Hicham Mouline wrote:
> Hello,
> I have posted this question elsewhere:
>>Given a std::vector<T> and a Compare<T> comparator, how can I:
>>
>> 1. determine the number of different elements in the vector?
>> 2. get iterators to the list of distinct elements in the vector?
>
> The answer given was:
>
>>With 2 iterators b and e,
>> std::sort(b, e);
>> m = std::unique(b, e, Compare<T>());
>>Then you have the std::distance(b, m) unique elements available in the
>>range [b, m).
>
> My question was incomplete.
> 1. I wish to keep the container const and it may be unsorted.
> 2. I wish to avoid making a copy if possible and would accept a slower
> algorithm if there is one.
> 3. Is it possible to then iterate (possibly by writing a new iterator)
> over the distinct elements?
Sure, with almost the same algorithm as above. You need to introduce yet
another level of indirection. Use a custom predicate like:
class ForwardCompare {
typedef std::vector<T> T_vector;
T_vector * reference; // point to the original vector
bool operator() ( T_vector::size_type i, T_vector::size_type j ) const {
return( Compare<T>()( reference->at(i), reference->at(j) ) );
}
};
Now, you create a vector of indices from 0 to size-1, sort that vector using
ForwardCompare, make it unique using ForwardCompare and what you end up with
is a vector containing indices to pairwise Compare<T>-different elements of
the original vector.
Best
Kai-Uwe Bux
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