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Generic QuickSort implemented with vector and iterators C++


By : Thomas
Date : October 14 2020, 02:23 PM
Hope this helps Example of a Hoare partition type quicksort. Normally Hoare inits indexes to -1 and size, but the iterator equivalent of -1 is not allowed, so the first instance uses the equivalent of 0 and size-1, before falling into the main loop.
code :
template <typename I>
void QuickSort(I beg, I end)
{
    if (end - beg < 2)
        return;
    I lft(beg);
    I rgt(end-1);
    auto pvt = *(lft + (rgt-lft)/2);
    if(*lft < pvt)
        while (*++lft < pvt) ;
    if(*rgt > pvt)
        while (*--rgt > pvt) ;
    while (lft < rgt)
    {
        std::iter_swap(lft, rgt);
        while (*++lft < pvt) ;
        while (*--rgt > pvt) ;
    }
    rgt++;
    QuickSort(beg, rgt);
    QuickSort(rgt, end);
}


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C++ iterators: Can iterators to an abstract class be implemented as a nested class?


By : Paweł Kalbarczyk
Date : March 29 2020, 07:55 AM
will be helpful for those in need There is no requirement where the type should be defined. It has to be accessible as container::iterator (and container::const_iterator) and there is no reason why it couldn't be defined right there. A nested type is a type just like any other.

Implement quicksort on bi-directional iterators


By : Christina
Date : March 29 2020, 07:55 AM
it should still fix some issue There are a few reasons why practical library sort functions need random access iterators.
The most obvious one is the well-known fact that choosing an endpoint of the partition for a pivot reduces quicksort to O(n2) if the data is sorted (or "mostly sorted"), so most real-life quicksort's actually use a more robust algorithm. I think the most common one is the Wirth algorithm: choose the median of the first, middle, and last element of the partition, which is robust against sorted vectors. (As Dieter Kühl points out, just selecting the middle element would work almost as well, but there is practically no extra cost for the median-of-three algorithm.) Selecting a random element would also be a good strategy, since it is harder to game, but the requirement for a PRNG might be discouraging. Any strategy for selecting the pivot other than taking an endpoint requires random-access iterators (or a linear scan).

Vector iterators incompatible error for a vector holding iterators of another vector


By : MANI BHAI
Date : March 29 2020, 07:55 AM
it should still fix some issue push_back might cause a reallocation of the data contained in the vector. And that reallocation will make all iterators to the vector invalid. Dereferencing invalid iterators leads to undefined behavior.
Indexes into the vector will continue to stay valid, unless you remove elements from the vector.

Why is self-implemented quicksort faster than internal quicksort?


By : Denton
Date : March 29 2020, 07:55 AM
it should still fix some issue Consider the difference between the arguments you pass to your QuickSort and those you pass to usort(). usort() has a much more generic interface, which operates in terms of a comparison function. Your QuickSort is specialized for your particular kind of data, and for performing comparisons via the > operator.
Very likely, then, the difference in performance is attributable to the much higher cost of evaluating function calls relative to evaluating individual > operations. That difference could easily swamp any inherent efficiency advantage that usort() might have. Consider, moreover, that because it relies on a comparison function written in PHP, usort()'s operation involves running a lot of PHP, not just compiled C code.

Managing a container of iterators (specifically, a vector of iterators of a list)


By : Neetu Kataria
Date : March 29 2020, 07:55 AM
Any of those help A vector has no elements by default, nor any memory allocated to hold the elements. Your line myvec[i] = listIter attempts to access the element i that hasn't been allocated. Change the declaration to tVecTest myvec(10) to pre-allocate 10 elements. Or better yet, change myvec[i] = listIter to myvec.push_back(listIter).
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