Question

I’m trying to work out the best method to search a vector of type “Tracklet” (a class I have built myself) to find the first and last occurrence of a given value for one of its variables. For example, I have the following classes (simplified for this example):

class Tracklet {
    TimePoint *start;
    TimePoint *end;
    int angle;

    public:
        Tracklet(CvPoint*, CvPoint*, int, int);
}

class TimePoint {
    int x, y, t;

    public:
        TimePoint(int, int, int);
        TimePoint(CvPoint*, int);
        // Relevant getters and setters exist here   
};

I have a vector “vector<Tracklet> tracklets” and I need to search for any tracklets with a given value of “t” for the end timepoint. The vector is ordered in terms of end time (i.e. tracklet.end->t).

I’m happy to code up a search algorithm, but am unsure of which route to take with it. I’m not sure binary search would be suitable, as I seem to remember it won’t necessarily find the first. I was thinking of a method where I use binary search to find an index of an element with the correct time, then iterate back to find the first and forward to find the last. I’m sure there’s a better way than that, since it wastes binary searches O(log n) by iterating.

Hopefully that makes sense: I struggled to explain it a bit!
Cheers!

Answer 1

If the vector is sorted and contains the value, std::lower_bound will give you an iterator to the first element with a given value and std::upper_bound will give you an iterator to one element past the last one containing the value. Compare the value with the returned element to see if it existed in the vector. Both these functions use binary search, so time is O(logN).

To compare on tracklet.end->t, use:

bool compareTracklets(const Tracklet &tr1, const Tracklet &tr2) {
    return (tr1.end->t < tr2.end->t);
}

and pass compareTracklets as the fourth argument to lower_bound or upper_bound