[SPARK-16005][R] Add randomSplit to SparkR

## What changes were proposed in this pull request?

This PR adds `randomSplit` to SparkR for API parity.

## How was this patch tested?

Pass the Jenkins tests (with new testcase.)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13721 from dongjoon-hyun/SPARK-16005.

R/pkg/NAMESPACE

@@ -81,6 +81,7 @@ exportMethods("arrange",
               "orderBy",
               "persist",
               "printSchema",
+              "randomSplit",
               "rbind",
               "registerTempTable",
               "rename",

R/pkg/R/DataFrame.R

@@ -2934,3 +2934,40 @@ setMethod("write.jdbc",
             write <- callJMethod(write, "mode", jmode)
             invisible(callJMethod(write, "jdbc", url, tableName, jprops))
           })
+
+#' randomSplit
+#'
+#' Return a list of randomly split dataframes with the provided weights.
+#'
+#' @param x A SparkDataFrame
+#' @param weights A vector of weights for splits, will be normalized if they don't sum to 1
+#' @param seed A seed to use for random split
+#'
+#' @family SparkDataFrame functions
+#' @rdname randomSplit
+#' @name randomSplit
+#' @export
+#' @examples
+#'\dontrun{
+#' sc <- sparkR.init()
+#' sqlContext <- sparkRSQL.init(sc)
+#' df <- createDataFrame(data.frame(id = 1:1000))
+#' df_list <- randomSplit(df, c(2, 3, 5), 0)
+#' # df_list contains 3 SparkDataFrames with each having about 200, 300 and 500 rows respectively
+#' sapply(df_list, count)
+#' }
+#' @note since 2.0.0
+setMethod("randomSplit",
+          signature(x = "SparkDataFrame", weights = "numeric"),
+          function(x, weights, seed) {
+            if (!all(sapply(weights, function(c) { c >= 0 }))) {
+              stop("all weight values should not be negative")
+            }
+            normalized_list <- as.list(weights / sum(weights))
+            if (!missing(seed)) {
+              sdfs <- callJMethod(x@sdf, "randomSplit", normalized_list, as.integer(seed))
+            } else {
+              sdfs <- callJMethod(x@sdf, "randomSplit", normalized_list)
+            }
+            sapply(sdfs, dataFrame)
+          })

R/pkg/R/generics.R

@@ -679,6 +679,10 @@ setGeneric("withColumnRenamed",
 #' @export
 setGeneric("write.df", function(df, path, ...) { standardGeneric("write.df") })
 
+#' @rdname randomSplit
+#' @export
+setGeneric("randomSplit", function(x, weights, seed) { standardGeneric("randomSplit") })
+
 ###################### Column Methods ##########################
 
 #' @rdname column

R/pkg/inst/tests/testthat/test_sparkSQL.R

@@ -2280,6 +2280,24 @@ test_that("createDataFrame sqlContext parameter backward compatibility", {
   expect_equal(collect(before), collect(after))
 })
 
+test_that("randomSplit", {
+  num <- 4000
+  df <- createDataFrame(data.frame(id = 1:num))
+
+  weights <- c(2, 3, 5)
+  df_list <- randomSplit(df, weights)
+  expect_equal(length(weights), length(df_list))
+  counts <- sapply(df_list, count)
+  expect_equal(num, sum(counts))
+  expect_true(all(sapply(abs(counts / num - weights / sum(weights)), function(e) { e < 0.05 })))
+
+  df_list <- randomSplit(df, weights, 0)
+  expect_equal(length(weights), length(df_list))
+  counts <- sapply(df_list, count)
+  expect_equal(num, sum(counts))
+  expect_true(all(sapply(abs(counts / num - weights / sum(weights)), function(e) { e < 0.05 })))
+})
+
 unlink(parquetPath)
 unlink(jsonPath)
 unlink(jsonPathNa)