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Sorting With Functors

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},
    {87654321, "Darrell Whitley",  0.5},
    {83333333, "Homecoming Queen", 2.2},
    {84444444, "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 Homecoming Queen
84444444 2.2 Joe Kolledge
87654321 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.