ARROW-12861 adds a sign kernel to the C++ library. Add a mapping to this in the R package.

I think this should be as simple as just adding

"sign" = "sign",

to .unary_function_map in expression.R, plus a simple test.

Note that the C++ library's sign kernel returns integer output whereas base R's sign function returns numeric (double) output, so a test written with expect_dplyr_equal() might fail unless it explicitly converts the output from sign() using as.numeric() or as.integer().

Reporter: Ian Cook / @ianmcook
Assignee: Neal Richardson / @nealrichardson

Related issues:

• [R] bindings for sign() (duplicates)
• [C++][Compute] Add sign function kernels (depends upon)

_{Note: This issue was originally created as ARROW-13094. Please see the migration documentation for further details.}