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POBlitz++ arrays as keys for maps
 

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  1. POBlitz++ arrays as keys for maps
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    <p>I am trying to use blitz++ arrays, since I understand they generally offer higher performance than other forms of arrays. Is it possible to use blitz++ arrays as keys in a map? Trying </p> <pre><code>#include &lt;map&gt; #include &lt;blitz/array.h&gt; using namespace std; map&lt;blitz::Array&lt;int,1&gt;,int&gt; testmap; blitz::Array&lt;int,1&gt; B(3); B = 1,2,3; testmap.insert(make_pair(B,2)); </code></pre> <p>does not compile. Here's the error:</p> <blockquote> <p>In file included from /usr/include/c++/4.6/string:50:0,</p> <p>/usr/include/c++/4.6/bits/stl_function.h: In member function ‘bool std::less&lt;_Tp>::operator()(const _Tp&amp;, const _Tp&amp;) const [with _Tp = blitz::Array]’:</p> <p>/usr/include/c++/4.6/bits/stl_function.h:236:22: error: cannot convert ‘blitz::BzBinaryExprResult, blitz::Array >::T_result {aka blitz::_bz_ArrayExpr, blitz::FastArrayIterator, blitz::Less > >}’ to ‘bool’ in return</p> </blockquote> <p>Is this a matter of requiring a definition of the <code>&lt;</code> operator, and if so, can/should I define it myself?</p> <p><strong>ANSWER</strong></p> <p>As suggested by Jimmy Thompson, a possible solution is to define:</p> <pre><code>struct MyComparison { bool operator() (const blitz::Array&lt;int, 1&gt; &amp;lhs, const blitz::Array&lt;int, 1&gt; &amp;rhs) const { if (lhs.size() &lt; rhs.size()) {return true;} else if (lhs.size() &gt; rhs.size()) {return false;} else { for (int i=0; i&lt;lhs.size(); i++) { if (lhs(i)&lt;rhs(i)) {return true;} else if(lhs(i)&gt;rhs(i)) {return false;} } } } }; </code></pre> <p>Then</p> <pre><code>map&lt;blitz::Array&lt;int,1&gt;,int, MyComparison&gt; testmap; </code></pre>
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    1. COWhen you say it "does not work" it is very important to say how it does not work.
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    2. COI edited in the compiler error. I really don't know what it means though.
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    3. COYou are right. I edited the namespace std in.
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