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• Understanding MongoDB (and NoSQL in general) and how to make the best use of it

  - by Earlz

  Hello, I am beginning to think that my next project I am wanting to do would work better with a NoSQL solution. The project would either involve a ton of 2-column tables or a ton of dynamic queries with dynamically generated columns in a traditional SQL database. So I feel a NoSQL database would be much cleaner. I'm looking at MongoDB and it looks pretty promising. Anyway, I'm attempting to make sense of it all. Also, I will be using MongoMapper in Ruby. Anyway though, I'm confused as to how to layout things in such a freeform database. I've read http://stackoverflow.com/questions/2170152/nosql-best-practices and the answer there says that normalization is usually bad in a NoSQL DB. So how would be the best way of laying out say a simple blog with users, posts, and comments? My natural thought was to have three collections for each and then link them by a unique ID. But this apparently is wrong? So, what are some of the ways to lay out such a thing? My concern with the answer given in the other question is, what if the author's name changed? You'd have to go through updating a ton of posts and comments. But is this an okay thing to do with NoSQL?

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• Need help in understanding the mapping of user-space send, sendto, sendmsg to kernel-space sendmsg

  - by bala1486

  Hello, I am trying to implement my own transport layer protocol in Linux for an experiment. I am going to use socket interface and add my protocol using sock_register. For the proto_ops i can see that the parameters for the sendmsg and recvmsg are (struct kiocb *iocb, struct socket *sock, struct msghdr *msg, size_t len, int flags). But there are three types of user api's send, sendto, sendmsg. Of these three only sendmsg contains a parameter for msghdr. I find that the other two api's are incompatible with the parameters supplied by the kernel to my kernel-space sendmsg function. So what happens when we use send and sendto user-space api's? Hope i am clear.. Thanks, Bala

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• Understanding this matrix transposition function in Haskell

  - by dmindreader

  This matrix transposition function works, but I'm trying to understand its step by step execurtion and I don't get it. transpose:: [[a]]->[[a]] transpose ([]:_) = [] transpose x = (map head x) : transpose (map tail x) with transpose [[1,2,3],[4,5,6],[7,8,9]] it returns: [[1,4,7],[2,5,8],[3,6,9]] I don't get how the concatenation operator is working with map. It is concatenating each head of x in the same function call? How?

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• Understanding Haskell's filter

  - by dmindreader

  I understand that Haskell's filter is a high order function (meaning a function that takes another function as a parameter) that goes through a list checking which element fulfills certain boolean condition. I don't quite understand its definition: filter:: (a->Bool)->[a]->[a] filter p [] = [] filter p (x:y) | p x = x:filter p y | otherwise = filter p y I understand that if I pass an empty list to the function, it would just return an empty list, but how do I read the last two lines?

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• Understanding Device Contexts

  - by Smashery

  As a relative newcomer to MFC, I see Device Contexts (DCs) a lot. I vaguely understand that it's something to do with drawing, but the specifics are not very well explained anywhere that I can find. What does creating a "compatible Device Context" mean, and why is it important? What does SelectObject do, and how must I make a DC compatible first?

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• Understanding Virtualization

  - by mohit

  Hi, I wana to know about Virtualization in detail. But start from basics, like what is virtualization, with real world examples and scenarios. When ever i search, i got Virtualization technologies, but i want to learn and in fcat know what exactly is virtualization, its types n all..... Please help me in getting practical details of term "Virtualization"

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• Understanding many to many relationships and Entity Framework

  - by Anders Svensson

  I'm trying to understand the Entity Framework, and I have a table "Users" and a table "Pages". These are related in a many-to-many relationship with a junction table "UserPages". First of all I'd like to know if I'm designing this relationship correctly using many-to-many: One user can visit multiple pages, and each page can be visited by multiple users..., so am I right in using many2many? Secondly, and more importantly, as I have understood m2m relationships, the User and Page tables should not repeat information. I.e. there should be only one record for each user and each page. But then in the entity framework, how am I able to add new visits to the same page for the same user? That is, I was thinking I could simply use the Count() method on the IEnumerable returned by a LINQ query to get the number of times a user has visited a certain page. But I see no way of doing that. In Linq to Sql I could access the junction table and add records there to reflect added visits to a certain page by a certain user, as many times as necessary. But in the EF I can't access the junction table. I can only go from User to a Pages collection and vice versa. I'm sure I'm misunderstanding relationships or something, but I just can't figure out how to model this. I could always have a Count column in the Page table, but as far as I have understood you're not supposed to design database tables like that, those values should be collected by queries... Please help me understand what I'm doing wrong...

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• some pointer to understanding GCC source code

  - by user299570

  hi, I'm student working on optimizing GCC for multi-core processor. I tried going through the source code, it is difficult to follow through it since I need to add some code to the back end. Can anyone suggest some good resource which explains the code flow through the different phases. Also suggest some development environment for debugging GCC mainly to step through the code. Is it possible on windows?

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• Problem understanding treesort in Haskell

  - by Jerry

  I am trying to figure out how exactly does treesort from here work (I understand flatten, insert and foldr). I suppose what's being done in treesort is applying insert for each element on the list thus generating a tree and then flattening it. The only problem I can't overcome here is where the list (that is the argument of the function) is hiding (because it is not written anywhere as an argument except for the function type declaration). One more thing: since dot operator is function composition, why is it an error when I change: treesort = flatten . foldr insert Leaf to treesort = flatten( foldr insert Leaf )?

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• Understanding run time code interpretation and execution

  - by Bob

  I'm creating a game in XNA and was thinking of creating my own scripting language (extremely simple mind you). I know there's better ways to go about this (and that I'm reinventing the wheel), but I want the learning experience more than to be productive and fast. When confronted with code at run time, from what I understand, the usual approach is to parse into a machine code or byte code or something else that is actually executable and then execute that, right? But, for instance, when Chrome first came out they said their JavaScript engine was fast because it compiles the JavaScript into machine code. This implies other engines weren't compiling into machine code. I'd prefer not compiling to a lower language, so are there any known modern techniques for parsing and executing code without compiling to low level? Perhaps something like parsing the code into some sort of tree, branching through the tree, and comparing each symbol and calling some function that handles that symbol? (Wild guessing and stabbing in the dark)

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• Understanding Ruby Enumerable#map (with more complex blocks)

  - by mstksg

  Let's say I have a function def odd_or_even n if n%2 == 0 return :even else return :odd end end And I had a simple enumerable array simple = [1,2,3,4,5] And I ran it through map, with my function, using a do-end block: simple.map do |n| odd_or_even(n) end # => [:odd,:even,:odd,:even,:odd] How could I do this without, say, defining the function in the first place? For example, # does not work simple.map do |n| if n%2 == 0 return :even else return :odd end end # Desired result: # => [:odd,:even,:odd,:even,:odd] is not valid ruby, and the compiler gets mad at me for even thinking about it. But how would I implement an equivalent sort of thing, that works?

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• Understanding the concept of inheritance in Java

  - by Nirmal

  Hello All.... I am just refreshing the oops features of the java. So, I have a little confusion regarding inheritance concept. For that I have a following sample code : class Super{ int index = 5; public void printVal(){ System.out.println("Super"); } } class Sub extends Super{ int index = 2; public void printVal(){ System.out.println("Sub"); } } public class Runner { public static void main(String args[]){ Super sup = new Sub(); System.out.println(sup.index+","); sup.printVal(); } } Now above code is giving me output as : 5,Sub. Here, we are overriding printVal() method, so that is understandable that it is accessing child class method only. But I could not understand why it's accessing the value of x from Super class... Thanks in advance....

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• understanding the anatomy of file formats and image formats

  - by numerical25

  I am trying to do some research on file formats especially image formats. Information such as header layouts for particular types of image formats and how to parse them using C++. If anyone can point me in the right direction of some good tutorials or books. that would be helpful. thanks

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• Need some help understanding this problem

  - by Legend

  I was wondering if someone could help me understand this problem. I prepared a small diagram because it is much easier to explain it visually. Problem I am trying to solve: 1. Constructing the dependency graph Given the connectivity of the graph and a metric that determines how well a node depends on the other, order the dependencies. For instance, I could put in a few rules saying that node 3 depends on node 4 node 2 depends on node 3 node 3 depends on node 5 But because the final rule is not "valuable" (again based on the same metric), I will not add the rule to my system. 2. Execute the request order Once I built a dependency graph, execute the list in an order that maximizes the final connectivity. First and foremost, I am wondering if I constructed the problem correctly and if I should be aware of any corner cases. Secondly, is there a closely related algorithm that I can look at? Currently, I am thinking of something like Feedback Arc Set or the Secretary Problem but I am a little confused at the moment. Any suggestions? PS: I am a little confused about the problem myself so please don't flame on me for that. If any clarifications are needed, I will try to update the question.

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• Java Matcher groups: Understanding The difference between "(?:X|Y)" and "(?:X)|(?:Y)"

  - by user358795

  Can anyone explain: Why the two patterns used below give different results? (answered below) Why the 2nd example gives a group count of 1 but says the start and end of group 1 is -1? public void testGroups() throws Exception { String TEST_STRING = "After Yes is group 1 End"; { Pattern p; Matcher m; String pattern="(?:Yes|No)(.*)End"; p=Pattern.compile(pattern); m=p.matcher(TEST_STRING); boolean f=m.find(); int count=m.groupCount(); int start=m.start(1); int end=m.end(1); System.out.println("Pattern=" + pattern + "\t Found=" + f + " Group count=" + count + " Start of group 1=" + start + " End of group 1=" + end ); } { Pattern p; Matcher m; String pattern="(?:Yes)|(?:No)(.*)End"; p=Pattern.compile(pattern); m=p.matcher(TEST_STRING); boolean f=m.find(); int count=m.groupCount(); int start=m.start(1); int end=m.end(1); System.out.println("Pattern=" + pattern + "\t Found=" + f + " Group count=" + count + " Start of group 1=" + start + " End of group 1=" + end ); } } Which gives the following output: Pattern=(?:Yes|No)(.*)End Found=true Group count=1 Start of group 1=9 End of group 1=21 Pattern=(?:Yes)|(?:No)(.*)End Found=true Group count=1 Start of group 1=-1 End of group 1=-1

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• SQL SERVER - Understanding how MIN(text) works.

  - by tmercer

  I'm doing a little digging and looking for a explanation on how SQL server evaluates MIN(Varchar). I found this remark in BOL: MIN finds the lowest value in the collating sequence defined in the underlying database So if I have a table that has one row with the following values: Data AA AB AC Doing a SELECT MIN(DATA) would return back AA. I just want to understand the why behind this and understand the BOL a little better. Thanks!

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• help understanding the concept of javascript callbacks with node.js, especially in loops

  - by Mr JSON

  hi I am just starting with node.js. I have done a little ajax stuff but nothing too complicated so callbacks are still kind of over my head. I looked at async but all I need is to run a few functions sequentially. I basically have something that pulls some json from an api, creates a new one and then does something with that. obviously i can't just run it because it runs everything at once and has an empty json. mostly they have to go sequentially but if while pulling a json from the api it can pull other json while it's waiting that is fine. I just got confused when putting the callback in a loop. what do I do with the index? i think i have seen some places that use callback inside the loop as kind of a recusive function and don't use for loops at all. simple examples would help alot thanks!

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• Need help understanding the MVC design pattern

  - by Doron Sinai

  Hi, I am trying to find a ood example of MVC design pattern in java. This is what i understood from reading about it, please correct me if I am wrong: I have the Model part which is the logic behind the program, let's say if we have a phonebook, so adding and removing contact from the Array will be the model. The Gui is the view and it contains buttons that upon clicking them, the model is changing. What I am trying to undersand what is the controller part, is it the ActionListeners? how to you seperate those modules in practice. thank you

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• Perl code -need some understanding.

  - by benjamin button

  Hi i have a perl code below: foreach (@tmp_cycledef) { chomp; my ($cycle_code, $close_day, $first_date) = split(/\|/, $_,3); $cycle_code =~ s/^\s*(\S*(?:\s+\S+)*)\s*$/$1/; $close_day =~ s/^\s*(\S*(?:\s+\S+)*)\s*$/$1/; $first_date =~ s/^\s*(\S*(?:\s+\S+)*)\s*$/$1/; #print "$cycle_code, $close_day, $first_date\n"; $cycledef{$cycle_code} = [ $close_day, split(/-/,$first_date) ]; } the value of tmp_cycledef comes from output of an sql query: select cycle_code,cycle_close_day,to_char(cycle_first_date,'YYYY-MM-DD') from cycle_definition d order by cycle_code; can anybody tell me what exactly is happening inside the for loop? thanks in advance.

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• Trying to verify understanding of Foreign Keys MSSQL

  - by msarchet

  So I'm working on just a learning project to expose myself to doing some things I do not get to do at work. I'm just making a simple bug and case tracking app (I know there are a million this is just to work with some tools I don't get to). So I was designing my database and realized I've never actually used Foreign Keys before in any of my projects, I've used them before but never actually setting up a column as a FK. So I've designed my database as follows, which I think is close to correct (at least for the initial layout). However When I try to add the FK's to the linking Tables I get an error saying, "The tables present in the relationship must have the same number of columns". I'm doing this by in SQLSMS by going to the Keys 'folder' and adding a FK. Is there something that I am doing wrong here, I don't understand why the tables would have to have the same number of columns for me to add a FK relationship between the tables?

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• Understanding Wordpress database schema - querying from 3rd party app

  - by deceze

  Is there an easy way to grab the latest posts out of a Wordpress wp_posts table using a simple SQL query? I have a Wordpress 2.9.2 installation as part of, but separate from, a larger system. It has a customized theme to look like the rest of the site but has otherwise nothing to do with it. I want to display the latest handful of headlines of posts made using Wordpress on a site of that other system. Preferably I do not want to mess around with importing any of the Wordpress library files. Looking at the database structure I can't see an easy, straight-forward query to simply get the latest revision of the latest posts. The post_status can either be "post" or "inherit", the post_type "post" or "revision" and the parent "0" or the id of the original post of a revision. I can't figure out how to reliably filter different revisions of the same post, drafts, attachments and pages out of this mess and just get the latest revision of the latest posts. I'm aware that the database schema is subject to change in subsequent versions of Wordpress, so shouldn't be relied upon, but that's a minor concern, since it's such a minor feature that could easily be fixed. If I understood how that database is supposed to work, that is.

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• Understanding the workflow of the messages in a generic server implementation in Erlang

  - by Chiron

  The following code is from "Programming Erlang, 2nd Edition". It is an example of how to implement a generic server in Erlang. -module(server1). -export([start/2, rpc/2]). start(Name, Mod) -> register(Name, spawn(fun() -> loop(Name, Mod, Mod:init()) end)). rpc(Name, Request) -> Name ! {self(), Request}, receive {Name, Response} -> Response end. loop(Name, Mod, State) -> receive {From, Request} -> {Response, State1} = Mod:handle(Request, State), From ! {Name, Response}, loop(Name, Mod, State1) end. -module(name_server). -export([init/0, add/2, find/1, handle/2]). -import(server1, [rpc/2]). %% client routines add(Name, Place) -> rpc(name_server, {add, Name, Place}). find(Name) -> rpc(name_server, {find, Name}). %% callback routines init() -> dict:new(). handle({add, Name, Place}, Dict) -> {ok, dict:store(Name, Place, Dict)}; handle({find, Name}, Dict) -> {dict:find(Name, Dict), Dict}. server1:start(name_server, name_server). name_server:add(joe, "at home"). name_server:find(joe). I tried so hard to understand the workflow of the messages. Would you please help me to understand the workflow of this server implementation during the executing of the functions: server1:start, name_server:add and name_server:find?

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• Understanding max JVM heap size

  - by Marcus

  I've read the max heap size on 32bit Windows is ~1.5GB which is due to the fact that the JVM requires contiguous memory. Can someone explain the concept of "contiguous memory" and why you only have max 1.5GB on Windows? Secondly, what then is the max heap size on 64 bit Windows and why is this different than what's available on 32 bit?

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• Need help understanding this MATLAB function implementation

  - by Runner

  http://www.cc.gatech.edu/classes/AY2000/cs7495_fall/participants/sashag/ps3/getchaincode.m offset = N/2; %offset between ajacent pixels I don't understand what the above mean?

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• understanding list[i-1] vs list[i]-1

  - by user3720527

  Hopefully this is a simple answer that I am just failing to understand. Full code is public static void mystery(int[] list) { for( int i = list.length - 1; i>1; i --) { if (list[i] > list[i - 1]) { list[i -1] = list[i] - 2; list[i]++; } } } } and lets say we are using a list of [2,3,4]. I know that it will output 2,2,5 but I am unclear how to actually work through it. I understand that the list.length is 3 here, and I understand that the for loop will only run once, but I am very unclear what happens at the list[i - 1] = list[i] - 2; area. Should it be list[2-1] = list[2] - 2? How does the two being outside the bracket effect it differently? Much thanks.

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