Feb 14, 2012

C/C++ Code Bites

When one has several vectors, and need to sort them simultaneously by one of them, one can contract a vector with the indices of the sorted entries.

#include &lt;iostream&gt;

#include &lt;vector&gt;

#include &lt;utility&gt;


using namespace std;

bool pairsort( const pair&lt;float, int&gt;&amp; i, const pair&lt;float, int&gt;&amp; j ) {
	if( i.first &lt; j.first ) return false;
	if( j.first &lt; i.first ) return true;
	return j.second &lt; i.second;
}

vector&lt;int&gt;  sortmap(vector&lt;float&gt; org){
	vector&lt; pair&lt;float, int&gt;  &gt; p;
	//create a pair &lt;value, index&gt;
	for(unsigned int i= 0; i!= org.size(); i++) {
		pair&lt;float, int&gt; a ( org.at(i), i);
		p.push_back( a );
	}

	sort(p.begin(), p.end(), pairsort);

	vector&lt; int &gt; ind;
	//create a pair &lt;value, index&gt;
	for(unsigned int i= 0; i!= p.size(); i++) {
		ind.push_back( p.at(i).second );
	}
	return ind;
}


int main(){

 vector&lt;float&gt; Jet_pt;
 vector&lt;float&gt; Jet_eta;
 vector&lt;float&gt; Jet_phi;

 // Fill vectors...
 vector&lt;int&gt; m = sortmap(Jet_eta);
 cout &lt;&lt; &quot;Jets ordered by eta&quot; &lt;&lt; endl;
 for(int i = 0; i!= m.size(); i++)
   cout &lt;&lt; i &lt;&lt; &quot; pt=&quot;&lt;&lt; Jets_pt.at(m.at(i))&lt;&lt; &quot;eta=&quot;&lt;&lt; Jets_eta.at(m.at(i)) &lt;&lt; endl;
 return 0;
}