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copied!<p>Well, the short answer would be to not use <a href="http://en.cppreference.com/w/cpp/algorithm/qsort" rel="nofollow"><code>std::qsort</code></a> at all, but <a href="http://en.cppreference.com/w/cpp/algorithm/sort" rel="nofollow"><code>std::sort</code></a>. But unfortunately the latter won't work, since <code>unsigned int[3]</code> is not assignable. So here's the easiest <code>std::qsort</code> solution.</p> <p>First we define a custom comparator function:</p> <pre><code>// return -1 if a before b, 1 if after, 0 if equal int compare(const void *a, const void *b) { const unsigned int *arg1 = reinterpret_cast<const unsigned int*>(a); const unsigned int *arg2 = reinterpret_cast<const unsigned int*>(b); if(arg1[2] > arg2[2]) return -1; if(arg1[2] < arg2[2]) return 1; return 0; } </code></pre> <p>Which we then use to sort the array. Keep in mind that <code>work</code> is an array of arrays, and thus <code>work[0]</code> is an array of 3 <code>unsigned int</code>s, there's no pointer indirection involved in any way. So it's perfectly suited for being sorted by <code>std::qsort</code>:</p> <pre><code>std::qsort(work, sizeof(work)/sizeof(work[0]), sizeof(work[0]), compare); </code></pre> <p>By the way, the third element is indexed with <code>2</code>, since we usually start to count at <code>0</code> in C++ (and many other programming languages).</p> <p><strong>EDIT:</strong> Though, the best solution would indeed be to drop this array of arrays and use something more suited to C++, like a <code>std::vector</code> of <code>std::array<unsigned int,3></code>s (or any other datastructure that fits a bit more to the actual context):</p> <pre><code>typedef std::array<unsigned int,3> uint3; std::vector<uint3> work(N); </code></pre> <p>Which can then be sorted with a simple:</p> <pre><code>std::sort(std::begin(work), std::end(work), [](const uint3 &a, const uint3 &b) { return a[2] > b[2]; }); </code></pre> <p>Or, if you don't have C++11 (though in this case you won't have <code>std::array</code> either and need to start thinking about a resonable datastructure apart from a mere 3-array):</p> <pre><code>struct compare { bool operator()(const uint3 &a, const uint3 &b) const { return a[2] > b[2]; } }; std::sort(work.begin(), work.end(), compare()); </code></pre> <p>As a bonus to much clearer code, you also most probably get a slight performance boost of <code>std::sort</code> over <code>std::qsort</code>.</p>

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