[SPARK-10075] [SPARKR] Add `when` expressino function in SparkR

- Add `when` and `otherwise` as `Column` methods
- Add `When` as an expression function
- Add `%otherwise%` infix as an alias of `otherwise`

Since R doesn't support a feature like method chaining, `otherwise(when(condition, value), value)` style is a little annoying for me. If `%otherwise%` looks strange for shivaram, I can remove it. What do you think?

### JIRA
[[SPARK-10075] Add `when` expressino function in SparkR - ASF JIRA](https://issues.apache.org/jira/browse/SPARK-10075)

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #8266 from yu-iskw/SPARK-10075.

R/pkg/NAMESPACE

@@ -152,6 +152,7 @@ exportMethods("abs",
               "n_distinct",
               "nanvl",
               "negate",
+              "otherwise",
               "pmod",
               "quarter",
               "reverse",
@@ -182,6 +183,7 @@ exportMethods("abs",
               "unhex",
               "upper",
               "weekofyear",
+              "when",
               "year")
 
 exportClasses("GroupedData")

R/pkg/R/column.R

@@ -203,3 +203,17 @@ setMethod("%in%",
             jc <- callJMethod(x@jc, "in", table)
             return(column(jc))
           })
+
+#' otherwise
+#'
+#' If values in the specified column are null, returns the value. 
+#' Can be used in conjunction with `when` to specify a default value for expressions.
+#'
+#' @rdname column
+setMethod("otherwise",
+          signature(x = "Column", value = "ANY"),
+          function(x, value) {
+            value <- ifelse(class(value) == "Column", value@jc, value)
+            jc <- callJMethod(x@jc, "otherwise", value)
+            column(jc)
+          })

R/pkg/R/functions.R

@@ -165,3 +165,17 @@ setMethod("n", signature(x = "Column"),
           function(x) {
             count(x)
           })
+
+#' when
+#'
+#' Evaluates a list of conditions and returns one of multiple possible result expressions.
+#' For unmatched expressions null is returned.
+#'
+#' @rdname column
+setMethod("when", signature(condition = "Column", value = "ANY"),
+          function(condition, value) {
+              condition <- condition@jc
+              value <- ifelse(class(value) == "Column", value@jc, value)
+              jc <- callJStatic("org.apache.spark.sql.functions", "when", condition, value)
+              column(jc)
+          })

R/pkg/R/generics.R

@@ -651,6 +651,14 @@ setGeneric("rlike", function(x, ...) { standardGeneric("rlike") })
 #' @export
 setGeneric("startsWith", function(x, ...) { standardGeneric("startsWith") })
 
+#' @rdname column
+#' @export
+setGeneric("when", function(condition, value) { standardGeneric("when") })
+
+#' @rdname column
+#' @export
+setGeneric("otherwise", function(x, value) { standardGeneric("otherwise") })
+
 
 ###################### Expression Function Methods ##########################
 

R/pkg/inst/tests/test_sparkSQL.R

@@ -727,6 +727,13 @@ test_that("greatest() and least() on a DataFrame", {
   expect_equal(collect(select(df, least(df$a, df$b)))[, 1], c(1, 3))
 })
 
+test_that("when() and otherwise() on a DataFrame", {
+  l <- list(list(a = 1, b = 2), list(a = 3, b = 4))
+  df <- createDataFrame(sqlContext, l)
+  expect_equal(collect(select(df, when(df$a > 1 & df$b > 2, 1)))[, 1], c(NA, 1))
+  expect_equal(collect(select(df, otherwise(when(df$a > 1, 1), 0)))[, 1], c(0, 1))
+})
+
 test_that("group by", {
   df <- jsonFile(sqlContext, jsonPath)
   df1 <- agg(df, name = "max", age = "sum")