Question

I want to write a function that slices a ‘string’ into a vector, sequentially, at a given index. I have a fairly adequate R solution for it; however, I figure that writing the code in C/C++ would likely be faster. For example, I’d like to be able to write a function ‘strslice’ that operates as follows:

x <- "abcdef"
strslice( x, 2 ) ## should return c("ab", "cd", "ef")

However, I’m not sure how to handle treating elements of the ‘CharacterVector’ passed around in the Rcpp code as strings. This is what I imagine might work (given my lack of C++/Rcpp knowledge I’m sure there’s a better approach):

f <- rcpp( signature(x="character", n="integer"), '
  std::string myString = Rcpp::as<std::string>(x);
  int cutpoint = Rcpp::as<int>(n);
  vector<std::string> outString;
  int len = myString.length();
  for( int i=0; i<len/n; i=i+n ) {
    outString.push_back( myString.substr(i,i+n-1 ) );
    myString = myString.substr(i+n, len-i*n);
  }
  return Rcpp::wrap<Rcpp::CharacterVector>( outString );
  ')

For the record, the corresponding R code I have is:

strslice <- function(x, n) {
  x <- as.data.frame( stringsAsFactors=FALSE, 
                      matrix( unlist( strsplit( x, "" ) ), ncol=n, byrow=T )
  )

  do.call( function(...) { paste(..., sep="") }, x )

}

…but I figure jumping around between data structures so much will slow things down with very large strings.

(Alternatively: is there a way to coerce ‘strsplit’ into behaving as I want?)

Answer 1

I would use substring. Something like this:

strslice <- function( x, n ){   
    starts <- seq( 1L, nchar(x), by = n )
    substring( x, starts, starts + n-1L )
}
strslice( "abcdef", 2 )
# [1] "ab" "cd" "ef"

About your Rcpp code, maybe you can allocate the std::vector<std::string> with the right size, so that you avoid resizing it which might mean memory allocations, … or perhaps directly use a Rcpp::CharacterVector. Something like this:

strslice_rcpp <- rcpp( signature(x="character", n="integer"), '
    std::string myString = as<std::string>(x);
    int cutpoint = as<int>(n);
    int len = myString.length();
    int nout = len / cutpoint ;
    CharacterVector out( nout ) ;
    for( int i=0; i<nout; i++ ) {
      out[i] = myString.substr( cutpoint*i, 2 ) ;
    }
    return out ;
')
strslice_rcpp( "abdcefg", 2 )
# [1] "ab" "cd" "ef"