**Rcpp Gallery**, and kindly contributed to R-bloggers)

Since the 0.10.* release series, Rcpp contains a new sugar function `clamp`

which can be used to limit vectors to both a minimum and maximim value. This recent StackOverflow question permitted `clamp`

to shine. We retake some of the answers, including the `clamp`

entry by Romain.

We first define the three R versions.

```
pminpmaxClamp <- function(x, a, b) {
pmax(a, pmin(x, b) )
}
ifelseClamp <- function(x, a, b) {
ifelse(x <= a, a, ifelse(x >= b, b, x))
}
operationsClamp <- function(x, a, b) {
a + (x-a > 0)*(x-a) - (x-b > 0)*(x-b)
}
```

We then define some data, and ensure that these versions all producing identical results.

```
set.seed(42)
x <- rnorm(100000)
a <- -1.0
b <- 1.0
stopifnot(all.equal(pminpmaxClamp(x,a,b), ifelseClamp(x,a,b), operationsClamp(x,a,b)))
```

Next is the C++ solution: a one-liner thanks to the existing sugar function.

```
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericVector rcppClamp(NumericVector x, double mi, double ma) {
return clamp(mi, x, ma);
}
```

We can then check and benchmark the new C++ version.

```
stopifnot(all.equal(pminpmaxClamp(x,a,b), rcppClamp(x,a,b)))
library(rbenchmark)
benchmark(pminpmaxClamp(x, a, b),
ifelseClamp(x, a, b),
operationsClamp(x, a, b),
rcppClamp(x, a, b),
order="relative")[,1:4]
```

test replications elapsed relative 4 rcppClamp(x, a, b) 100 0.119 1.000 3 operationsClamp(x, a, b) 100 0.505 4.244 1 pminpmaxClamp(x, a, b) 100 0.530 4.454 2 ifelseClamp(x, a, b) 100 5.268 44.269

We see a decent gain of the Rcpp version even relative to these vectorised R solutions. Among these, the simplest (based on `ifelse`

) is by far the slowest. The parallel min/max version is about as faster as the clever-but-less-readable expression-based solution.

Real “production” solutions will of course need some more testing of inputs etc. However, as an illustration of `clamp`

this example has hopefully been compelling.

**leave a comment**for the author, please follow the link and comment on his blog:

**Rcpp Gallery**.

R-bloggers.com offers

**daily e-mail updates**about R news and tutorials on topics such as: visualization (ggplot2, Boxplots, maps, animation), programming (RStudio, Sweave, LaTeX, SQL, Eclipse, git, hadoop, Web Scraping) statistics (regression, PCA, time series, trading) and more...