[SPARK-10905] [SPARKR] Export freqItems() for DataFrameStatFunctions

[SPARK-10905][SparkR]: Export freqItems() for DataFrameStatFunctions
- Add function (together with roxygen2 doc) to DataFrame.R and generics.R
- Expose the function in NAMESPACE
- Add unit test for the function

Author: Rerngvit Yanggratoke <rerngvit@kth.se>

Closes #8962 from rerngvit/SPARK-10905.

R/pkg/NAMESPACE

@@ -40,6 +40,7 @@ exportMethods("arrange",
               "fillna",
               "filter",
               "first",
+              "freqItems",
               "group_by",
               "groupBy",
               "head",

R/pkg/R/generics.R

@@ -63,6 +63,10 @@ setGeneric("countByValue", function(x) { standardGeneric("countByValue") })
 # @export
 setGeneric("crosstab", function(x, col1, col2) { standardGeneric("crosstab") })
 
+# @rdname statfunctions
+# @export
+setGeneric("freqItems", function(x, cols, support = 0.01) { standardGeneric("freqItems") })
+
 # @rdname distinct
 # @export
 setGeneric("distinct", function(x, numPartitions = 1) { standardGeneric("distinct") })

R/pkg/R/stats.R

@@ -100,3 +100,30 @@ setMethod("corr",
             statFunctions <- callJMethod(x@sdf, "stat")
             callJMethod(statFunctions, "corr", col1, col2, method)
           })
+
+#' freqItems
+#'
+#' Finding frequent items for columns, possibly with false positives.
+#' Using the frequent element count algorithm described in
+#' \url{http://dx.doi.org/10.1145/762471.762473}, proposed by Karp, Schenker, and Papadimitriou.
+#'
+#' @param x A SparkSQL DataFrame.
+#' @param cols A vector column names to search frequent items in.
+#' @param support (Optional) The minimum frequency for an item to be considered `frequent`. 
+#'                Should be greater than 1e-4. Default support = 0.01.
+#' @return a local R data.frame with the frequent items in each column
+#'
+#' @rdname statfunctions
+#' @name freqItems
+#' @export
+#' @examples
+#' \dontrun{
+#' df <- jsonFile(sqlContext, "/path/to/file.json")
+#' fi = freqItems(df, c("title", "gender"))
+#' }
+setMethod("freqItems", signature(x = "DataFrame", cols = "character"),
+          function(x, cols, support = 0.01) {
+            statFunctions <- callJMethod(x@sdf, "stat")
+            sct <- callJMethod(statFunctions, "freqItems", as.list(cols), support)
+            collect(dataFrame(sct))
+          })

R/pkg/inst/tests/test_sparkSQL.R

@@ -1350,6 +1350,27 @@ test_that("cov() and corr() on a DataFrame", {
   expect_true(abs(result - 1.0) < 1e-12)
 })
 
+test_that("freqItems() on a DataFrame", {
+  input <- 1:1000
+  rdf <- data.frame(numbers = input, letters = as.character(input),
+                    negDoubles = input * -1.0, stringsAsFactors = F)
+  rdf[ input %% 3 == 0, ] <- c(1, "1", -1)
+  df <- createDataFrame(sqlContext, rdf)
+  multiColResults <- freqItems(df, c("numbers", "letters"), support=0.1)
+  expect_true(1 %in% multiColResults$numbers[[1]])
+  expect_true("1" %in% multiColResults$letters[[1]])
+  singleColResult <- freqItems(df, "negDoubles", support=0.1)
+  expect_true(-1 %in% head(singleColResult$negDoubles)[[1]])
+
+  l <- lapply(c(0:99), function(i) {
+    if (i %% 2 == 0) { list(1L, -1.0) }
+    else { list(i, i * -1.0) }})
+  df <- createDataFrame(sqlContext, l, c("a", "b"))
+  result <- freqItems(df, c("a", "b"), 0.4)
+  expect_identical(result[[1]], list(list(1L, 99L)))
+  expect_identical(result[[2]], list(list(-1, -99)))
+})
+
 test_that("SQL error message is returned from JVM", {
   retError <- tryCatch(sql(sqlContext, "select * from blah"), error = function(e) e)
   expect_equal(grepl("Table Not Found: blah", retError), TRUE)