| title | author | license | tags | summary | layout | src |
|---|---|---|---|---|---|---|
Finding the minimum of a vector |
Dirk Eddelbuettel |
GPL (>= 2) |
stl featured |
Demonstrates how STL's min_element can be used. |
post |
2012-12-21-vector-minimum.cpp |
This example was motivated by http://stackoverflow.com/questions/5158219/find-minimum-of-vector-in-rcpp and addresses how to find the minumum value and its position index in a vector.
{% highlight cpp %} #include <Rcpp.h> using namespace Rcpp;
// [[Rcpp::export]] double vecmin(NumericVector x) { // Rcpp supports STL-style iterators NumericVector::iterator it = std::min_element(x.begin(), x.end()); // we want the value so dereference return *it; } {% endhighlight %}
We can also use the iterator to compute the position, simply by taking the offset to the vector beginning.
{% highlight cpp %} // [[Rcpp::export]] int vecminInd(NumericVector x) { // Rcpp supports STL-style iterators NumericVector::iterator it = std::min_element(x.begin(), x.end()); // we want the value so dereference return it - x.begin(); } {% endhighlight %}
A quick illustration follows. Note that we pad the position by one to adjust for the 0-based versus 1-based indexing between C++ and R.
{% highlight r %} set.seed(5) x <- sample(1:100, 10) # ten out 100 x {% endhighlight %}
[1] 21 68 90 28 11 67 50 76 88 96
{% highlight r %} cat("Min is ", vecmin(x), " and at position ", vecminInd(x)+1, "\n") {% endhighlight %}
Min is 11 and at position 5
Of course, we subsequently added min and which_min as sugar
functions, but this example still illustrated how useful the STL
algoritms can be.