

Pick an item by position

list[n] is the item at position n, list[-1] is the last item, and list[a:b] is a new list of the items in between. The positions work just as they do for text.

You'll be able to

Read the first, last and nth item, and take a slice of a list
Loop over positions with range(len(items)) when you need an item's neighbour
Recognise IndexError and the off-by-one mistakes that cause it

The idea

A list's items have positions, just as a text's characters do. ratings[0] is the first item, ratings[-1] is the last, and ratings[1:3] is a new list holding the items at positions 1 and 2. All the rules from the Strings & Text module carry over. Positions start at 0, the end of a slice is not included, and a slice past the end is shorter instead of an error.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · Positions in a list of ratings

ratings = [4.5, 3, 5, 2.5, 4]
print(ratings[0], ratings[-1], len(ratings))
print(ratings[1:3])
print(ratings[-2:])

example-2.py · Reading past the last item

ratings = [4.5, 3, 5, 2.5, 4]
print(ratings[len(ratings) - 1])
print(ratings[5])

Some questions compare an item with its neighbour: *how much did the temperature change since yesterday?* for t in highs gives you each item but not the one before it. Loop over the positions instead, with range(len(highs)), and read highs[i] and highs[i - 1]. Start at 1, because position 0 has no day before it.

example-3.py · Day-to-day temperature change

highs = [6.7, 7.2, 3.3, 7.2, 3.9, -2.1]
for i in range(1, len(highs)):
    change = highs[i] - highs[i - 1]
    print(f"Day {i + 1}: {change:+.1f}")

When to use it: use positions for the first or last item, a fixed window such as the last 3 days, or a comparison with a neighbour. When you only need each item, for item in items is simpler. Mistakes to avoid:

Starting a neighbour loop at 0. Then highs[0 - 1] quietly reads the *last* item.
Using len(items) as a position.

Review

items[n], items[-1] and items[a:b] work on lists as they do on text. A slice of a list is a new list.
Loop over range(1, len(items)) when each item is compared with the one before it.
Five items sit at positions 0 to 4. Asking for position 5 raises IndexError.
Next: change a list in place by adding, replacing and removing items.