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POMergeSort. Implementation with Iterators
 

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  1. POMergeSort. Implementation with Iterators
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    <p>I need to implement the algorithm of mergesort with following interface:</p> <pre><code>template &lt;typename T, typename Comp&gt; void mergesort(T begin, T end, Comp comp); </code></pre> <p>where <em>begin</em> and <em>end</em> - iterators for elements of some container of type T, comp - is function of comparing elements, which are stored in container. And</p> <pre><code>template &lt;typename T, typename Comp&gt; void merge(T beginLeft, T endLeft, T beginRight, T endRight, T beginResult, Comp comp); </code></pre> <p>is function of merging of two sorted containers (<em>beginLeft</em> and <em>endLeft</em> - iterators of the first container, <em>beginRight</em> and <em>endRight</em> - of the second) and result should be iterator to the begining of new merged container <em>beginResult</em>.</p> <p>It should be look like <a href="http://www.cplusplus.com/reference/algorithm/merge/" rel="nofollow">this</a>. </p> <p>I've wrote recursive function for merge sort, which calls the procedure of merging. </p> <pre><code>template &lt;typename T, typename Comp&gt; void mergesort(T begin, T end, Comp comp) { if (begin == std::prev(end)) { return; } T middle = begin + std::distance(begin, end) / 2; mergesort&lt;T, Comp&gt;(begin, middle, comp); mergesort&lt;T, Comp&gt;(middle, end, comp); merge&lt;T, Comp&gt;(begin, middle, middle, end, begin, comp); } template &lt;typename T, typename Comp&gt; void merge(T beginLeft, T endLeft, T beginRight, T endRight, T beginResult, Comp comp) { while (beginLeft != endLeft &amp;&amp; beginRight != endRight) { if (comp(*beginLeft, *beginRight)) { *beginResult = *beginLeft; ++beginResult; ++beginLeft; } else { *beginResult = *beginRight; ++beginResult; ++beginRight; } } while (beginLeft != endLeft) { *beginResult = *beginLeft; ++beginResult; ++beginLeft; } while (beginRight != endRight) { *beginResult = *beginRight; ++beginResult; ++beginRight; } } </code></pre> <p>Function <em>merge</em> works correctly, but I don't quite understand, how <em>mergesort</em> should work. What argument should I pass to </p> <pre><code>merge&lt;T, Comp&gt;(begin, middle, middle, end, /?...?/, comp); </code></pre> <p>If I pass just <em>begin</em> there, than the result of sorting is not correct. Or should I pass there some temporary container. But how can I create it, if I have only type of iterator and don't know the type of elements?</p> <p>I will be grateful for your help. Thank you!</p> <p>--ADDED---</p> <p>Example of merging:</p> <pre><code>vector&lt;int&gt; vec1; vector&lt;int&gt; vec2; vector&lt;int&gt; vec3(6); vec1.push_back(1); vec1.push_back(3); vec1.push_back(5); vec2.push_back(2); vec2.push_back(4); vec2.push_back(6); merge(vec1.begin(), vec1.end(), vec2.begin(), vec2.end(), vec3.begin(), std::less&lt;int&gt;()); for (int i = 0; i &lt; vec3.size(); ++i) { std::cout &lt;&lt; vec3[i] &lt;&lt; std::endl; } </code></pre> <p>Output is: 1 2 3 4 5 6</p>
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    1. COhow dyou know that *merge* works correctly if you don't even know how to call it?
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      1. USKaroly Horvath
    2. COWhat you require is an in-place merge sort. This is not easy to implement as this link states: http://stackoverflow.com/questions/2571049/how-to-sort-in-place-using-the-merge-sort-algorithm
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      1. USAbhishek Bansal
 

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