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copied!<p>There are two problems in your code.</p> <p><em>First</em>, you need return a value in the following function, or simply make it return void.</p> <pre><code>// you should return a value here or make it return void int outputSet(int i) { cout << i << endl; } </code></pre> <p><em>Second</em>, the iterators in the following iterations of your for-loop are <strong><em>invalidated</em></strong> once you remove the current one. Once an element is removed, its iterator i is also invalidated, so as to the following iterators based on ++i;</p> <p>And you'll get run-time error because iterator i now points to You need somehow "reset" it.</p> <p><strong>MSVC Implementation</strong></p> <pre><code>for (set<int>::iterator i = n1Ar.begin(); i != n1Ar.end(); ++i) { it = n2Ar.find(*i); if (it != n2Ar.end()) { list.insert(*i); // the following iterators become invalidated after the // current one is removed. You need reset it like // i = n1Ar.erase(i); n1Ar.erase(i); n2Ar.erase(it); } else { it = n3Ar.find(*i); if (it != n3Ar.end()) { list.insert(*i); // the following iterators become invalidated after the // current one is removed. You need reset it like // i = n1Ar.erase(i); n1Ar.erase(i); n3Ar.erase(it); } } } </code></pre> <p><strong>Edit</strong>: Note that returning a new iterator from set::erase() is not a Standard way. That's mainly for the purpose of performance.</p> <p><strong>A More Portable Solution</strong></p> <p>The basic idea is to correctly set the next iterator before removing the current one.</p> <pre><code> set<int>::iterator i = n1Ar.begin(); while (i != n1Ar.end()) { it = n2Ar.find(*i); if (it != n2Ar.end()) { // the trick is to use "i++" where i is incremented by one while "old" i // is removed. list.insert(*i); n1Ar.erase(i++); n2Ar.erase(it); } else { it = n3Ar.find(*i); if (it != n3Ar.end()) { list.insert(*i); n1Ar.erase(i++); n3Ar.erase(it); } else { ++i; } } } </code></pre>

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