R/lazy_table_generics.R

In MarcusKlik/lazyplyr: a most lazy dplyr implementation for remote data frames

# lazyplyr - a most lazy dplyr implementation for remote data frames
#
#  Copyright (C) 2021-present, Mark AJ Klik
#
#  This file is part of the lazyplyr R package.
#
#  This Source Code Form is subject to the terms of the Mozilla Public
#  License, v. 2.0. If a copy of the MPL was not distributed with this file,
#  You can obtain one at https://mozilla.org/MPL/2.0/.
#
#  https://www.mozilla.org/en-US/MPL/2.0/FAQ/
#
#  You can contact the author at:
#  - lazyplyr R package source repository : https://github.com/fstpackage/lazyplyr


#' Read data from a lazy column
#'
#' @param x a custom object with a lazy table API such as a `lazy_frame` (see method `lazy_frame()`)
#' @param col_name lazy table column to retrieve data from
#' @param index integer vector specifying the index to use from the vector, a single integer specifying
#' the starting index position of the subset or NULL. If a single integer is used, length should be equal to
#' the total number of elements. If NULL, the full column will be read.
#'
#' @return a subset of a single lazy table column vector
#' @export
read_row_index <- function(x, col_name, index) {
  UseMethod("read_row_index", x)
}


#' Read data from a lazy column
#'
#' @param x a custom object with a lazy table API such as a `lazy_frame` (see method `lazy_frame()`)
#' @param col_name lazy table column to retrieve data from
#' @param from starting element
#' @param length length of the vector returned
#'
#' @return a subset of a single lazy table column vector
#' @export
read_row_range <- function(x, col_name, from, length) {
  UseMethod("read_row_range", x)
}


#' Get column names
#'
#' @param x a custom object with a lazy table API such as a `lazy_frame` (see method `lazy_frame()`)
#'
#' @return character vector of column names
#' @export
column_names <- function(x) {
  UseMethod("column_names", x)
}