[SPARKR-150] phase 2: implement takeOrdered() and top().

Author: Sun Rui

pkg/NAMESPACE

@@ -49,7 +49,9 @@ exportMethods(
               "sortBy",
               "sortByKey",
               "take",
+              "takeOrdered",
               "takeSample",
+              "top",
               "unionRDD",
               "unpersist",
               "value",

pkg/R/RDD.R

@@ -1294,6 +1294,79 @@ setMethod("sortBy",
             values(sortByKey(keyBy(rdd, func), ascending, numPartitions))
           })
 
+# Helper function to get first N elements from an RDD in the specified order.
+# Param:
+#   rdd An RDD.
+#   num Number of elements to return.
+#   ascending A flag to indicate whether the sorting is ascending or descending.
+# Return:
+#   A list of the first N elements from the RDD in the specified order.
+#
+takeOrderedElem <- function(rdd, num, ascending = TRUE) {          
+  if (num <= 0L) {
+    return(list())
+  }
+  
+  partitionFunc <- function(part) {
+    if (num < length(part)) {
+      # R limitation: order works only on primitive types!
+      ord <- order(unlist(part, recursive = FALSE), decreasing = !ascending)
+      part[ord[1:num]]
+    } else {
+      part
+    }
+  }
+  
+  newRdd <- mapPartitions(rdd, partitionFunc)
+  take(sortBy(newRdd, function(x) { x }, ascending = ascending), num)
+}
+
+#' Returns the first N elements from an RDD in ascending order.
+#'
+#' @param rdd An RDD.
+#' @param num Number of elements to return.
+#' @return The first N elements from the RDD in ascending order.
+#' @rdname takeOrdered
+#' @export
+#' @examples
+#'\dontrun{
+#' sc <- sparkR.init()
+#' rdd <- parallelize(sc, list(10, 1, 2, 9, 3, 4, 5, 6, 7))
+#' takeOrdered(rdd, 6L) # list(1, 2, 3, 4, 5, 6)
+#'}
+setGeneric("takeOrdered", function(rdd, num) { standardGeneric("takeOrdered") })
+
+#' @rdname takeOrdered
+#' @aliases takeOrdered,RDD,RDD-method
+setMethod("takeOrdered",
+          signature(rdd = "RDD", num = "integer"),
+          function(rdd, num) {          
+            takeOrderedElem(rdd, num)
+          })
+
+#' Returns the top N elements from an RDD.
+#'
+#' @param rdd An RDD.
+#' @param num Number of elements to return.
+#' @return The top N elements from the RDD.
+#' @rdname top
+#' @export
+#' @examples
+#'\dontrun{
+#' sc <- sparkR.init()
+#' rdd <- parallelize(sc, list(10, 1, 2, 9, 3, 4, 5, 6, 7))
+#' top(rdd, 6L) # list(10, 9, 7, 6, 5, 4)
+#'}
+setGeneric("top", function(rdd, num) { standardGeneric("top") })
+
+#' @rdname top
+#' @aliases top,RDD,RDD-method
+setMethod("top",
+          signature(rdd = "RDD", num = "integer"),
+          function(rdd, num) {          
+            takeOrderedElem(rdd, num, FALSE)
+          })
+
 ############ Shuffle Functions ############
 
 #' Partition an RDD by key

pkg/inst/tests/test_rdd.R

@@ -278,6 +278,30 @@ test_that("sortBy() on RDDs", {
   expect_equal(actual, as.list(nums))
 })
 
+test_that("takeOrdered() on RDDs", {
+  l <- list(10, 1, 2, 9, 3, 4, 5, 6, 7)
+  rdd <- parallelize(sc, l)
+  actual <- takeOrdered(rdd, 6L)
+  expect_equal(actual, as.list(sort(unlist(l)))[1:6])
+
+  l <- list("e", "d", "c", "d", "a")
+  rdd <- parallelize(sc, l)
+  actual <- takeOrdered(rdd, 3L)
+  expect_equal(actual, as.list(sort(unlist(l)))[1:3])
+})
+
+test_that("top() on RDDs", {
+  l <- list(10, 1, 2, 9, 3, 4, 5, 6, 7)
+  rdd <- parallelize(sc, l)
+  actual <- top(rdd, 6L)
+  expect_equal(actual, as.list(sort(unlist(l), decreasing = TRUE))[1:6])
+  
+  l <- list("e", "d", "c", "d", "a")
+  rdd <- parallelize(sc, l)
+  actual <- top(rdd, 3L)
+  expect_equal(actual, as.list(sort(unlist(l), decreasing = TRUE))[1:3])
+})
+
 test_that("keys() on RDDs", {
   keys <- keys(intRdd)
   actual <- collect(keys)

pkg/man/takeOrdered.Rd

@@ -0,0 +1,31 @@
+% Generated by roxygen2 (4.0.2): do not edit by hand
+\docType{methods}
+\name{takeOrdered}
+\alias{takeOrdered}
+\alias{takeOrdered,RDD,RDD-method}
+\alias{takeOrdered,RDD,integer-method}
+\title{Returns the first N elements from an RDD in ascending order.}
+\usage{
+takeOrdered(rdd, num)
+
+\S4method{takeOrdered}{RDD,integer}(rdd, num)
+}
+\arguments{
+\item{rdd}{An RDD.}
+
+\item{num}{Number of elements to return.}
+}
+\value{
+The first N elements from the RDD in ascending order.
+}
+\description{
+Returns the first N elements from an RDD in ascending order.
+}
+\examples{
+\dontrun{
+sc <- sparkR.init()
+rdd <- parallelize(sc, list(10, 1, 2, 9, 3, 4, 5, 6, 7))
+takeOrdered(rdd, 6L) # list(1, 2, 3, 4, 5, 6)
+}
+}
+

pkg/man/top.Rd

@@ -0,0 +1,31 @@
+% Generated by roxygen2 (4.0.2): do not edit by hand
+\docType{methods}
+\name{top}
+\alias{top}
+\alias{top,RDD,RDD-method}
+\alias{top,RDD,integer-method}
+\title{Returns the top N elements from an RDD.}
+\usage{
+top(rdd, num)
+
+\S4method{top}{RDD,integer}(rdd, num)
+}
+\arguments{
+\item{rdd}{An RDD.}
+
+\item{num}{Number of elements to return.}
+}
+\value{
+The top N elements from the RDD.
+}
+\description{
+Returns the top N elements from an RDD.
+}
+\examples{
+\dontrun{
+sc <- sparkR.init()
+rdd <- parallelize(sc, list(10, 1, 2, 9, 3, 4, 5, 6, 7))
+top(rdd, 6L) # list(10, 9, 7, 6, 5, 4)
+}
+}
+