sorted, keys and counting

Sorting in Python is one line: sorted(values) gives a new, sorted list; values.sort() sorts the list in place and returns nothing. The real power is in the key parameter: from each element you produce the value it is to be sorted by.

1people = [("Ece", 31), ("Can", 25), ("Ada", 31)]
2sorted(people, key=lambda p: p[1])
3# [("Can", 25), ("Ece", 31), ("Ada", 31)]
4sorted(people, key=lambda p: (-p[1], p[0]))
5# [("Ada", 31), ("Ece", 31), ("Can", 25)]

The first sort is by age, and those of equal age keep the order they came in (Python's sort is stable). In the second the key is a pair: by age descending first (the minus sign), and by name ascending on a tie.

When the values come in only a few kinds (0, 1 and 2, say) there is no need to sort by comparing: count how many of each there are, then write that many out in order. That is one pass, faster than the general n log n sort.

Tasks