# subset vectors in Rcpp11

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Under the impulsion of `@kevin_ushey`

who already did something similar for `Rcpp`

, we’ve been adding subsetting behavior into `Rcpp11`

.

The idea is given a vector `y`

and a vector `x`

we want to give meaning to `y[x]`

.

The first legitimate question is what kind of `x`

do we want to allow. This has been discussed since january. So far, we’ve settled to allow `x`

to be integer, logical and character vectors. The main source of anxiety here being the typical Cornelian Dilemma

Do we use 0-based or 1-based indices ?

We decided to use 0-based indices, as this is what we do when `x`

is a scalar `int`

, and this is C++ : **indexing starts at 0** .

## rhs use

Given that, here is a first example:

NumericVector y = sqrt( seq_len(10) ) ; IntegerVector x {0,1,2} ; NumericVector res = y[x] ; // [1] 1.000000 1.414214 1.732051

The way we implemented this, `y[x]`

does not yet return a `NumericVector`

, that would have been too easy, instead it gives us a lovely sugar expression.

NumericVector y = sqrt( seq_len(10) ) ; IntegerVector x {0,1,2} ; auto res = y[x] ; Rprintf( "type(res) = %s\n", DEMANGLE(decltype(res)) ) ; // type(res) = Rcpp::SubsetProxy<Rcpp::Vector<14, Rcpp::PreserveStorage>, int, Rcpp::Vector<13, Rcpp::PreserveStorage> > return res ; // [1] 1.000000 1.414214 1.732051

This is relevant because we don’t need to materialize the data too early, we can send it to whatever `sugar`

function:

NumericVector y = sqrt( seq_len(10) ) ; IntegerVector x {0,1,2} ; auto res = sapply( y[x], [](double x){ return x*x; }) ; Rprintf( "type(res) = %s\n", DEMANGLE(decltype(res)) ) ; // type(res) = Rcpp::sugar::Sapply<double, Rcpp::SubsetProxy<Rcpp::Vector<14, Rcpp::PreserveStorage>, int, Rcpp::Vector<13, Rcpp::PreserveStorage> >, test()::$_0> return res ; // [1] 1 2 3

`x`

may also be a sugar expression, it does not necessarily need to be a materialized vector. For example:

NumericVector y = sqrt( seq_len(10) ) ; auto res = sapply( y[seq(0, 4)], [](double x){ return x*x; }) ; Rprintf( "type(res) = %s\n", DEMANGLE(decltype(res)) ) ; // type(res) = Rcpp::sugar::Sapply<double, Rcpp::SubsetProxy<Rcpp::Vector<14, Rcpp::PreserveStorage>, int, Rcpp::sugar::Seq>, test()::$_0> return res ; // [1] 1 2 3 4 5

And it can be a logical or character expression. For example `y[ y < 2.0 ]`

...

## lhs use

In addition to being a sugar expression, that knows how to apply itself to a vector, the object that is created by `y[x]`

may also be used on the lhs of the expression.

For example :

NumericVector y = sqrt( seq_len(10) ) ; IntegerVector x {0,1,2} ; y[x] = - y[x] ; return y ; // [1] -1.000000 -1.414214 -1.732051 2.000000 2.236068 2.449490 2.645751 // [8] 2.828427 3.000000 3.162278

And of course, handling sugar :

NumericVector y = sqrt( seq_len(10) ) ; IntegerVector x {0,1,2} ; y[2*x] = - y[x] ; return y ; // [1] -1.000000 1.414214 -1.414214 2.000000 1.414214 2.449490 2.645751 // [8] 2.828427 3.000000 3.162278

Although the feature has been discussed for a few months, it is pretty new so things might change. Actually I came up with a few ideas while writing this post.

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