Chapter4 working with key/value pairs key/values pairs键值对是Spark中非常常见的一种数据类型(type),RDD有时经常操作键值对数据类型.第四章的第一部分内容就是介绍有关键值对的ETL操作:extract,transform,load.第二部分内容介绍的是Spark中的一种高级特征,分布在多个节点nodespairs RDD布局layout:partioning,即分区.使用可控制的分区,可以降低节点间的通信成本,可以是spark的计算…

Working with key/value Pairs Motivation Pair RDDs are a useful building block in many programs, as they expose operations that allow u to act on each key in parallel or regroup data across network. Eg: pair RDDs have a reduceByKey() method that can a…

MongoDB is very powerful, but it is still easy to get started with. In this chapter we’ll introduce some of the basic concepts of MongoDB: • A document is the basic unit of data for MongoDB, roughly equivalent to a row in a relational database manage…

2.1 Introduction MapReduce framework sorts input to reducers by key, but values of reducers are arbitrarily ordered. This means that if all mappers generated the following (key-value) pairs for key = K: (K, V1), (K, V2), ..., (K, Vn). Then all these…

先看一段简单的代码: local mytable = { , , aa = "abc", subtable = {}, , } --for循环1 print("for --- index") ,#mytable do print(i) end --for循环2 print("for ---- index-value") for i,v in ipairs(mytable) do print(i,v) end --for循环3 print(&quo…

标准库提供了集中迭代器,包括迭代文件每行的(io.lines),迭代table元素的(pairs),迭代数组元素的(ipairs),迭代字符串中单词的 (string.gmatch)等等.LUA手册中对与pairs,ipairs解释如下: ipairs (t) Returns three values: an iterator function, the table t, and 0, so that the construction for i,v in ipairs(t) do body e…

lua 中pairs 和 ipairs区别 标准库提供了集中迭代器,包括迭代文件每行的(io.lines),迭代table元素的(pairs),迭代数组元素的(ipairs),迭代字符串中单词的 (string.gmatch)等等.LUA手册中对与pairs,ipairs解释如下: ipairs (t) Returns three values: an iterator function, the table t, and 0, so that the construction for i,v…

什么是Pair 关于类Pair的介绍,下面是引自<C++ Standard Library>的一段话: The class pair is provided to treat two values as a single unit. It is used in several places within the C++ standard library. In particular, the container classes map and multimap use pairs to man…

ipairs (t) Returns three values: an iterator function, the table t, and 0, so that the construction for i,v in ipairs(t) do body end will iterate over the pairs (1,t[1]), (2,t[2]), ···, up to the first integer key absent from the table. pairs (t) Ret…

pairs Returns three values: the next function, the table t, and nil, so that the construction for k,v in pairs(t) do body end will iterate over all key–value pairs of table t. See function next for the caveats of modifying the table during its traver…