STL Sort Analysis
xuwenzhi@cityuniversity.edu
What's Sort?
Common used sort algorithms
Quick Sort
Merge Sort
Heap Sort
Insertion Sort
Counting Sort
Bubble Sort
Selection Sort
Care about what?
-
Time and Space complexity
-
Stable or Unstable
-
In-place or not In-place
-
Frequently Swap or not
How C++ implement Sort?
std::sort(a.begin(), a.end());
Introspective Sort
- A hybrid sort method.
Quick Sort + Heap Sort + Insertion Sort
Quick Sort vs. Heap Sort
- Generally faster
- Unstable Worst Time : O(n^2)
- In-place sorting
- Generally slower
- Stable (nlogn)
- In-place sorting
Heap Sort
Quick Sort
5
5
10
0
3
1
Pivot
i
j
Insertion Sort
Behind Principles
- At first std::sort() use Quick Sort. Time complexity is O(nlogn).
- When the current range is less than threshold, it will use insertion sort.
- In the first step, if the recursion level is too deep, it will change to use heap sort.
Source Code
template<typename _RandomAccessIterator>
inline void
sort(_RandomAccessIterator __first, _RandomAccessIterator __last)
{
//...
if (__first != __last)
{
std::__introsort_loop(__first, __last,
std::__lg(__last - __first) * 2);
std::__final_insertion_sort(__first, __last);
}
}template<typename _RandomAccessIterator, typename _Size>
void
__introsort_loop(_RandomAccessIterator __first,
_RandomAccessIterator __last,
_Size __depth_limit)
{
while (__last - __first > int(_S_threshold))
{
if (__depth_limit == 0)
{
_GLIBCXX_STD_A::partial_sort(__first, __last, __last);
return;
}
--__depth_limit;
_RandomAccessIterator __cut =
std::__unguarded_partition_pivot(__first, __last);
std::__introsort_loop(__cut, __last, __depth_limit);
__last = __cut;
}
}Reference
https://www.geeksforgeeks.org/sorting-algorithms/
https://gcc.gnu.org/onlinedocs/gcc-4.7.2/libstdc++/api/a01462_source.html
http://www.student.montefiore.ulg.ac.be/~merciadri/docs/papers/heap-quick-comparison.pdf
Thanks!
Stlsort
By xuwenzhi
