Collections
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| list/lɪst/ | danh sách |
| dictionary/ˈdɪkʃənəri/ | từ điển |
| key/kiː/ | A value used to identify or look up an associated item, such as a key in a dictionary or database. |
| tuple/ˈtuːpl/ | cặp có thứ tự |
| set/set/ | tập hợp |
| aliasing/ˈeɪlɪəsɪŋ/ | tên gọi chung |
Which drink sells best?
- A café till records every order:
"tea","latte","tea"… 200 times a day. One variable per order would be useless. You need one name that holds many values. - Python has four built-in collections, and each one suits a different job. Choosing the right one often makes the rest of the program short.
A list keeps items in order
- A list 列表 is an ordered sequence that you can change:
marks = [72, 55, 90, 48, 81]. Positions are counted from 0, somarks[0]is72andmarks[-1], the last item, is81. - A slice takes a part:
marks[1:3]is[55, 90], from position 1 up to, but not including, position 3.len(marks)is5.
With s = [10, 20, 30, 40, 50], what is s[1:4]?
Độ khó 1/5 · 100 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
Position 1 holds 20 (counting from 0), and the slice stops before position 4, so it takes positions 1, 2 and 3.
Changing a list
marks.append(66)adds to the end;marks.remove(48)deletes the first48;marks[1] = 58replaces one item.marks.sort()sorts the list in place and returnsNone. To keep the original and get a sorted copy, writesorted(marks).
nums = [3, 1, 2]
result = nums.sort()
print(result, nums) # None [1, 2, 3]
After nums = [3, 1, 2] and nums = nums.sort(), what is nums?
Độ khó 1/5 · 100 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
sort() sorts the list in place and returns None, and that None is what gets stored in nums. Write nums.sort() alone, or nums = sorted(nums).
A dictionary looks things up by key
- A dictionary 字典 maps each key 键 to a value:
stock = {"tea": 40, "latte": 25}.stock["tea"]is40. - A missing key raises a
KeyError.stock.get("mocha", 0)returns the default0instead, so it is safe for counting.
- Count how many times each drink was ordered.
- Start with
tally = {}. For eachitemin the orders, writetally[item] = tally.get(item, 0) + 1. - For
["tea", "flat white", "tea", "latte", "tea"]the result is{'tea': 3, 'flat white': 1, 'latte': 1}. The first sighting of a drink gets0 + 1; later sightings add one more.
A tally is a dictionary
Each key is a drink and each value is how many were ordered. Step through the pairs; looking up tally["tea"] goes straight to its value without scanning the others.
The tally loop runs over ["tea", "latte", "tea", "mocha", "tea", "latte"]. What is tally["tea"] at the end?
Độ khó 2/5 · 140 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
"tea" appears three times. The first time tally.get("tea", 0) + 1 gives 1; each later sighting adds one more.
Tuples and sets
- A tuple 元组 is a fixed group, such as a point
p = (3, 4). You can unpack it,x, y = p, butp[0] = 5raises aTypeError: it cannot change. - A set 集合 holds each value once, in no fixed order:
{"tea", "latte", "tea"}has only2items. Use it to remove duplicates or to test membership quickly.
Choose the collection by the job
- Order matters, or you need positions → list. Look up a value by a name or code → dictionary.
- A fixed group of parts, such as a date or a point → tuple. Only "is it there?" and no duplicates → set.
Match each job to the best collection.
Độ khó 2/5 · 140 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
Order → list; lookup by key → dictionary; fixed parts → tuple; uniqueness → set.
Two names, one list
b = adoes not copy a list. It puts a second name on the same list, sob.append(2)changes whatashows too. This sharing is called aliasing 别名.- When you need an independent copy, make one:
b = a[:]orb = list(a).
After a = [1, 2], b = a, b.append(3), what is a?
Độ khó 1/5 · 100 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
b = a put a second name on the same list, so the change made through b shows through a. With b = a[:], a would stay [1, 2].
Don't remove items while you loop over them
- Removing from a list while a
forloop walks through it skips items: each removal shifts the later items left, but the loop still moves on to the next position. - With
m = [40, 45, 70, 30], removing every mark below 50 inside the loop leaves[45, 70], and45is missed. Build a new list instead:[x for x in m if x >= 50]gives[70].
Removing every item below 50 inside for x in m: always leaves only the items of 50 or more.
Độ khó 1/5 · 100 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
Each removal shifts the later items left, so the loop skips one. [40, 45, 70, 30] ends as [45, 70]. Build a new list with [x for x in m if x >= 50].
Marks that slip away
- Counting from 1. The first item is
[0]; the last is[len(x) - 1]or[-1]. x = x.sort().sort()returnsNone, soxis lost. Callx.sort()alone, or usesorted(x).d[key]for a key that may be missing. Used.get(key, default)or testif key in d:first.- Thinking
b = acopies. It shares; copy witha[:]when you need two lists.
d = {"tea": 2}. Which lines run without raising an error? Select all that apply.
Độ khó 2/5 · 140 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
Reading a missing key with d["mocha"] raises a KeyError. .get returns the default, in returns False, and assigning to a new key simply adds it.
You've got it
- list: ordered and changeable, positions from 0, slices
[a:b]stop beforeb. dictionary: key → value,.get(key, default)never raises. - tuple: a fixed group you can unpack. set: unique values, fast membership tests.
b = ashares one list (aliasing); copy witha[:]. Never remove from a list you are looping over; build a new one.