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05.15.2008 at 04:53PM PDT, ID: 23407083

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9.4

Find Common Elements in Multiple Arrays

Zone: C++ Programming Language

I'm trying to write a generic algorithm that's similar to std::set_intersection, except it works for any number of arrays. So if you have N arrays, it will produce a result array which contains only the common elements found in all N arrays.

I've implemented the algorithm, but there must be something wrong with the logic because it only works correctly sometimes. With larger arrays it often fails to identify all the common elements correctly. I've looked over the code, but I can't identify anything wrong with it.

         
           
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             template <class Iterator>
struct array_intersection_iterator 
{
	Iterator idx;
	Iterator end;
};
 
template <class InputContainer, class OutputContainer>
inline void find_intersection(InputContainer* array, const size_t n, 
	OutputContainer& result)
{
	typedef typename InputContainer::iterator sub_iterator;
	typedef array_intersection_iterator<sub_iterator> iterator_type;
 
	iterator_type* it = new iterator_type[n];
 
	// First sort all the arrays
	for (std::size_t i = 0; i < n; ++i) {
		std::sort(array[i].begin(), array[i].end());
		it[i].idx = array[i].begin();
		it[i].end = array[i].end();
	}
	
	// Now find all the common elements
	std::size_t match, i;
 
	for (; it[0].idx < it[0].end; ++it[0].idx) {
		for (match = i = 1; i < n; ++i) {
			while (!(*it[i].idx > *it[0].idx) && it[i].idx < it[i].end) {
				if (*it[i].idx == *it[0].idx) ++match;
				++it[i].idx;
			}
		}
		if (match == n)
			result.push_back(*it[0].idx);
	}
 
	delete[] it;
}
           
         

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Question Stats

Zone: Programming

Question Asked By: chsalvia

Solution Provided By: SeanDurkin

Participating Experts: 3

Solution Grade: A

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