Python practice

Python sorting practice questions

8 problems, ordered from easiest. They run in your browser, and 4 are free.

Start with Second largest order total
On this page12 sections
  1. The idea in a minute
  2. You almost never write a sort yourself
  3. sorted or .sort()?
  4. The key: what should I compare?
  5. Ties, and breaking them
  6. Sorting dictionaries
  7. Why sorting solves other problems
  8. Sorting strings and mixed data
  9. Cost
  10. Questions people ask
  11. Now try it yourself
  12. More Python topics

The idea in a minute

You almost never write a sort. You call sorted and tell it what to sort by. That second part, the key, is where the thinking is.

Below: sorted versus sort, keys and lambdas, ties and stability, sorting by two things at once, and how a sorted list turns hard questions into easy ones.

You almost never write a sort yourself

In an interview you may be asked to write bubble sort. In real work you call sorted(). Your job is to tell it what to sort by. That "what by" is the key, and it is the thing worth learning.

Pythonsorted gives a new list
nums = [4, 1, 3, 9, 2]
print(sorted(nums))
print(sorted(nums, reverse=True))
print(sorted(["pune", "Delhi", "goa"]))
print(nums)
[1, 2, 3, 4, 9]
[9, 4, 3, 2, 1]
['Delhi', 'goa', 'pune']
[4, 1, 3, 9, 2]

Two things to notice. Capital letters sort before lowercase ones (Delhi came before goa and pune because D comes before g in the character table). And nums itself is unchanged: sorted returns a new list.

sorted or .sort()?

Pythonlist.sort() changes the list and returns None
nums = [4, 1, 3]
result = nums.sort()
print(result)
print(nums)
None
[1, 3, 4]

A classic bug: writing nums = nums.sort() and ending up with None. sort() works in place and returns nothing. Use sorted(nums) when you want a new list, and nums.sort() when you want to change the existing one.

The key: what should I compare?

The key is a function. Python calls it on each item once, and sorts the items by what the function returns. Sort words by length? The key is len. Sort rows by the second column? The key picks the second column.

PythonSorting by length, and by a column
words = ["banana", "fig", "apple", "kiwi"]
print(sorted(words, key=len))
rows = [("Asha", 92), ("Ravi", 85), ("Dev", 70)]
print(sorted(rows, key=lambda r: r[1]))
['fig', 'kiwi', 'apple', 'banana']
[('Dev', 70), ('Ravi', 85), ('Asha', 92)]

lambda r: r[1] is a tiny unnamed function that takes a row and returns its second item. You could write it as a normal def, it is only shorter.

Ties, and breaking them

Two people scored 92. Which one comes first? Python's sort is stable, which means items with equal keys keep the order they had before. That is a promise you can rely on.

PythonAsha comes before Meera because she came first
rows = [("Asha", 92), ("Ravi", 85), ("Meera", 92), ("Dev", 70)]
print(sorted(rows, key=lambda r: r[1], reverse=True))
[('Asha', 92), ('Meera', 92), ('Ravi', 85), ('Dev', 70)]

When you want a rule instead of leaving it to the original order, make the key a tuple. Python compares tuples left to right: first the first item, and only if those tie, the second.

How the key decides

Step 1 / 3
1rows = [("Asha", 92), ("Ravi", 85), ("Meera", 92), ("Dev", 70)]
2print(sorted(rows, key=lambda r: (-r[1], r[0])))
Sort keys
Asha 92 · Ravi 85 · Meera 92 · Dev 70
Line 1
The rows as they arrive. Two people share the score 92.
PythonHighest score first, then name A to Z
rows = [("Asha", 92), ("Ravi", 85), ("Meera", 92), ("Dev", 70)]
print(sorted(rows, key=lambda r: (-r[1], r[0])))
[('Asha', 92), ('Meera', 92), ('Ravi', 85), ('Dev', 70)]

The minus sign in -r[1] is the trick for mixed directions. Sorting by -score ascending puts the highest score first, while the name stays ascending. For text you cannot negate, sort twice (the stable property makes that work) or use a different key.

Sorting dictionaries

PythonA dictionary has to be turned into pairs first
stock = {"pen": 12, "book": 3, "bag": 0, "ink": 12}
by_qty = sorted(stock.items(), key=lambda kv: (-kv[1], kv[0]))
print(by_qty)
print(dict(by_qty))
print(sorted(stock))
[('ink', 12), ('pen', 12), ('book', 3), ('bag', 0)]
{'ink': 12, 'pen': 12, 'book': 3, 'bag': 0}
['bag', 'book', 'ink', 'pen']

stock.items() gives (key, value) pairs, and those are what you sort. sorted(stock) on its own sorts just the keys.

Why sorting solves other problems

Once things are in order, a lot of questions stop being hard. Equal items sit side by side. The smallest and largest are at the ends. You can binary search. You can use two pointers.

PythonDuplicates become neighbours
nums = [5, 3, 5, 1, 3, 3]
s = sorted(nums)
dups = sorted({s[i] for i in range(1, len(s)) if s[i] == s[i - 1]})
print(s)
print(dups)
[1, 3, 3, 3, 5, 5]
[3, 5]
PythonMerge overlapping meetings
meetings = [(9, 11), (10, 12), (14, 15), (8, 9)]
meetings.sort()
merged = [meetings[0]]
for start, end in meetings[1:]:
    if start <= merged[-1][1]:
        merged[-1] = (merged[-1][0], max(merged[-1][1], end))
    else:
        merged.append((start, end))
print(merged)
[(8, 12), (14, 15)]

Sorted by start time, a meeting can only overlap with the one just before it. That one fact turns a messy comparison of every pair into a single pass.

Sorting strings and mixed data

PythonCase and numbers stored as text
names = ["asha", "Ravi", "meera", "Dev"]
print(sorted(names))
print(sorted(names, key=str.lower))
nums_as_text = ["10", "9", "100"]
print(sorted(nums_as_text))
print(sorted(nums_as_text, key=int))
['Dev', 'Ravi', 'asha', 'meera']
['asha', 'Dev', 'meera', 'Ravi']
['10', '100', '9']
['9', '10', '100']

Text sorts character by character, so '100' comes before '9' because '1' is less than '9'. If the values are really numbers, sort with key=int.

Cost

Sorting takes about n log n steps. For a million items that is about twenty million, which is nothing. It is only worth avoiding when you can answer the question without it in a single pass (like finding the maximum).

Questions people ask

What is the difference between sorted and sort?

sorted(x) returns a new sorted list and works on anything you can loop over. x.sort() changes a list in place and returns None.

What does stable mean?

Items that compare equal stay in their original relative order. It is why sorting by name and then by score gives you scores in order, with names in order inside each score.

Can I sort a list with mixed numbers and text?

Not directly, Python raises a TypeError. Convert them, or give a key that returns a comparable value.

Keep this checklist handy

  • sorted returns a new list and list.sort changes the list in place.
  • Python sorting is stable: items with equal keys keep their original order.
  • To sort in descending order on one field only, negate a number in the key instead of using reverse for everything.

Now try it yourself

8 sorting problems. They run right here in your browser, and you can start anywhere.

Getting comfortable

Two ideas together, or a trap to spot.

  1. 5Top-K most active accounts
  2. 6Custom sort by multiple derived keys

Want a guided order across every topic? Open the practice sheets or the Python hub.

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