The Javadoc description for the optional class is as follows:
This is a container object that can be null. If the value exists, the Ispresent () method returns True, and the call to the Get () method returns the object.
OfCreates a optional for a
MapReduce is a software framework released by Google in 2004 to support distributed computing for large-scale data, see here for details.
MongoDB is an open source document-oriented NoSQL database system, written in C + +, please see here for
1. Aggregation Frameworkusing the aggregation framework to transform and combine documents in a collection, you can create a pipeline (pipeline) with multiple artifacts to process a series of documents. These artifacts include filtering (filtering),
MapReduce Programming Basics
1. WordCount Sample and MapReduce program framework
2. MapReduce Program Execution Flow
3. Deep Learning MapReduce Programming (1)
4. Reference and code download . WordCount Sample and MapReduce program framework
First
1, first see what is the Iterable object
Take the built-in Max function as an example to view its doc:
Copy Code code as follows:
>>> Print max.__doc__
Max (iterable[, Key=func])-> value
Max (A, B, C, ...) [, Key=func]) -> value
1. IntroductionMongoDB's mapreduce is equivalent to the group by in MySQL, so it's easy to use map/reduce on MongoDB, using MapReduce to implement two function map functions and the reduce function, the map function calls emit (Key,value),
Mongdb's mapreduce is equivalent to "group by" in MySQL, so it's easy to use map/reduce for parallel "stats" on MongoDB.Using MapReduce to implement the two function map functions and the reduce function, the map function calls emit (Key,value),
In addition to the basic query functionality, MONGODB also has powerful aggregation tools, including: count (), distinct (), group (), MapReduce.counting function countCount is the simplest aggregation tool that returns the number of
Test data is inserted into the group of MongoDB: for (vari1; i20; I ++) {varnumi % 6; db. test. insert ({_ id: I, name: user _ + I, age: num});} 1. queries the database by common groups. test. group ({key: {age: true}, initial: {num: 0}, $ reduce:
Use a function as a parameter of another function to run javascript code first:
The Code is as follows:
Var a = [1, 2, 3];Function map (fn, a, init ){Var s = init;For (I = 0; I S = fn (s, a [I]);}Return s;}Alert (map (function (x, y) {return
Source:linkWe'll looking some opreators for combining stream in RxJS:
Merge
Combinelatest
Withlatestfrom
Concat
Forkjoin
Flatmap/switchmap
Merge:Observable.mergeBehaves like a "logical or" to has your stream handle
1. Clustering IssuesThe so-called clustering problem is that given an element set D, where each element has n observable attributes, using an algorithm to divide D into k subsets requires that the elements within each subset be as low-level as
Python map and reduce function usage example, pythonreduce
First look at map. The map () function receives two parameters. One is a function, and the other is a sequence. map sequentially applies the input function to each element of the sequence
The new version of PHP5.3 follows many new features. One of the most eye-catching features is the support for closures. In the future, can we write cool code like those who write Ruby, Javascript, and other "high-tech languages? Er, in most cases,
In all of Python's data structures, list is important and convenient, and this article is mainly about the advanced application of list lists, and the basics can be viewed in blogs.
This article is a translated version of the Python English document,
This is a creation in
Article, where the information may have evolved or changed.
Objective
The recent use of MongoDB in the project for simple data analysis, the use of MongoDB to drive MgO encountered some problems, such as the execution of the
Some time ago, a friend asked me to help him write a product comparison function. at that time, he was just visiting the green forest digital mall and clicked on the product list to see the product comparison function, then he directly downloaded
This article is a detailed introduction of Python in the list of the advanced functions, interested in the small partners can refer to
Use a list as a stack: #像栈一样使用列表
stack = [3, 4, 5] stack.append (6) stack.append (7) Stack [3, 4, 5, 6, 7] Stack.
Introduction
Functional programming (functional programming) concept originated in Lisp, founded by John McCarthy in 1958, the idea of automatic garbage collection was first proposed, and this concept is now being used for reference by
This article mainly introduces the usage of map function traversal array in jquery. It analyzes the skills related to map function traversal array in jQuery and provides a custom traversal Array Function for reference, for more information about how
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