kevinushey

---
title: Handling Strings with Rcpp
author: Kevin Ushey
license: GPL (>= 2)
tags: string vector
summary: Demonstrates how one might handle a vector of strings with `Rcpp`,
  in addition to returning output.
---
 
This is a quick example of how you might use Rcpp to send and receive R

kevinushey

// the column-wise implementation is just as fast as colMeans,
// but the row-wise operation is not quite as fast as rowMeans.

// can I do better?

#include <Rcpp.h>
using namespace Rcpp;

template <class T>
inline double do_mean( T& x ) {

kevinushey

---
title: Make your own 'apply' functions for matrices
author: Kevin Ushey
license: GPL (>= 2)
tags: Rcpp apply matrix
summary: Clever use of sourceCpp can allow you to define your own 'apply' functions on the fly.
---

Make your own 'apply' functions
==========

kevinushey

---
title: Using Boost's foreach macro
author: Kevin Ushey
license: GPL (>= 2)
tags: basics boost
summary: Boost's BOOST_FOREACH can enable a more functional programming style.
---

Boost provides a macro, `BOOST_FOREACH`, that allows us to easily iterate
over elements in a container, similar to what we might

kevinushey

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
StringVector check( StringVector x ) {
  Rcout << "The type of x is: " << ::TYPEOF( x ) << std::endl;
  Rcout << "The type of x(0) is: " << ::TYPEOF( x(0) ) << std::endl;
  Rcout << "The integer associated with STRSXPs is: " << STRSXP << std::endl;
  Rcout << "The integer associated with CHARSXPs is: " << CHARSXP << std::endl;
  Rcout << "Is x a string?: " << ::Rf_isString(x) << std::endl;

kevinushey

---
title: Fast factor generation with Rcpp
author: Kevin Ushey
license: GPL (>= 2)
tags: factor tapply sugar
summary: We can make use of Rcpp sugar to implement a faster factor generator.
---

Recall that factors are really just integer vectors with 'levels', 
i.e., character labels that get mapped to each integer in the vector. 

kevinushey

pymat <- function(string, ...) {
  i <- 0
  for( arg in list(...) ) {
    to_replace <- paste( sep='', "\\{", i, "\\}" )
    string <- gsub( to_replace, arg, string )
    i <- i + 1
  }
  return( string )
}

kevinushey

---
title: Dynamic Wrapping and Recursion with Rcpp
author: Kevin Ushey
license: GPL (>= 2)
tags: basics
summary: We can use parts of R's API alongside Rcpp to recurse through 
  lists and dynamically wrap objects as needed.
---
 
We can leverage small parts of the R's C API in order to

kevinushey

swap <- function(vec, from, to) {
  tmp <- to[ match(vec, from) ]
  tmp[is.na(tmp)] <- vec[is.na(tmp)]
  return(tmp)
}

## example
swap( 1:10, c(5, 7), c(50, 70) ) ## exchange occurrences of 5 w/ 50, 7 w/ 70

kevinushey

a <- 1
b <- 2

f <- function(x) {
  call <- match.call()
  len <- length( call[[2]] )
  output <- x
  names(output) <- sapply( call[[2]][2:len], as.character )
  return(output)
}