Question

I’ve been looking at the std::nth_element algorithm which apparently:

Rearranges the elements in the range [first,last), in such a way that
the element at the resulting nth position is the element that would be
in that position in a sorted sequence, with none of the elements
preceding it being greater and none of the elements following it
smaller than it. Neither the elements preceding it nor the elements
following it are guaranteed to be ordered.

However, with my compiler, running the following:

    vector<int> myvector;
    srand(GetTickCount());

    // set some values:
    for ( int i = 0; i < 10; i++ )
        myvector.push_back(rand());

    // nth_element around the 4th element
    nth_element (myvector.begin(), myvector.begin()+4, myvector.end());

    // print results
    for (auto it=myvector.begin(); it!=myvector.end(); ++it)
        cout << " " << *it;

    cout << endl;

Always returns a completely sorted list of integers in exactly the same way as std::sort does. Am I missing something? What is this algorithm useful for?

EDIT: Ok the following example using a much larger set shows that there is quite a difference:

    vector<int> myvector;
    srand(GetTickCount());

    // set some values:
    for ( int i = 0; i < RAND_MAX; i++ )
        myvector.push_back(rand());

    // nth_element around the 4th element
    nth_element (myvector.begin(), myvector.begin()+rand(), myvector.end());

    vector<int> copy = myvector;
    std::sort(myvector.begin(), myvector.end());

    cout << (myvector == copy ? "true" : "false") << endl;

Answer 1

It’s perfectly valid for std::nth_element to sort the entire range for fulfilling the documented semantic – however, doing so will fail at meeting the required complexity (linear). The key point is that it may do so, but it doesn’t have to.

This means that std::nth_element can bail out early – as soon as it can tell what the n'th element of your range is going to be, it can stop. For instance, for a range

[9,3,6,2,1,7,8,5,4,0]

asking it to give you the fourth element may yield something like

[2,0,1,3,8,5,6,9,7,4]

The list was partially sorted, just good enough to be able to tell that the fourth element in order will be 3.

Hence, if you want to answer ‘which number is the fourth-smallest’ or ‘which are the four smallest’ numbers then std::nth_element is your friend.

If you want to get the four smallest numbers in order you may want to consider using std::partial_sort.