2.1 Lists

Big ideas

• A list holds items in order: scores = [90, 85, 77]. Index and slice like strings, but lists are mutable: scores[0] = 95 changes it in place.
• Growing and shrinking: append(x), insert(i, x), remove(x) (first match), pop() (last item, returned) or pop(i). len, sum, min, max, sorted work on lists.
• lst.sort() sorts in place and returns None; sorted(lst) returns a new sorted list and leaves the original alone. Same with reverse() versus lst[::-1].
• Two names can share one list (b = a). To copy, use a[:] or list(a). Passing a list to a function lets the function change it.
• Lists of lists make tables: grid[row][col]. text.split() turns a string into a list of words; " ".join(lst) goes back.

Code you should know

nums = [4, 1, 7]
nums.append(10)
nums.insert(0, 99)
print(nums, len(nums))
last = nums.pop()
nums.remove(1)
print(nums, last)
print(nums[1:3], nums[-1], 7 in nums)
print(sum(nums), max(nums), sorted(nums), nums)

[99, 4, 1, 7, 10] 5
[99, 4, 7] 10
[4, 7] 7 True
110 99 [4, 7, 99] [99, 4, 7]

a = [3, 1, 2]
b = a
c = a[:]
a.sort()
print(a, b, c)
result = a.sort()
print(result)
print(sorted(c, reverse=True), c)

[1, 2, 3] [1, 2, 3] [3, 1, 2]
None
[3, 2, 1] [3, 1, 2]

grades = [88, 92, 79, 95]
print(f"average {sum(grades) / len(grades):.1f}")

table = [[1, 2, 3], [4, 5, 6]]
print(table[1][2], len(table), len(table[0]))
for row in table:
    print(sum(row))

words = "the quick brown fox".split()
print(words[-1], "_".join(words))

average 88.5
6 2 3
6
15
fox the_quick_brown_fox

Worked example

a = [5, 3, 8]
b = a
b.append(1)
a.sort()
print(a, b, a is b)

Common mistakes

• lst = lst.sort() throws away the list (sort returns None).
• Removing items from a list while looping over it skips elements.
• Confusing append([4, 5]) (adds one item, a list) with extend([4, 5]) (adds two items).
• Expecting b = a to copy.