Question

I get this problem while doing some experiment with set.

I use a structure with 2 integers as the key:

struct Key {
    int k1;
    int k2;
};

And use a class to build the ordering between keys:

struct keyComp {
    bool operator () (const struct Key& lhs, const struct Key& rhs) const {
        if (lhs.k1 < rhs.k1)
            return true;
        else if (lhs.k2 < rhs.k2)
            return true;
        else
            return false;
    }
};

But with this comparator, the set failed to find some existing keys. For example, in this program I store 9 keys in the set, from (0, 0) to (2, 2):

Key pos;
set <Key, keyComp> aset;

// insert elements from (0, 0) to (2, 2)
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        pos.k1 = i;
        pos.k2 = j;
        aset.insert (pos);
    }
}

// now try to find each of them
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        pos.k1 = i;
        pos.k2 = j;
        set <Key, keyComp> :: iterator it = aset.find (pos);
        if (it != aset.end ())
            cout << "v "; // element found
        else
            cout << "! "; // not found
    }
    cout << endl;
}

// print the set
cout << "element in set : ";
for (set <Key, keyComp> :: iterator it = aset.begin (); it != aset.end (); it++) {
    cout << "(" << it->k1 << ", " << it->k2 << ") ";
}
cout << endl;

I expect it would print 9 v‘s, which means all keys are found. But instead I got:

v v v 
! ! v 
! ! v 
element in set : (0, 0) (1, 0) (2, 0) (0, 1) (1, 1) (2, 1) (0, 2) (1, 2) (2, 2)

Some keys can be found, but some can’t even they are in the set.

Also, if I change the comparator to:

struct keyComp {
    bool operator () (const struct Key& lhs, const struct Key& rhs) const {
        // combine the two keys for comparison
        // say LARGE_NUMBER is a number bigger than all k2
        return lhs.k1 * LARGE_NUMBER + lhs.k2 < rhs.k1 * LARGE_NUMBER + rhs.k2;
    }
};

Then all keys are found.

Why is this happening? Is it because the original comparator failed to construct the order between keys?

Answer 1

Your comparator doesn’t provide a proper ordering, which means that the internals of set will do all manner of weird things (aka “undefined behaviour”) when trying to figure out where to insert or look-up things.

You need something like this:

    if (lhs.k1 < rhs.k1)
        return true;
    if (lhs.k1 > rhs.k1)
        return false;
    return (lhs.k2 < rhs.k2);