r/R/table.R

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#' @title Table class
#' @description A Table is a sequence of [chunked arrays][ChunkedArray]. They
#' have a similar interface to [record batches][RecordBatch], but they can be
#' composed from multiple record batches or chunked arrays.
#' @usage NULL
#' @format NULL
#' @docType class
#'
#' @section Factory:
#'
#' The `Table$create()` function takes the following arguments:
#'
#' * `...` arrays, chunked arrays, or R vectors, with names; alternatively,
#'    an unnamed series of [record batches][RecordBatch] may also be provided,
#'    which will be stacked as rows in the table.
#' * `schema` a [Schema], or `NULL` (the default) to infer the schema from
#'    the data in `...`
#'
#' @section S3 Methods and Usage:
#' Tables are data-frame-like, and many methods you expect to work on
#' a `data.frame` are implemented for `Table`. This includes `[`, `[[`,
#' `$`, `names`, `dim`, `nrow`, `ncol`, `head`, and `tail`. You can also pull
#' the data from an Arrow table into R with `as.data.frame()`. See the
#' examples.
#'
#' A caveat about the `$` method: because `Table` is an `R6` object,
#' `$` is also used to access the object's methods (see below). Methods take
#' precedence over the table's columns. So, `tab$Slice` would return the
#' "Slice" method function even if there were a column in the table called
#' "Slice".
#'
#' A caveat about the `[` method for row operations: only "slicing" is
#' currently supported. That is, you can select a continuous range of rows
#' from the table, but you can't filter with a `logical` vector or take an
#' arbitrary selection of rows by integer indices.
#'
#' @section R6 Methods:
#' In addition to the more R-friendly S3 methods, a `Table` object has
#' the following R6 methods that map onto the underlying C++ methods:
#'
#' - `$column(i)`: Extract a `ChunkedArray` by integer position from the table
#' - `$ColumnNames()`: Get all column names (called by `names(tab)`)
#' - `$GetColumnByName(name)`: Extract a `ChunkedArray` by string name
#' - `$field(i)`: Extract a `Field` from the table schema by integer position
#' - `$select(spec)`: Return a new table with a selection of columns.
#'    This supports the usual `character`, `numeric`, and `logical` selection
#'    methods as well as "tidy select" expressions.
#' - `$Slice(offset, length = NULL)`: Create a zero-copy view starting at the
#'    indicated integer offset and going for the given length, or to the end
#'    of the table if `NULL`, the default.
#' - `$serialize(output_stream, ...)`: Write the table to the given
#'    [OutputStream]
#' - `$cast(target_schema, safe = TRUE, options = cast_options(safe))`: Alter
#'    the schema of the record batch.
#'
#' There are also some active bindings
#' - `$num_columns`
#' - `$num_rows`
#' - `$schema`
#' - `$columns`: Returns a list of `ChunkedArray`s
#' @rdname Table
#' @name Table
#' @examples
#' \donttest{
#' tab <- Table$create(name = rownames(mtcars), mtcars)
#' dim(tab)
#' dim(head(tab))
#' names(tab)
#' tab$mpg
#' tab[["cyl"]]
#' as.data.frame(tab[4:8, c("gear", "hp", "wt")])
#' }
#' @export
Table <- R6Class("Table", inherit = Object,
  public = list(
    column = function(i) {
      assert_is(i, c("numeric", "integer"))
      assert_that(length(i) == 1)
      shared_ptr(ChunkedArray, Table__column(self, i))
    },
    ColumnNames = function() Table__ColumnNames(self),
    GetColumnByName = function(name) {
      assert_is(name, "character")
      assert_that(length(name) == 1)
      shared_ptr(ChunkedArray, Table__GetColumnByName(self, name))
    },
    field = function(i) shared_ptr(Field, Table__field(self, i)),

    serialize = function(output_stream, ...) write_table(self, output_stream, ...),
    ToString = function() ToString_tabular(self),

    cast = function(target_schema, safe = TRUE, options = cast_options(safe)) {
      assert_is(target_schema, "Schema")
      assert_is(options, "CastOptions")
      assert_that(identical(self$schema$names, target_schema$names), msg = "incompatible schemas")
      shared_ptr(Table, Table__cast(self, target_schema, options))
    },

    select = function(spec) {
      spec <- enquo(spec)
      if (quo_is_null(spec)) {
        self
      } else {
        all_vars <- self$ColumnNames()
        vars <- vars_select(all_vars, !!spec)
        indices <- match(vars, all_vars)
        shared_ptr(Table, Table__select(self, indices))
      }
    },

    Slice = function(offset, length = NULL) {
      if (is.null(length)) {
        shared_ptr(Table, Table__Slice1(self, offset))
      } else {
        shared_ptr(Table, Table__Slice2(self, offset, length))
      }
    },

    Equals = function(other) {
      Table__Equals(self, other)
    }
  ),

  active = list(
    num_columns = function() Table__num_columns(self),
    num_rows = function() Table__num_rows(self),
    schema = function() shared_ptr(Schema, Table__schema(self)),
    columns = function() map(Table__columns(self), shared_ptr, class = ChunkedArray)
  )
)

Table$create <- function(..., schema = NULL){
  dots <- list2(...)
  # making sure there are always names
  if (is.null(names(dots))) {
    names(dots) <- rep_len("", length(dots))
  }
  stopifnot(length(dots) > 0)
  shared_ptr(Table, Table__from_dots(dots, schema))
}

#' @export
as.data.frame.Table <- function(x, row.names = NULL, optional = FALSE, use_threads = TRUE, ...){
  Table__to_dataframe(x, use_threads = option_use_threads())
}

#' @export
dim.Table <- function(x) {
  c(x$num_rows, x$num_columns)
}

#' @export
names.Table <- function(x) x$ColumnNames()

#' @export
`[.Table` <- `[.RecordBatch`

#' @export
`[[.Table` <- `[[.RecordBatch`

#' @export
`$.Table` <- `$.RecordBatch`

#' @export
head.Table <- head.RecordBatch

#' @export
tail.Table <- tail.RecordBatch