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• Are .NET's regular expressions Turing complete?

  - by Robert

  Regular expressions are often pointed to as the classical example of a language that is not Turning complete. For example "regular expressions" is given in as the answer to this SO question looking for languages that are not Turing complete. In my, perhaps somewhat basic, understanding of the notion of Turning completeness, this means that regular expressions cannot be used check for patterns that are "balanced". Balanced meaning have an equal number of opening characters as closing characters. This is because to do this would require you to have some kind of state, to allow you to match the opening and closing characters. However the .NET implementation of regular expressions introduces the notion of a balanced group. This construct is designed to let you backtrack and see if a previous group was matched. This means that a .NET regular expressions: ^(?<p>a)*(?<-p>b)*(?(p)(?!))$ Could match a pattern that: ab aabb aaabbb aaaabbbb ... etc. ... Does this means .NET's regular expressions are Turing complete? Or are there other things that are missing that would be required for the language to be Turing complete?

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• Should I use a regular server instead of AWS?

  - by Jon Ramvi

  Reading about and using the Amazon Web Services, I'm not really able to grasp how to use it correctly. Sorry about the long question: I have a EC2 instance which mostly does the work of a web server (apache for file sharing and Tomcat with Play Framework for the web app). As it's a web server, the instance is running 24/7. It just came to my attention that the data on the EC2 instance is non persistent. This means I lose my database and files if it's stopped. But I guess it also means my server settings and installed applications are lost as they are just files in the same way as the other data. This means that I will either have to rewrite the whole app to use amazon CloudDB or write some code which stores the db on S3 and make my own AMI with the correct applications installed and configured. Or can this be quick-fixed by using EBS somehow? My question is 1. is my understanding of aws is correct? and 2. is it's worth it? It could be a possibility to just set up a regular dedicated server where everything is persistent, as you would expect. Would love to have the scaleability of aws though..

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• A typical mysql query( how to use subquery column into main query)

  - by I Like PHP

  I HAVE TWO TABLES shown below table_joining id join_id(PK) transfer_id(FK) unit_id transfer_date joining_date 1 j_1 t_1 u_1 2010-06-05 2010-03-05 2 j_2 t_2 u_3 2010-05-10 2010-03-10 3 j_3 t_3 u_6 2010-04-10 2010-01-01 4 j_5 NULL u_3 NULL 2010-06-05 5 j_6 NULL u_4 NULL 2010-05-05 table_transfer id transfer_id(PK) pastUnitId futureUnitId effective_transfer_date 1 t_1 u_3 u_1 2010-06-05 2 t_2 u_6 u_1 2010-05-10 3 t_3 u_5 u_3 2010-04-10 now i want to know total employee detalis( using join_id) which are currently working on unit u_3 . means i want only join_id j_1 (has transfered but effective_transfer_date is future date, right now in u_3) j_2 ( tansfered and right now in `u_3` bcoz effective_transfer_date has been passed) j_6 ( right now in `u_3` and never transfered) what i need to take care of below steps( as far as i know ) <1> first need to check from table_joining whether transfer_id is NULL or not <2> if transfer_id= is NULL then see unit_id=u_3 where joining_date <=CURDATE() ( means that person already joined u_3) <3> if transfer_id is NOT NULL then go to table_transfer using transfer_id (foreign key reference) <4> now see the effective_transfer_date regrading that transfer_id whether effective_transfer_date<=CURDATE() <5> if transfer date has been passed(means transfer has been done) then return futureUnitID otherwise return pastUnitID i used two separate query but don't know how to join those query?? for step <1 ans <2 SELECT unit_id FROM table_joining WHERE joining_date<=CURDATE() AND transfer_id IS NULL AND unit_id='u_3' for step<5 SELECT IF(effective_transfer_date <= CURDATE(),futureUnitId,pastUnitId) AS currentUnitID FROM table_transfer // here how do we select only those rows which have currentUnitID='u_3' ?? please guide me the process?? i m just confused with JOINS. i think using LEFT JOIN can return the data i need, or if we use subquery value to main query? but i m not getting how to implement ...please help me. Thanks for helping me alwayz

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• Treating differential operator as algebraic entity

  - by chappar

  I know that this question is offtopic and don't belong here. But i didn't know somewhere else to ask. So here is the question. I was reading e:the story of a number by Eli Maor, where he treats differential operator as just like any algebraic entity. For example if we have a differential equation like y’’ + 5y’ - 6y = 0. This can be treaed as (D^2 + 5D – 6)y = 0. So, either y = 0 (trivial solution) or (D^2 + 5D – 6) = 0. Factoring out above equation we get (D-1)(D+6)= 0 with solutions as D = 1 and D = -6. Since D does not have any meaning on its own, multiplying by y on both the sides we get Dy = y and Dy = -6y for which the solutions are Ae^x and Be^-6x. Combining these 2 solutions we get Ae^x + Be^-6x. Now my doubt is this approach break when we have an equation like D^2y = 0. Which means y = 0 (again trivial) or D^2 = 0 which means D = 0. Now Dy = y*0 = 0. That means y = C ( a constant). The actual answer should be Cx. I know that it is stupidity to treat D^2 = 0 as D = 0, it led me to doubt the entire process of treating differential equation as algebraic equation. Can someone throw light on this? Or any other site where i might get answer?

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• #include - brackets vs quotes in XCode?

  - by Chris Becke

  In MSVC++ #include files are searched for differently depending on whether the file is enclosed in "" or <. The quoted form searches first in the local folder, then in /I specified locations, The angle bracket form avoids the local folder. This means, in MSVC++, its possible to have header files with the same name as runtime and SDK headers. So, for example, I need to wrap up the windows sdk windows.h file to undefine some macro's that cause trouble. With MSVS I can just add a (optional) windows.h file to my project as long as I include it using the quoted form :- // some .cpp file #include "windows.h" // will include my local windows.h file And in my windows.h, I can pull in the real one using the angle bracket form: // my windows.h #include <windows.h> // will load the real one #undef ConflictingSymbol Trying this trick with GCC in XCode didn't work. angle bracket #includes in system header files in fact are finding my header files with similar names in my local folder structure. The MSVC system means its quite safe to have a "String.h" header file in my own folder structre. On XCode this seems to be a major no no. Is there some way to control this search path behaviour in XCode to be more like MSVC's? Or do I just have to avoid naming any of my headers anything that might possibly conflict with a system header. Writing cross platform code and using lots of frameworks means the possibility of incidental conflicts seems large.

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• pushd - handling multiple drives from cmd

  - by user673600

  I'm trying to figure out how to install some programs where the components reside on two different drives on a networked path. However whenever I use pushd \\xyz\c$ I get a mapped drive which means I cannot use any knowledge of using for example c:\install e:\mycomponents.dll. Is there anyway that I can do this once I have used the pushd command? How can I ensure that I keep the drives the same for example. I'm in the process of installing services. So it seems that when I install the service, I need to keep the path as the same as the actual location of the .exe which means that I'm running into issues. Is there a way to simply use pushd but at the sametime not actually map drives? As when installing services, when I've been using net use, I've found that there is an issue with installing on drives which are mapped, as the service whilst can be installed doesn't find the actual .exe when it comes to starting up the service. So to expand this, is there a way to solve this using net use or pushd or a combination that lets me install a service as such: c:\windows\..\installutil e:\mynode? So to clarify, I need to somehow be able to see both drives on the remote machine by their relative drives i.e. E:\ and C:\ - if I use a mapped drive letter then it means installing service is a pain because I cannot use the path.

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• Create a model that switches between two different states using Temporal Logic?

  - by NLed

  Im trying to design a model that can manage different requests for different water sources. Platform : MAC OSX, using latest Python with TuLip module installed. For example, Definitions : Two water sources : w1 and w2 3 different requests : r1,r2,and r3 - Specifications : Water 1 (w1) is preferred, but w2 will be used if w1 unavailable. Water 2 is only used if w1 is depleted. r1 has the maximum priority. If all entities request simultaneously, r1's supply must not fall below 50%. - The water sources are not discrete but rather continuous, this will increase the difficulty of creating the model. I can do a crude discretization for the water levels but I prefer finding a model for the continuous state first. So how do I start doing that ? Some of my thoughts : Create a matrix W where w1,w2 ? W Create a matrix R where r1,r2,r3 ? R or leave all variables singular without putting them in a matrix I'm not an expert in coding so that's why I need help. Not sure what is the best way to start tackling this problem. I am only interested in the model, or a code sample of how can this be put together. edit Now imagine I do a crude discretization of the water sources to have w1=[0...4] and w2=[0...4] for 0, 25, 50, 75,100 percent respectively. == means implies Usage of water sources : if w1[0]==w2[4] -- meaning if water source 1 has 0%, then use 100% of water source 2 etc if w1[1]==w2[3] if w1[2]==w2[2] if w1[3]==w2[1] if w1[4]==w2[0] r1=r2=r3=[0,1] -- 0 means request OFF and 1 means request ON Now what model can be designed that will give each request 100% water depending on the values of w1 and w2 (w1 and w2 values are uncontrollable so cannot define specific value, but 0...4 is used for simplicity )

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• please suggest mysql query for this

  - by I Like PHP

  I HAVE TWO TABLES shown below table_joining id join_id(PK) transfer_id(FK) unit_id transfer_date joining_date 1 j_1 t_1 u_1 2010-06-05 2010-03-05 2 j_2 t_2 u_3 2010-05-10 2010-03-10 3 j_3 t_3 u_6 2010-04-10 2010-01-01 4 j_5 NULL u_3 NULL 2010-06-05 5 j_6 NULL u_4 NULL 2010-05-05 table_transfer id transfer_id(PK) pastUnitId futureUnitId effective_transfer_date 1 t_1 u_3 u_1 2010-06-05 2 t_2 u_6 u_1 2010-05-10 3 t_3 u_5 u_3 2010-04-10 now i want to know total employee detalis( using join_id) which are currently working on unit u_3 . means i want only join_id j_1 (has transfered but effective_transfer_date is future date, right now in u_3) j_2 ( tansfered and right now in `u_3` bcoz effective_transfer_date has been passed) j_6 ( right now in `u_3` and never transfered) what i need to take care of below steps( as far as i know ) <1> first need to check from table_joining whether transfer_id is NULL or not <2> if transfer_id= is NULL then see unit_id=u_3 where joining_date <=CURDATE() ( means that person already joined u_3) <3> if transfer_id is NOT NULL then go to table_transfer using transfer_id (foreign key reference) <4> now see the effective_transfer_date regrading that transfer_id whether effective_transfer_date<=CURDATE() <5> if transfer date has been passed(means transfer has been done) then return futureUnitID otherwise return pastUnitID i used two separate query but don't know how to join those query?? for step <1 ans <2 SELECT unit_id FROM table_joining WHERE joining_date<=CURDATE() AND transfer_id IS NULL AND unit_id='u_3' for step<5 SELECT IF(effective_transfer_date <= CURDATE(),futureUnitId,pastUnitId) AS currentUnitID FROM table_transfer // here how do we select only those rows which have currentUnitID='u_3' ?? please guide me the process?? i m just confused with JOINS. i think using LEFT JOIN can return the data i need, or if we use subquery value to main query? but i m not getting how to implement ...please help me. Thanks for helping me alwayz

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• Separating code logic from the actual data structures. Best practices?

  - by Patrick

  I have an application that loads lots of data into memory (this is because it needs to perform some mathematical simulation on big data sets). This data comes from several database tables, that all refer to each other. The consistency rules on the data are rather complex, and looking up all the relevant data requires quite some hashes and other additional data structures on the data. Problem is that this data may also be changed interactively by the user in a dialog. When the user presses the OK button, I want to perform all the checks to see that he didn't introduce inconsistencies in the data. In practice all the data needs to be checked at once, so I cannot update my data set incrementally and perform the checks one by one. However, all the checking code work on the actual data set loaded in memory, and use the hashing and other data structures. This means I have to do the following: Take the user's changes from the dialog Apply them to the big data set Perform the checks on the big data set Undo all the changes if the checks fail I don't like this solution since other threads are also continuously using the data set, and I don't want to halt them while performing the checks. Also, the undo means that the old situation needs to be put aside, which is also not possible. An alternative is to separate the checking code from the data set (and let it work on explicitly given data, e.g. coming from the dialog) but this means that the checking code cannot use hashing and other additional data structures, because they only work on the big data set, making the checks much slower. What is a good practice to check user's changes on complex data before applying them to the 'application's' data set?

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• What does 'Highest active time' for disk activity in Windows resource monitor mean?

  - by Nick R

  I know what the disk io, disk queue length and other measures are, but what does 'Highest active time' mean? Is it the amount of time it is busy handling requests, or something else? When it is high, does it mean the CPU is busy doing some IO work, or is it just indicating that the disk is busy handling requests? I'm trying to work out if 50% active time means that 50% of the time the disk is either seeking, reading or writing, rather than the kernel is spending 50% of it's time servicing IO requests. Edit Another quick data point here. If you look at the difference between an SSD and a physical disk, the SSD has significantly less activity, so I guess this really means the amount of time the operating system is waiting for the disk to respond and returning data.

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• Does Hotspot Shield hide my activity from my ISP?

  - by test

  Can Hotspot Shield make your activities invisible to your ISP? Or can they still see what you're downloading if they so choose? Here's the text from the product description: Hotspot Shield protects your entire web surfing session; securing your connection at both your home Internet network & Public Internet networks (both wired and wireless). Hotspot Shield protects your identity by ensuring that all web transactions (shopping, filling out forms, downloads) are secured through HTTPS. Hotspot Shield also makes you private online making your identity invisible to third party websites and ISP’s. I'm just not sure what it means by "invisible to third-party websites and ISPs" and if that means the ISP can still see what I'm doing.

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• What does %st mean in top?

  - by Ben

  Here is an example from my top: Cpu(s): 6.0%us, 3.0%sy, 0.0%ni, 78.7%id, 0.0%wa, 0.0%hi, 0.3%si, 12.0%st I am trying to figure out the significance of the %st field. I read that it means steal cpu and it represents time spent by the hypervisor, but I want to know what that actually means to me. Does it mean I may be on a busy physical server and someone else is using too much CPU on the server and they are taking from my VM? If I am using EBS could it be related to handling EBS I/O at the hypervisor level? Is it related to things running on my VM or is it completely unaffected by me?

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• AWS RDS connection count

  - by wmarbut

  I am using AWS RDS with MySQL for a project and have a "large" instance. The documentation is clear on what this means as far as compute resources and RAM goes, but I can't find anything that documents how many open database connections that I can have. The app that I am using is PHP and it utilizes PDO with persistent connections. This means that the number of open connections could reach the maximum number of PHP child processes running at any given point. How do I ensure that my RDS instance has a max connections setting high enough to be comfortable with this?

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• What is meant by "streaming data access" in HDFS?

  - by Van Gale

  According to the HDFS Architecture page HDFS was designed for "streaming data access". I'm not sure what that means exactly, but would guess it means an operation like seek is either disabled or has sub-optimal performance. Would this be correct? I'm interested in using HDFS for storing audio/video files that need to be streamed to browser clients. Most of the streams will be start to finish, but some could have a high number of seeks. Maybe there is another file system that could do this better?

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• Clickonce applications being allowed through firewall

  - by DanielF

  The company I work for uses an application that is deployed through clickonce. This means that everytime they update said program the program regenerates in a new location ie: C:\Users\%username%\AppData\Local\Apps\2.0\89XCT43D.10T\WBZ8WE7L.927\trm8..tion_1ad39bb503bcb5df_0008.0005_14829906c9aa8611\%appname%.exe Recently the computers were updated to windows 7 machines and now everytime an update is pushed out the windows firewall see this as a new application and prompts the users to allow the program through the firewall. THis means I have to walk around to every computer and type in admin credentials. Does anyone know of a way to allow the application regardless of the path it is in? Google has not been very helpful so far.

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• OpenVPN performance: how many concurrent clients are possible?

  - by Steffen Müller

  I am evaluating a system for a client where many OpenVPN clients connect to a OpenVPN server. "Many" means 50000 - 1000000. Why do I do that? The clients are distributed embedded systems, each sitting behind the system owners dsl router. The server needs to be able to send commands to the clients. My first naive approach is to make the clients connect to the server via an openvpn network. This way, the secure communication tunnel can be used in both directions. This means that all clients are always connected to the server. There are many clients summing up over the years. The question is: does the OpenVPN server explode when reaching a certain number of clients? I am already aware of a maximum TCP connection number limit, therefore (and for other reasons) the VPN would have to use UDP transport. OpenVPN gurus, what is your opinion?

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• Gmail & yahoo - offline mail viewer for mac osx

  - by sagar

  Hello ! Every one. My question are divided into two parts ----------------------- 1 ---------------------------- I am having a difficulty regarding receiving my mails. I have 3 gmail accounts & 1 yahoo account. In windows, I use Microsoft outlook or microsoft outlook express. But In mac is there any inbuilt software for it ? If not, is that any freely available mail client for mac osx ? ----------------------- 2 ---------------------------- One more thing, My system administrators has banned regular gmail & yahoo sites. Means I can't use gmail or yahoo site for mailing. Ok. I don't want to break my office rules - but what about my mails on gmail & yahoo, they are really very important for me. Means that - I just want to access incoming mails nothing more than that.

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• Could we build a mega-processor out of superconductors?

  - by Carson Myers

  A superconductor, once cooled below a critical temperature, loses all of its electrical resistance and therefore becomes 100% efficient. This means that when a current flows through a superconductor, none of the energy is lost to heat or light. Theoretically, could we build a processor out of superconductive materials, that could effectively run at, oh I don't know, say, 300ghz? or 5,000ghz? Since a superconductive circuit is 100% efficient, this means that once supplied with electricity, the source of power could be completely removed from the circuit and the current would continue to flow forever. So if we made all the components inside a computer out of superconductive materials, could we get away with only supplying power to the peripherals and save a-whole-lot on energy, while dramatically increasing computing speed? Might this be one of the next big breakthroughs in computing? What do you think?

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• SQL Server High Availability - Mirroring with MSCS?

  - by David

  I'm looking at options for high-availability for my SQL Server-powered application. The requirements are: HA protection from storage failure. Data accessibility when one of the DB servers is undergoing software updates (e.g. planned outage for Windows Update / SQL Server service-packs). Must not involve much in the way of hardware procurement. The application is an ASP.NET web application. The web application's users have their own database instances. I've seen two main options: SQL Server failover clustering, and SQL Server mirroring. I understand that SQL Server Failover Clustering requires the purchasing of a shared disk array and doesn't offer any protection if the shared storage goes down (so the documentation recommends to set up a Mirroring between two clusters). Database Mirroring seems the cheaper option (as it only requires two database servers and a simple witness box) - but I've heard it doesn't work well when you have a large number of databases. The application I'm developing involves giving each client their own database for their application - there could be hundreds of databases. Setting up the mirroring is no problem thanks to the automation systems we have in place. My final point concerns how failover works with respect to client connections - SQL Server Failover Clustering uses MSCS which means that the cluster is invisible to clients - a connection attempt might fail during the failover, but a simple reconnect will have it working again. However mirroring, as far as I know, requires that the client be aware of the mirrored partners: if the client cannot connect to the primary server then it tries the secondary server. I'm wondering how this work with respect to Connection Pooling in ASP.NET applications - does the client connection failovering mean that there's a potential 2-second (assuming 2000ms TCP timeout policy) pause when the connection pool tries the primary server on every connection attempt? I read somewhere that Mirroring can be used on top of MSCS which means that the client does not need to be aware of mirroring (so there wouldn't be any potential delays during connection, and also that no changes would need to be made to the client, not even the connection string) - however I'm finding it hard to get documentation or white papers on this approach. But if true, then it means the best method is then Mirroring (for HA) with MSCS (for client ignorance and connection performance). ...but how does this scale to a server instance that might contain hundreds of mirrored databases?

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• Browsers with good keyboard support

  - by egon

  What browsers have good keyboard support? That means you don't have to use the mouse so much. Or what is a good setup for browser + keyboard interaction. I know the basic keyboard shortcuts (tab switching, new tabs etc.) but I don't know a good way to switch between links and search areas - usually that means TAB, Shift-TAB one at a time. Is there maybe some plugin that'll add numbers to links and I could just do something like 12 to go to the link 12?

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• What does S in linux file properties mean?

  - by penguin

  I'm creating directories and changing the permissions of them in perl with the following code: umask 0000; mkdir $path, 0770; chown $userid, $groupid, $path; Now when I do ls -l on a directory I've just created, they are as follows: drwxrws--- 2 user group 4096 Nov 3 15:34 test1 I notice for the group permissions, there's an s instead of x. Even if I chmod manually to remove all permissions for the user and group ("chmod g=" and "chmod u=", it's still there: d-----S--- 2 user group 4096 Nov 3 15:36 test2 The internet suggests S means everything in the folder is run as su or something? I don't quite understand what it means but I figure I should understand seeing as these are webroots so if there's a security implication, I ought to be aware of it. Many thanks for your help!

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• Web authentication using LDAP and Apache?

  - by Stephen R

  I am working on a project of setting up a web administered inventory database for my work (or if they don't want it then i'll enjoy learning about it) and hit the problem of allowing only authorized users to access the website (In its testing/development phase, I allow all people to navigate to the website to add entries to the database and query it). I am trying to make it so only particular users in the domain (Active Directory) are allowed to access the website after they are queried about their credentials. I read that Apache (I am using a LAMP server) has a means of asking visitors to the website to provide LDAP credentials in order to gain access to the site, but I wasn't sure if that was exactly what I was looking for. If anyone has experience in the LDAP configurations for Apache that I mentioned or any other means of securely authenticating with websites I would greatly appreciate advice or a direction to go Thank you!

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• Will open files limit in Centos affect HTTP connections? Does the limit apply to a single session or all sessions?

  - by forestclown

  When I do a ulimit -n I got 256, I assume it means I can open 256 files at the sametime. Does it means I can open 256 files with one single session? or all sessions? For example, I logined to my server with username "abc" (via putty/ssh), and open 200 files, with the session still running, I logined to the same server again with the same username "abc" (via putty/ssh), I can open only another 56 files? or I can open another 256 files? Lastly, does this limit also restrict number of http connections? e.g. with the above example, I have opened 200 files, and then I use "wget" or "curl" to make http connections. Thanks

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• Send individual e-mails to each contact in Gmail?

  - by Robert C. Cartaino

  I'm trying to send an e-mail to a group of contacts in Gmail (200 recipients, no spam). Is there a way to force Gmail to send the e-mail as 200 individual e-mails, each addressed to that specific person in the list? But I'm trying to protect their privacy: Sending to a contact group puts all their e-mail addresses in the To: field. Adding their addresses to the cc: field means everyone can see all the addresses. Adding their addresses to the bcc: field means that no one sees their address (not even their own) in the to: field. That looks odd and seems like that would trigger a lot of spam filters. So how can I force Gmail to send the e-mail addressed specifically to each contact in the list?

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• Proper 16:9 video size for non-HD 4:3 video (for youtube/vimeo)

  - by Xeoncross

  Since High Definition video came out on all the online sites it has changed the default aspect ratio of the player from 4:3 to 16:9. This means that for people posting SD video you have to resize some of your videos to get them to fit right. For example, NTSC DVD quality (aka 480i/p) is 720x480 pixels (width x height). However, low-end High Definition (720i/p) is 1280x720. Anyway, now that the video players are built for HD you will find that uploading standard quality videos will result in videos that are "letter boxed" which means they have extra black bars on the top and bottom (or sides). Correct me if I'm wrong, but in order to get a 720x480 video to fit a box that is designed for HD the best practice would be to crop some of it off so that it fits as 720x404 since: 16/9 = 1.78 (1.7777777777778) 720/405 = 1.78 405x1.78 = 720.9 The same would stand for 640x480 (old TV quality) video that would need to be 640x360 correct? I'm asking because I'm not sure about all this and whether this is the proper way to fix these letter-boxing/display problems.

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