[SPARK-10807] [SPARKR] Added as.data.frame as a synonym for collect

Created method as.data.frame as a synonym for collect().

Author: Oscar D. Lara Yejas <olarayej@mail.usf.edu>
Author: olarayej <oscar.lara.yejas@us.ibm.com>
Author: Oscar D. Lara Yejas <oscar.lara.yejas@us.ibm.com>

Closes #8908 from olarayej/SPARK-10807.

R/pkg/NAMESPACE

@@ -247,3 +247,5 @@ export("structField",
        "structType.jobj",
        "structType.structField",
        "print.structType")
+
+export("as.data.frame")
\ No newline at end of file

R/pkg/R/DataFrame.R

@@ -1848,3 +1848,28 @@ setMethod("crosstab",
             sct <- callJMethod(statFunctions, "crosstab", col1, col2)
             collect(dataFrame(sct))
           })
+
+
+#' This function downloads the contents of a DataFrame into an R's data.frame.
+#' Since data.frames are held in memory, ensure that you have enough memory
+#' in your system to accommodate the contents.
+#' 
+#' @title Download data from a DataFrame into a data.frame
+#' @param x a DataFrame
+#' @return a data.frame
+#' @rdname as.data.frame
+#' @examples \dontrun{
+#' 
+#' irisDF <- createDataFrame(sqlContext, iris)
+#' df <- as.data.frame(irisDF[irisDF$Species == "setosa", ])
+#' }
+setMethod("as.data.frame",
+          signature(x = "DataFrame"),
+          function(x, ...) {
+            # Check if additional parameters have been passed
+            if (length(list(...)) > 0) {
+              stop(paste("Unused argument(s): ", paste(list(...), collapse=", ")))
+            }
+            collect(x)
+          }
+)

R/pkg/R/generics.R

@@ -983,3 +983,7 @@ setGeneric("glm")
 #' @rdname rbind
 #' @export
 setGeneric("rbind", signature = "...")
+
+#' @rdname as.data.frame
+#' @export
+setGeneric("as.data.frame")
\ No newline at end of file

R/pkg/inst/tests/test_sparkSQL.R

@@ -1327,6 +1327,13 @@ test_that("SQL error message is returned from JVM", {
   expect_equal(grepl("Table Not Found: blah", retError), TRUE)
 })
 
+test_that("Method as.data.frame as a synonym for collect()", {
+  irisDF <- createDataFrame(sqlContext, iris)
+  expect_equal(as.data.frame(irisDF), collect(irisDF))
+  irisDF2 <- irisDF[irisDF$Species == "setosa", ]
+  expect_equal(as.data.frame(irisDF2), collect(irisDF2))
+})
+
 unlink(parquetPath)
 unlink(jsonPath)
-unlink(jsonPathNa)
+unlink(jsonPathNa)
\ No newline at end of file