scala

Initializing Generic Variables in Scala

由 ぃ、小莉子 提交于 2021-02-07 05:28:29
问题 How do I declare a generic variable in Scala without initializing it (or initializing to any value)? def foo[T] { var t: T = ???? // tried _, null t } 回答1: def foo[T] { var t: T = null.asInstanceOf[T] t } And, if you don't like the ceremony involved in that, you can ease it this way: // Import this into your scope case class Init() implicit def initToT[T](i: Init): T = { null.asInstanceOf[T] } // Then use it def foo[T] { var t: T = Init() t } 回答2: You can't not initialize local variables, but

Scala Slick Cake Pattern: over 9000 classes?

由 守給你的承諾、 提交于 2021-02-07 05:27:09
问题 I'm developing a Play! 2.2 application in Scala with Slick 2.0 and I'm now tackling the data access aspect, trying to use the Cake Pattern. It seems promising but I really feel like I need to write a huge bunch of classes/traits/objects just to achieve something really simple. So I could use some light on this. Taking a very simple example with a User concept, the way I understand it is we should have: case class User(...) //model class Users extends Table[User]... //Slick Table object users

Scala copy case class with generic type

由 假如想象 提交于 2021-02-07 05:27:05
问题 I have two classes PixelObject , ImageRefObject and some more, but here are just these two classes to simplify things. They all are subclasses of a trait Object that contains an uid. I need universal method which will copy a case class instance with a given new uid . The reason I need it because my task is to create a class ObjectRepository which will save instance of any subclass of Object and return it with new uid . My attempt: trait Object { val uid: Option[String] } trait UidBuilder[A <:

Why is Option not Traversable?

由 天大地大妈咪最大 提交于 2021-02-07 05:11:28
问题 Is there any rational for Option not being Traversable ? In Scala 2.9, Seq(Set(1,3,2),Seq(4),Option(5)).flatten doesn't compile and simply having it to implement the Traversable trait seams rational to me. If it's not the case, there must be something I don't see that don't allow it. What is it? PS: While trying to understand, I achieved awful things that compile, like: scala> Seq(Set(1,3,2),Seq(4),Map("one"->1, 2->"two")).flatten res1: Seq[Any] = List(1, 3, 2, 4, (one,1), (2,two)) PS2: I

Why is Option not Traversable?

由 ￣綄美尐妖づ 提交于 2021-02-07 05:04:34
问题 Is there any rational for Option not being Traversable ? In Scala 2.9, Seq(Set(1,3,2),Seq(4),Option(5)).flatten doesn't compile and simply having it to implement the Traversable trait seams rational to me. If it's not the case, there must be something I don't see that don't allow it. What is it? PS: While trying to understand, I achieved awful things that compile, like: scala> Seq(Set(1,3,2),Seq(4),Map("one"->1, 2->"two")).flatten res1: Seq[Any] = List(1, 3, 2, 4, (one,1), (2,two)) PS2: I

AWS S3 : Spark - java.lang.IllegalArgumentException: URI is not absolute… while saving dataframe to s3 location as json

由 非 Y 不嫁゛ 提交于 2021-02-07 04:28:21
问题 I am getting strange error while saving dataframe to AWS S3. df.coalesce(1).write.mode(SaveMode.Overwrite) .json(s"s3://myawsacc/results/") In the same location I was able to insert the data from spark-shell . and is working... spark.sparkContext.parallelize(1 to 4).toDF.write.mode(SaveMode.Overwrite) .format("com.databricks.spark.csv") .save(s"s3://myawsacc/results/") My question is why its working in spark-shell and is not working via spark-submit ? Is there any logic/properties

Replace groupByKey with reduceByKey in Spark

由 大憨熊 提交于 2021-02-07 04:28:19
问题 Hello I often need to use groupByKey in my code but I know it's a very heavy operation. Since I'm working to improve performance I was wondering if my approach to remove all groupByKey calls is efficient. I was used to create an RDD from another RDD and creating pair of type (Int, Int) rdd1 = [(1, 2), (1, 3), (2 , 3), (2, 4), (3, 5)] and since I needed to obtain something like this: [(1, [2, 3]), (2 , [3, 4]), (3, [5])] what I used was out = rdd1.groupByKey but since this approach might be

Why prefer implicit val over implicit object

由 时光毁灭记忆、已成空白 提交于 2021-02-07 04:14:51
问题 When asking questions about implicits a common suggestion / recommendation / advice that is given together with the answer (or sometimes that is the answer itself) is to use implicit vals with excplicit type signatures instead of using implicit objects . But, what is the reason behind that? 回答1: "TL;DR;" The reason is that an implici val with an explicit type signature has the exact type you want whereas an implicit object has a different type. The best way to show why that could be a problem

Why prefer implicit val over implicit object

由 回眸只為那壹抹淺笑 提交于 2021-02-07 04:12:30
问题 When asking questions about implicits a common suggestion / recommendation / advice that is given together with the answer (or sometimes that is the answer itself) is to use implicit vals with excplicit type signatures instead of using implicit objects . But, what is the reason behind that? 回答1: "TL;DR;" The reason is that an implici val with an explicit type signature has the exact type you want whereas an implicit object has a different type. The best way to show why that could be a problem

Spark学习之路 （十五）SparkCore的源码解读（一）启动脚本

由 做~自己de王妃 提交于 2021-02-07 04:03:28
一、启动脚本分析 独立部署模式下，主要由master和slaves组成，master可以利用zk实现高可用性，其driver，work，app等信息可以持久化到zk上；slaves由一台至多台主机构成。Driver通过向Master申请资源获取运行环境。 启动master和slaves主要是执行/usr/dahua/spark/sbin目录下的start-master.sh和start-slaves.sh，或者执行 start-all.sh，其中star-all.sh本质上就是调用start-master.sh和start-slaves.sh 1.1　start-all.sh #1.判断SPARK_HOME是否有值，没有将其设置为当前文件所在目录的上级目录 if [ -z "${SPARK_HOME}" ]; then export SPARK_HOME ="$(cd "`dirname "$0"`"/..; pwd)" fi # 2.执行${SPARK_HOME}/sbin/spark- config.sh，见以下分析 . "${SPARK_HOME}/sbin/spark-config.sh" #3.执行"${SPARK_HOME}/sbin"/start-master.sh，见以下分析 "${SPARK_HOME}/sbin"/start- master.sh # 4.执行"
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