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POHow to access/iterate over all non-unique keys in an unordered_multimap?
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  1. POHow to access/iterate over all non-unique keys in an unordered_multimap?
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    copied!<p>I would like to access/iterate over all non-unique keys in an unordered_multimap. The hash table basically is a map from a signature <code>&lt;SIG&gt;</code> that does indeed occur more than once in practice to identifiers <code>&lt;ID&gt;</code>. I would like to find those entries in the hash table where occurs once.</p> <p>Currently I use this approach:</p> <pre><code>// map &lt;SIG&gt; -&gt; &lt;ID&gt; typedef unordered_multimap&lt;int, int&gt; HashTable; HashTable&amp; ht = ...; for(HashTable::iterator it = ht.begin(); it != ht.end(); ++it) { size_t n=0; std::pair&lt;HashTable::iterator, HashTable::iterator&gt; itpair = ht.equal_range(it-&gt;first); for ( ; itpair.first != itpair.second; ++itpair.first) { ++n; } if( n &gt; 1 ){ // access those items again as the previous iterators are not valid anymore std::pair&lt;HashTable::iterator, HashTable::iterator&gt; itpair = ht.equal_range(it-&gt;first); for ( ; itpair.first != itpair.second; ++itpair.first) { // do something with those items } } } </code></pre> <p>This is certainly not efficient as the outer loop iterates over all elements of the hash table (via <code>ht.begin()</code>) and the inner loop tests if the corresponding key is present more than once.</p> <p>Is there a more efficient or elegant way to do this?</p> <p>Note: I know that with a <code>unordered_map</code> instead of <code>unordered_multimap</code> I wouldn't have this issue but due to application requirements I must be able to store multiple keys <code>&lt;SIG&gt;</code> pointing to different identifiers <code>&lt;ID&gt;</code>. Also, an <code>unordered_map&lt;SIG, vector&lt;ID&gt; &gt;</code> is not a good choice for me as it uses roughly 150% of memory as I have many unique keys and <code>vector&lt;ID&gt;</code> adds quite a bit of overhead for each item.</p>
 

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