# STL Transform

December 28, 2012
By

(This article was first published on Rcpp Gallery, and kindly contributed to R-bloggers)

The STL transform function can be used to pass a single function over a vector. Here we use a simple function `square()`.

``````#include <Rcpp.h>

using namespace Rcpp;

inline double square(double x) { return x*x ; }

// [[Rcpp::export]]
std::vector<double> transformEx(const std::vector<double>& x) {
std::vector<double> y(x.size());
std::transform(x.begin(), x.end(), y.begin(), square);
return y;
}
``````
``````  x <- c(1,2,3,4)
transformEx(x)
``````
```[1]  1  4  9 16
```

A second variant combines two input vectors.

``````inline double squaredNorm(double x, double y) { return sqrt(x*x + y*y); }

// [[Rcpp::export]]
NumericVector transformEx2(NumericVector x, NumericVector y) {
NumericVector z(x.size());
std::transform(x.begin(), x.end(), y.begin(), z.begin(), squaredNorm);
return z;
}
``````
``````  x <- c(1,2,3,4)
y <- c(2,2,3,3)
transformEx2(x,y)
``````
```[1] 2.236 2.828 4.243 5.000
```

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