Question

If a std::set or std::list contains a sequence of natural numbers (1, 2, 3..). would there be a function in standard library to find the missing number?

Answer 1

You can find all missing numbers using set_difference and a custom iterator:

class seqIter : public std::iterator<std::input_iterator_tag, int> {
public:
    seqIter(int n) : num(n) {}
    seqIter(const seqIter & n) : num(n.num) {}
    int & operator *() {return num;}
    seqIter & operator ++() { ++num; return *this; }
    bool operator !=(const seqIter & n) { return n.num != num; }
private:
    int num;
};

int main(int argc, char *argv[])
{
    int n[] = { 1, 3, 4, 7, 10 };
    std::set<int>   numbers(n, n + sizeof(n)/sizeof(n[0]));
    std::set<int>   missing;
    std::set_difference(    seqIter(*numbers.begin()+1), seqIter(*numbers.rbegin()), 
                            numbers.begin(), numbers.end(),
                            std::insert_iterator<std::set<int> >(missing, missing.begin())
                        );
}

It’s probably not any faster than going thru the numbers with a for loop though.