CS253 Sorting With Functors

Sorting

How does a container like set know the sorting order?
Sure, it’s easy to answer for int and string, but what about class Student?
- Sort by major?
- Sort by eId?
- Sort by last name, then first name, then CSUID?
Besides, even for int, you might want to sort in a different order.
You might even want multi-key sorting, like a phone book.

The Idiot Savant

Imagine that you had a semi-brilliant assistant who was very good at sorting.
However, your assistant doesn’t know alphabetical order.
He can do everything that’s needed for sorting except actually determine if, say, “Jay” comes before “David”.
Therefore, he frequently comes to you, asking if this word comes before this other word.
set is like that. It knows the mechanics of sorting, but it doesn’t know what order to use. You need to help it.

Comparison functor

You provide set with a comparison functor.
The set code makes comparisons using the functor.
The functor only compares two items at a time.
- I say again, two items at a time. Only two.
The functor returns true iff the first item should come before the second item.
Why can’t you use a comparison function, as opposed to a comparison functor?

Implicit/explicit

Implicit sorting criterion:

set<int> s = {31, 41, 59, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

23 26 31 41 53 58 59 84 93 97

Explicit sorting criterion:

set<int, less<int> > s = {31, 41, 59, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

23 26 31 41 53 58 59 84 93 97

What’s this > > folderol?

Reversed

Use greater<int> to sort from biggest to smallest:

set<int, greater<int>> s = {31, 41, 59, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

97 93 84 59 58 53 41 31 26 23

Explicitly explicit

Use a comparison functor that behaves the same as less<int>:

struct compare {                            // 😱😱struct‽😱😱
    bool operator()(int a, int b) const {
        return a < b;
    }
};

set<int, compare> s = {31, 41, 59, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

23 26 31 41 53 58 59 84 93 97

Two-level

Use a comparison functor that does a two-level comparison, like last name/first name.

struct compare {
    bool operator()(int a, int b) const {
        int da = abs(a-50), db = abs(b-50);
        if (da != db)
            return da < db; // Primary sort: distance from 50
        return a < b;       // Secondary sort: value of number
    }
};

set<int, compare> s = {31, 41, 59, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

53 58 41 59 31 26 23 84 93 97

Ternary operator

Another way to write a two-level comparison functor:

struct compare {
    bool operator()(int a, int b) const {
        int da = abs(a-50), db = abs(b-50);
        return (da != db) ? da < db : a < b;
    }
};

set<int, compare> s = {31, 41, 59, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

53 58 41 59 31 26 23 84 93 97

Curious criteria

This sorts by different criteria:

struct compare {
    bool operator()(int a, int b) const {
        int last_a = a%10, last_b = b%10;
        if (last_a != last_b)
            return last_a < last_b; // Primary sort: by last digit
        return a < b;               // Secondary sort: value of number
    }
};

set<int, compare> s = {31, 41, 59, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

31 41 23 53 93 84 26 97 58 59

Multi-level

struct student {
    int id; string name; float gpa;
};
struct compare {
    bool operator()(const student &a, const student &b) const {
        return (a.gpa != b.gpa) ? a.gpa > b.gpa : a.id < b.id;
    }
};
set<student,compare> s = {
    {81234567, "Jack Applin",      3.9},
    {82828282, "Darrell Whitley",  0.5},
    {84444444, "Homecoming Queen", 2.2},
    {83333333, "Joe Kolledge",     2.2},
};
for (const auto &v : s)
    cout << v.id << ' ' << v.gpa << ' ' << v.name << '\n';

81234567 3.9 Jack Applin
83333333 2.2 Joe Kolledge
84444444 2.2 Homecoming Queen
82828282 0.5 Darrell Whitley

Not all

Of course, not all collections require a comparison functor.
vector, array, and list just go in the order you specify, and so have no comparison functor.
unordered_set needs only an equality comparison (and a hash functor), so its comparison functor defaults to equal_to, not less.

CS253: Software Development with C++

Spring 2018

Sorting With Functors