Stop conditionally std::sort

[danjiun]

I want to ask : exist a way to stop (interrupt) a sorting process through std::sort given a comparation function ? Inside comparation function exist heavy calculations, but also it can happen that a absolute result of comparation (<) is not possible, in which case i want to interrupt this sorting process in order to start another type of sorting process.
Only it is possible using "trow" and "catch" or exist another way ?

thanks for any help, Daniel

[gunnerfunner]

Couldn't you just do the comparison first before attempting std::sort?

[danjiun]

You mean, compare every element with every element ?
My god, i told you this is expensive in termes of time.
My another idea of course, is to create a sorting algorithm or copy std::sort's code, and change it, with this extra, but my question is if or not exist a way to interrupt this process, with some kind of condition ?

[gunnerfunner]

True but what happens to your container if you interrupt sort mid-way?

[danjiun]

It will give up to have a order-result, in will conform to only contain groups : which elements are same (equivalents) , and which elements are different (non equivalents). This algorithm not will give results which are so precise as another ordering by <, because we only can sort this elements (which are agrouped by != and ==) by number of elements by group, which not is in all cases satisfactory.
So, i will first try to obtain a order off this elements, and if this fails, use results af agroupments.
greatings, and thanks

[JLBorges]

> a way to stop (interrupt) a sorting process through std::sort given a comparation function ?
> Only it is possible using "trow" and "catch" or exist another way ?

There is no way to interrupt std::sort mid-way and then later resume the sort from where it was interrupted.

std::sort can be aborted by throwing an exception from the comparison predicate; however the sequence would be in some unspecified order after that.

#include <iostream>
#include <algorithm>
#include <stdexcept>

struct cmp_dbl
{
    static double LessThanComparable( double v )
    {
        if( v != v ) throw std::domain_error( "NaN - not LessThanComparable" ) ;
        return v ;
    }

    bool operator() ( double a, double b ) const
    { ++cnt ; return LessThanComparable(a) < LessThanComparable(b) ; }

    static int cnt ;
};

int cmp_dbl::cnt = 0 ;

int main()
{
    double arr[] { 1.2, 3.4, 5,6, 7.8, 0.0, 9.1, 2.3, 4.5, 6.7, 8.9 } ;
    for( double& v : arr ) v /= 0.0 ;

    try { std::sort( std::begin(arr), std::end(arr), cmp_dbl() ) ; }
    catch( const std::exception& e )
    {
        std::cerr << "*** error: " << e.what()
                  << "\nsort was aborted after " << cmp_dbl::cnt << " comparisons\n" ; }
}

http://coliru.stacked-crooked.com/a/cbdca9e0f4844267

> My another idea of course, is to create a sorting algorithm or copy std::sort's code, and change it, with this extra

For anything more involved, that idea is the way to go.

[gunnerfunner]

Something like this?

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

struct BoundedSort
{
    bool operator ()(const int &lhs, const int &rhs)
    {
        if(abs(lhs - rhs) < 10)
        {
            return lhs < rhs;
        }

    }
}sort_by_bound;

auto main ()->int
{
    vector<int> v {1, 2, 4, 5, 3, 21, 4, 6, 9, 10};
    sort(v.begin(), v.end(), sort_by_bound);

    for (auto& elem : v)
    {
        cout << elem << " ";
    }
}

Output

4 21 1 2 3 4 5 6 9 10
Process returned 0 (0x0)   execution time : 0.719 s
Press any key to continue.

edit: OP: just seen JLBorges' and I want to stress once again what I have said before and s/he is saying as well, the container will be left in an unspecified state

[danjiun]

Thanks for both.
What JLBorges said have sense.
gunnerfunner thanks, but you code contains errors and also not help me.
You also forget to return a value in case
abs(lhs-rhs)>=10

Daniel