Mathematica's Null is interpreted as R NULL during conversion to RLink's internal representation:
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Of course, it also works in another direction:
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You can set a variable (or part of an expression, e.g. part of a list) to NULL:
Comparison to NULL does not really work—it returns a logical vector of length 0:
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You can, however, test the type of the value stored in the test variable:
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The proper way to test for NULL is via the is.null built-in R function:
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Vectors of zero length are also considered NULL in R:
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For this reason, assigning NULL to a vector's element is an error:
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Assigning NULL to an element of a list is not an error, and effectively deletes that element from a list. Here is a test list:
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This will assign NULL to its second element:
This is equivalent to removal of that element from a list:
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In some sense, R NULL therefore combines the behavior of Mathematica's Null and Mathematica's Sequence[].
is RLink's representation of an R NULL object.
