Home/ Questions/Q 287037
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-12T05:41:26+00:00

I was using stl::merge to put two sorted collections into one. But my object

  • 0

I was using stl::merge to put two sorted collections into one.

But my object has a natural key; and a defined addition semantic, so what I am after is a merge_and_sum that would not just merge the two collections into a single N+M length collection, but if the operator== on the object returned true, would then operator+ them.

I have implemented it thus 

template<class _InIt1, class _InIt2, class _OutIt> 
_OutIt merge_and_sum(_InIt1 _First1, _InIt1 _Last1, _InIt2 _First2, _InIt2 _Last2, _OutIt _Dest )
{   // copy merging ranges, both using operator<
    for (; _First1 != _Last1 && _First2 != _Last2; ++_Dest)
    {
        if ( *_First2 < *_First1 )
            *_Dest = *_First2, ++_First2;
        else if ( *_First2 == *_First1)
            *_Dest = *_First2 + *_First1, ++_First1, ++_First2;
        else
            *_Dest = *_First1, ++_First1;
    }
    _Dest = copy(_First1, _Last1, _Dest);   // copy any tail
    return (copy(_First2, _Last2, _Dest));
}

But was wondering if I have reinvented something that is composable from the other algorithms.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    It sounds like your collections are like multisets with duplicates collapsed by your + operator (maybe just summing the multiplicities instead of keeping redundant copies). I assume so, because you’re not changing the sorting order when you +, so + isn’t affecting your key.

    You should use your implementation. There’s nothing in STL that will do it as efficiently. The closest semantic I can think of is standard merge followed by unique_copy. You could almost get unique_copy to work with a side-effectful comparison operator, but that would be extremely ill advised, as the implementation doesn’t promise to only compare things directly vs. via a value-copied temporary (or even a given number of times).

    Your type and variable names are unpleasantly long 😉

    • 0