Letâs check that we get what we want: x <- rnorm ( 1000 ) res_cpp <- top_index ( x, 30L ) res_r <- tail ( order ( x ), 30L ) identical ( res_cpp, res_r )Īnd then let us benchmark: require ( microbenchmark ) We push the first n points into the queue, and finally we input the rest of the We handle theĬase where there are less than n data points which is not interesting, then With this, the implementation of top_index is straightforward. conversion to an IntegerVector for when we want to get the results.input : this first compares the top of the queue and replaces it if necessary.The IndexQueue template embeds a priority_queue with the right parameters, Of the operator : || (data = data & j > i ) Want to retain the value that has the smallest index. (int, float, or even any custom datatype) container (optional) : Container is passed as an underlying container to store the elements. The implementation of the operator takes into account ties. Syntax: priorityqueue ds Parameters: datatype (mandatory) : datatype that we are going to store in priorityqueue.We however need a specific implementationÄ«ecause we need a cast to String to use the operator. Type CHARSXP, which we can compare thanks to the implementation of the Raw pointer for better performance and defines the parenthesis operatorįor character vectors, data is internally stored into an array of SEXP of IndexCompare keeps a reference to the internal data of the vector, as a We will use the template implementation above for integer and numeric vectors. Using namespace Rcpp using namespace std // ] IntegerVector top_i_pq ( NumericVector v, int n ) In the queue and feed it by scanning the vector. The idea is then to only keep n such pairs Only keep the last n values: top, using theÄefault implementation of the âgreaterâ operator for pairs which justÄoes lexicographic ordering. The problem is to find the indices of top n elements from a vector.Īn inefficient way of doing this is to run order on the vector and then Getting indices of top elements from a vector using a priority queueĪnd more precisely on Martin Morganâs answer.
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