Function builtin_which 

fn builtin_which(
    args: &[RValue],
    named: &[(String, RValue)],
) -> Result<RValue, RError>

Indices of TRUE elements.

When arr.ind = TRUE and the input has a dim attribute (matrix), returns a matrix of row/column subscripts instead of linear indices.

@param x logical vector or matrix @param arr.ind if TRUE, return matrix subscripts for array input @return integer vector of 1-based indices, or integer matrix of subscripts