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2016-08-31
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Sorting and sequence containers

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Example

std::sort, found in the standard library header algorithm, is a standard library algorithm for sorting a range of values, defined by a pair of iterators. std::sort takes as the last parameter a functor used to compare two values; this is how it determines the order. Note that std::sort is not stable.

The comparison function must impose a Strict, Weak Ordering on the elements. A simple less-than (or greater-than) comparison will suffice.

A container with random-access iterators can be sorted using the std::sort algorithm:

C++11
#include <vector>
#include <algorithm>

std::vector<int> MyVector = {3, 1, 2}

//Default comparison of <
std::sort(MyVector.begin(), MyVector.end());

std::sort requires that its iterators are random access iterators. The sequence containers std::list and std::forward_list (requiring C++11) do not provide random access iterators, so they cannot be used with std::sort. However, they do have sort member functions which implement a sorting algorithm that works with their own iterator types.

C++11
#include <list>
#include <algorithm>

std::list<int> MyList = {3, 1, 2}

//Default comparison of <
//Whole list only.
MyList.sort();

Their member sort functions always sort the entire list, so they cannot sort a sub-range of elements. However, since list and forward_list have fast splicing operations, you could extract the elements to be sorted from the list, sort them, then stuff them back where they were quite efficiently like this:

void sort_sublist(std::list<int>& mylist, std::list<int>::const_iterator start, std::list<int>::const_iterator end) {
    //extract and sort half-open sub range denoted by start and end iterator 
    std::list<int> tmp;
    tmp.splice(tmp.begin(), list, start, end);
    tmp.sort();
    //re-insert range at the point we extracted it from
    list.splice(end, tmp);
}