Github user sun-rui commented on a diff in the pull request:
https://github.com/apache/spark/pull/10201#discussion_r47042545
--- Diff: R/pkg/R/DataFrame.R ---
@@ -1324,12 +1312,16 @@ setMethod("selectExpr",
#' path <- "path/to/file.json"
#' df <- jsonFile(sqlContext, path)
#' newDF <- withColumn(df, "newCol", df$col1 * 5)
+#' # Replace an existing column
+#' newDF2 <- withColumn(newDF, "newCol", newDF$col1)
--- End diff --
I don't know the reason. The original commit can be found at
https://github.com/amplab-extras/SparkR-pkg/pull/204.
I don't think it is related to supporting multiple columns with the same
name. Spark Core itself allows multiple columns with the same name:
```
scala> val df=sqlContext.createDataFrame(Seq((1,2,3))).toDF("a","a","c")
df: org.apache.spark.sql.DataFrame = [a: int, a: int, c: int]
scala> df.show
+---+---+---+
| a| a| c|
+---+---+---+
| 1| 2| 3|
+---+---+---+
scala> df.withColumn("a", df("c")).show
+---+---+---+
| a| a| c|
+---+---+---+
| 3| 3| 3|
+---+---+---+
```
You can see all columns of the same name will be replaced in the above
example.
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