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### Introduction

Often we need to have optional arguments in R of Rcpp functions with default values. Sometimes, the default value for the optional parameters is set to be NULL. Rcpp provides the Nullable <> to set default value as to be R_NilValue (equivalent of NULL in Rcpp). There have been several StackOverflow posts on using the Nullable behavior. As seen from quite a few posts, the key step in using Rcpp::Nullable<> is to cast it to the underlying type first (i.e., instantiation) after checking that the input is not NULL.

### Nullability of Vector, Matrix or Logical Vector

Running a few examples with setting NULL for a vector, matrix or a boolean value gives us the expected results.

Numeric Vector is set to not NULL.
1 2

Numeric Matrix is set to not NULL.
-0.500000 -0.500000 -0.500000
-0.500000 -0.500000 -0.500000
-0.500000 -0.500000 -0.500000

Logical Vector is set to not NULL.
0

Warning in nullable1(NULL, NULL, NULL): All arguments are set to NULL.

Warning in nullable1(c(), NULL, NULL): All arguments are set to NULL.


We get the same result when the input to the NumericVector argument is not NULL but an empty vector, i.e., c(), which is also expected since is.null(c()) is TRUE in R.

A stricter test whether the input is usable can be (aptly named) isUsable().

### Nullability of DataFrame and List

Rcpp::Nullable<> works for SEXP based Rcpp types, so Rcpp::DataFrame and Rcpp::List can also be set to Nullable and instantiated if not NULL.

Testing with Rcpp::List and Rcpp::DataFrame gives expected results, i.e.,

List is not NULL.
List length of 2 elements.

DataFrame is not NULL.
DataFrame of 20 rows and 2 columns.


### Nullability of RcppGSL::Matrix

In addition to Rcpp types, RcppGSL::Matrix can also be set with Nullable type (e.g, in the mvlabund package): e.g.,

Finally, testing with RcppGSL::Matrix which can also be set to Nullable<>, i.e.,

Warning in nullable4(NULL): Input GSL Matrix is NULL.

Input is not NULL.
Input GSL matrix has 3 and 3 columns.


### Summary

Rcpp provides a convenient construct to set datatypes to NULL using R_NilValue and application of the datatype if not set to NULL using the .isNotNull() check. This construct to applied to set datatypes to NULL as default values and possible simple simplification.