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  • stringindexoutofbounds with currency converter java program

    - by user1795926
    I am have trouble with a summary not showing up. I am supposed to modify a previous Java assignment by by adding an array of objects. Within the loop, instantiate each individual object. Make sure the user cannot keep adding another Foreign conversion beyond your array size. After the user selects quit from the menu, prompt if the user want to display a summary report. If they select ‘Y’ then, using your array of objects, display the following report: Item Conversion Dollars Amount 1 Japanese Yen 100.00 32,000.00 2 Mexican Peso 400.00 56,000.00 3 Canadian Dollar 100.00 156.00 etc. Number of Conversions = 3 There are no errors when I compile..but when I run the program it is fine until I hit 0 to end the conversion and have it ask if i want to see a summary. This error displays: Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0 at java.lang.String.charAt(String.java:658) at Lab8.main(Lab8.java:43) my code: import java.util.Scanner; import java.text.DecimalFormat; public class Lab8 { public static void main(String[] args) { final int Max = 10; String a; char summary; int c = 0; Foreign[] Exchange = new Foreign[Max]; Scanner Keyboard = new Scanner(System.in); Foreign.opening(); do { Exchange[c] = new Foreign(); Exchange[c].getchoice(); Exchange[c].dollars(); Exchange[c].amount(); Exchange[c].vertical(); System.out.println("\n" + Exchange[c]); c++; System.out.println("\n" + "Please select 1 through 4, or 0 to quit" + >"\n"); c= Keyboard.nextInt(); } while (c != 0); System.out.print("\nWould you like a summary of your conversions? (Y/N): "); a = Keyboard.nextLine(); summary = a.charAt(0); summary = Character.toUpperCase(summary); if (summary == 'Y') { System.out.println("\nCountry\t\tRate\t\tDollars\t\tAmount"); System.out.println("========\t\t=======\t\t=======\t\t========="); for (int i=0; i < Exchange.length; i++) System.out.println(Exchange[i]); Foreign.counter(); } } } I looked at line 43 and its this line: summary = a.charAt(0); But I am not sure what's wrong with it, can anyone point it out? Thank you.

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  • Oracle Enterprise Manager Extensibility News - June 2014

    - by Joe Diemer
    Introducing Extensibility Exchange Version 2 On the heals of Enterprise Manager 12c Release 4 this week comes version 2.0 of the Extensibility Exchange.  A new theme allows optimal viewing on a number of different computing devices from large monitor displays to tablets to smartphones.   One of the first things you'll notice is a scrollable banner with the latest news related to Enterprise Manager and extensibility.  Along with the "slider" and the latest entries from Oracle and the Partner community, new features like a tag cloud and an auto-complete search box provide a better way to find the plug-in, connector or other Enterprise Manager entity you are looking for.  Once you find it, a content details page with specific info related to that particular entity will enable you to access it at the provider's site and also rate and comment on that particular item. You can also send an email from the content details page which is routed to the developer.   And if you want to use version 1 of the Extensibility Exchange instead, you will be able to do so via the "Classic" option.  Check it out today at http://www.oracle.com/goto/emextensibility. Recent Additions from Oracle's Partner Community A number of important 3rd party plug-ins have been contributed by Oracle's partner community, which can be accessed via the Extensibility Exchange or by clicking the links in this blog: Dell Open Manage Fusion I-O ION Accelerator NetApp SANtricity E-Series PostgreSQL by Blue Medora You can also check out the following best practices and labs available via the Exchange: Riverbed Stingray Traffic Manager Reference Architecture Datavail Alert Optimizer Custom Templates Apps Associates' Oracle Enterprise Manager "Test Drives" for Oracle Database 12c Management Oracle Enterprise Manager Monitoring Essentials Oracle Application Management Suite for Oracle E-Business Suite

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  • What is recommended - UC or EV or EV UC certificate?

    - by Abdel Olakara
    We are implementing Exchange 2010 server and an eCommerce site. Both of these need certificates and I am confused what to use? I know Exchange need UC certificate. Can I use it for the ecommerce site as well? I did read EV is recommended for web sites.. I would like to know what to use and the recommended procedures. Here how we will be using the certificates: We are planning to use *.net for testing Exchange server Will be using *.com for Exchange server (Production) Will be using *.com for ecommerce site (Production) I also heard about certificates which are both EV UC.. please recommend the correct certificates to use.

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  • Failing report subscriptions

    - by DavidWimbush
    We had an interesting problem while I was on holiday. (Why doesn't this stuff ever happen when I'm there?) The sysadmin upgraded our Exchange server to Exchange 2010 and everone's subscriptions stopped. My Subscriptions showed an error message saying that the email address of one of the recipients is invalid. When you create a subscription, Reporting puts your Windows user name into the To field and most users have no permissions to edit it. By default, Reporting leaves it up to exchange to resolve that into an email address. This only works if Exchange is set up to translate aliases or 'short names' into email addresses. It turns out this leaves Exchange open to being used as a relay so it is disabled out of the box. You now have three options: Open up Exchange. That would be bad. Give all Reporting users the ability to edit the To field in a subscription. a) They shouldn't have to, it should just work. b) They don't really have any business subscribing anyone but themselves. Fix the report server to add the domain. This looks like the right choice and it works for us. See below for details. Pre-requisites: A single email domain name. A clear relationship between the Windows user name and the email address. eg. If the user name is joebloggs, then joebloggs@domainname needs to be the email address or an alias of it. Warning: Saving changes to the rsreportserver.config file will restart the Report Server service which effectively takes Reporting down for around 30 seconds. Time your action accordingly. Edit the file rsreportserver.config (most probably in the folder ..\Program Files[ (x86)]\Microsoft SQL Server\MSRS10_50[.instancename]\Reporting Services\ReportServer). There's a setting called DefaultHostName which is empty by default. Enter your email domain name without the leading '@'. Save the file. This domain name will be appended to any destination addresses that don't have a domain name of their own.

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  • What is recommended - UC or EV or EV UC certificate?

    - by Abdel Olakara
    Hi all, We are implementing Exchange 2010 server and an eCommerce site. Both of these need certificates and I am confused what to use? I know Exchange need UC certificate. Can I use it for the ecommerce site as well? I did read EV is recommended for web sites.. I would like to know what to use and the recommended procedures. Here how we will be using the certificates: We are planning to use *.net for testing Exchange server Will be using *.com for Exchange server (Production) Will be using *.com for ecommerce site (Production) I also heard about certificates which are both EV UC.. please recommend the correct certificates to use. Thanks in advance.

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  • AD account locks out when using Outlook 2007?

    - by Down Town
    Hi, I/we have a problem with our Windows Server 2008 forest and Exchange. We are buying Exchange hosting from another firm and Exchange Server is in their Windows Server 2008 forest. So, we have two forests and there isn't any trusts between these two forests. Our own forest logon name is [email protected] and we also use the same email address to logon to the Exchange mailbox. Everything works fine if both our AD account and Exchange mailbox account have the same password, but if the passwords don't match, our AD account gets locked out. I have tried to figure out why Outlook sends false logon attemps to our AD. If someone can help, please do.

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  • Scuttlebutt Reconciliation from "Efficient Reconciliation and Flow Control for Anti-Entropy Protocols"

    - by Maus
    This question might be more suited to math.stackexchange.com, but here goes: Their Version Reconciliation takes two parts-- first the exchange of digests, and then an exchange of updates. I'll first paraphrase the paper's description of each step. To exchange digests, two peers send one another a set of pairs-- (peer, max_version) for each peer in the network, and then each one responds with a set of deltas. The deltas look like: (peer, key, value, version), for all tuples for which peer's state maps the key to the given value and version, and the version number is greater than the maximum version number peer has seen. This seems to require that each node remember the state of each other node, and the highest version number and ID each node has seen. Question Why must we iterate through all peers to exchange information between p and q?

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  • How to make Evolution mail work with my work email address?

    - by Fady
    this is the 1st time to write here and the 1st time to use a mail client other than outlook. I tried to add my enterprise email address to evolution mail, I tried both server types exchange mapi and microsoft exchange. With exchange mapi i get this error message "Authentication failed. MapiLogonProvider: Failed to login into the server" With Microsoft Exchange I get this error "Could not connect to server . Make sure the URL is correct and try again." Although I'm sure of all the steps Server: ip address of the mail server Username: Domainname\Username Domain: domain name My system is Ubuntu Release 11.04 (natty) Kernel Linux 2.6.38-15-generic Genome 2.32.1 Evolution 2.32.2 Any kind of help is appreciated and thanks in advance

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  • addsubview animation works only first time

    - by Priya
    I am trying add subview with animation effect using the code which work fine for the first time below. CATransition *transition = [CATransition animation]; transition.duration = 1.0; transition.type = kCATransitionMoveIn; transition.subtype = kCATransitionFromRight; [newView.layer removeAllAnimations] [newView.layer addAnimation:transition forKey:nil]; [self.masterview addSubview:newView]; There is a back button in newView which removes the View from the superview. [newView removeFromSuperview]; Now when I try adding newView as subview again using the above code,its first adds the view as a subview(without animation) and again with animation.

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  • IIS SMTP server (Installed on local server) in parallel to Google Apps

    - by sharru
    I am currently using free version of Google Apps for hosting my email.It works great for my official mails my email on Google is [email protected]. In addition I'm sending out high volume mails (registrations, forgotten passwords, newsletters etc) from the website (www.mydomain.com) using IIS SMTP installed on my windows machine. These emails are sent from [email protected] My problem is that when I send email from the website using IIS SMTP to a mail address [email protected] I don’t receive the email to Google apps. (I only receive these emails if I install a pop service on the server with the [email protected] email box). It seems that the IIS SMTP is ignoring the domain MX records and just delivers these emails to my local server. Here are my DNS records for domain.com: mydomain.com A 82.80.200.20 3600s mydomain.com TXT v=spf1 ip4: 82.80.200.20 a mx ptr include:aspmx.googlemail.com ~all mydomain.com MX preference: 10 exchange: aspmx2.googlemail.com 3600s mydomain.com MX preference: 10 exchange: aspmx3.googlemail.com 3600s mydomain.com MX preference: 10 exchange: aspmx4.googlemail.com 3600s mydomain.com MX preference: 10 exchange: aspmx5.googlemail.com 3600s mydomain.com MX preference: 1 exchange: aspmx.l.google.com 3600s mydomain.com MX preference: 5 exchange: alt1.aspmx.l.google.com 3600s mydomain.com MX preference: 5 exchange: alt2.aspmx.l.google.com 3600s Please help! Thanks.

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  • Quickbooks integration: IPP/IDS: can these by used for actual data exchange?

    - by Parand
    Poking around options for integrating an online app with Quickbooks, I've made a lot of headway with QBWC, but it's fairly ugly. From an end user perspective the usability of QBWC is pretty low. Intuit is now pushing Intuit Partner Platform (IPP) and Intuit Data Services (IDS). I can't quite figure out what these are about: Is IPP limited to using Flex, or can it work with existing web apps? Are there APIs for actual data exchange? Is it possible to interact with desktop Quickbooks using IPP or IDS? If there is sample code, particularly in Python, some pointers would be great.

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  • Assigning two strings together getting Access Read Violation

    - by Jay Bell
    I am trying to pass a string to a class mutator and set the private member to that string here is the code that is sending the string void parseTradePairs(Exchange::Currency *curr, std::string *response, int begin, int exit) { int start; int end; string temp; string dataResponse; CURL *tempCurl; initializeCurl(tempCurl); int location = response->find("marketid", begin); if(location <= exit) { start = location + 11; begin = response->find("label", start); end = begin - start - 3; findStrings(start, end, temp, response); getMarketInfo(tempCurl, temp, dataResponse); curr->_coin->setExch(temp); // here is the line of code that is sending the string dataResponse >> *(curr->_coin); curr->_next = new Exchange::Currency(curr, curr->_position + 1); parseTradePairs(curr->_next, response, begin, exit); } } and here is the mutator within the coin class that is receiving the string and assigning it to _exch void Coin::setExch(string exch) { _exch = exch; } I have stepped through it and made sure that exch has the string in it. "105" but soon as it hits _exch = exch; I get the reading violation. I tried passing as pointer as well. I do not believe it should go out of scope. and the string variable in the class is initialized to zero in the default constructor but again that should matter unless I am trying to read from it instead of writing to it. /* defualt constructor */ Coin::Coin() { _id = ""; _label = ""; _code= ""; _name = ""; _marketCoin = ""; _volume = 0; _last = 0; _exch = ""; } Exchange::Exchange(std::string str) { _exch = str; _currencies = new Currency; std::string pair; std::string response; CURL *curl; initializeCurl(curl); getTradePairs(curl, response); int exit = response.find_last_of("marketid"); parseTradePairs(_currencies, &response, 0, exit); } int main(void) { CURL *curl; string str; string id; Coin coin1; initializeCurl(curl); Exchange ex("cryptsy"); curl_easy_cleanup(curl); system("pause"); return 0; } class Exchange { public: typedef struct Currency { Currency(Coin *coin, Currency *next, Currency *prev, int position) : _coin(coin), _next(next), _prev(prev), _position(position) {} Currency(Currency *prev, int position) : _prev(prev), _position(position), _next(NULL), _coin(&Coin()){} Currency() : _next(NULL), _prev(NULL), _position(0) {} Coin *_coin; Currency *_next; Currency *_prev; int _position; }; /* constructor and destructor */ Exchange(); Exchange(std::string str); ~Exchange(); /* Assignment operator */ Exchange& operator =(const Exchange& copyExchange); /* Parse Cryptsy Pairs */ friend void parseTradePairs(Currency *curr, std::string *response, int begin, int exit); private: std::string _exch; Currency *_currencies; }; here is what i changed it to to fix it. typedef struct Currency { Currency(Coin *coin, Currency *next, Currency *prev, int position) : _coin(coin), _next(next), _prev(prev), _position(position) {} Currency(Currency *prev, int position) : _prev(prev), _position(position), _next(NULL), _coin(&Coin()){} Currency() { _next = NULL; _prev = NULL; _position = 0; _coin = new Coin(); } Coin *_coin; Currency *_next; Currency *_prev; int _position; };

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  • How online-game clients are able to exchange data through internet so fast?

    - by Kirzilla
    Hello, Let's imagine really simple game... We have a labirinth and two players trying to find out exit in real time through internet. On every move game client should send player's coordinates to server and accept current coordinates of another client. How is it possible to make this exchange so fast (as all modern games do). Ok, we can use memcache or similar technology to reduce data mining operations on server side. We can also use fastest webserver etc., but we still will have problems with timings. So, the questions are... What protocol game clients are usually using for exchanging information with server? What server technologies are coming to solve this problem? What algorithms are applied for fighting with delays during game etc. PS: Sorry for my English and I hope that my question is clear. Thank you.

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  • Problem with creating a deterministic finite automata (DFA) - Mercury

    - by Jabba The hut
    I would like to have a deterministic finite automata (DFA) simulated in Mercury. But I’m s(t)uck at several places. Formally, a DFA is described with the following characteristics: a setOfStates S, an inputAlphabet E <-- summation symbol, a transitionFunction : S × E -- S, a startState s € S, a setOfAcceptableFinalStates F =C S. A DFA will always starts in the start state. Then the DFA will read all the characters on the input, one by one. Based on the current input character and the current state, there will be made to a new state. These transitions are defined in the transitions function. when the DFA is in one of his acceptable final states, after reading the last character, then will the DFA accept the input, If not, then the input will be is rejected. The figure shows a DFA the accepting strings where the amount of zeros, is a plurality of three. Condition 1 is the initial state, and also the only acceptable state. for each input character is the corresponding arc followed to the next state. Link to Figure What must be done A type “mystate” which represents a state. Each state has a number which is used for identification. A type “transition” that represents a possible transition between states. Each transition has a source_state, an input_character, and a final_state. A type “statemachine” that represents the entire DFA. In the solution, the DFA must have the following properties: The set of all states, the input alphabet, a transition function, represented as a set of possible transitions, a set of accepting final states, a current state of the DFA A predicate “init_machine (state machine :: out)” which unifies his arguments with the DFA, as shown as in the Figure. The current state for the DFA is set to his initial state, namely, 1. The input alphabet of the DFA is composed of the characters '0'and '1'. A user can enter a text, which will be controlled by the DFA. the program will continues until the user types Ctrl-D and simulates an EOF. If the user use characters that are not allowed into the input alphabet of the DFA, then there will be an error message end the program will close. (pred require) Example Enter a sentence: 0110 String is not ok! Enter a sentence: 011101 String is not ok! Enter a sentence: 110100 String is ok! Enter a sentence: 000110010 String is ok! Enter a sentence: 011102 Uncaught exception Mercury: Software Error: Character does not belong to the input alphabet! the thing wat I have. :- module dfa. :- interface. :- import_module io. :- pred main(io.state::di, io.state::uo) is det. :- implementation. :- import_module int,string,list,bool. 1 :- type mystate ---> state(int). 2 :- type transition ---> trans(source_state::mystate, input_character::bool, final_state::mystate). 3 (error, finale_state and current_state and input_character) :- type statemachine ---> dfa(list(mystate),list(input_character),list(transition),list(final_state),current_state(mystate)) 4 missing a lot :- pred init_machine(statemachine :: out) is det. %init_machine(statemachine(L_Mystate,0,L_transition,L_final_state,1)) :- <-probably fault 5 not perfect main(!IO) :- io.write_string("\nEnter a sentence: ", !IO), io.read_line_as_string(Input, !IO), ( Invoer = ok(StringVar), S1 = string.strip(StringVar), (if S1 = "mustbeabool" then io.write_string("Sentenceis Ok! ", !IO) else io.write_string("Sentence is not Ok!.", !IO)), main(!IO) ; Invoer = eof ; Invoer = error(ErrorCode), io.format("%s\n", [s(io.error_message(ErrorCode))], !IO) ). Hope you can help me kind regards

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  • How to import a text file into powershell and email it, formatted as HTML

    - by Don
    I'm trying to get a list of all Exchange accounts, format them in descending order from largest mailbox and put that data into an email in HTML format to email to myself. So far I can get the data, push it to a text file as well as create an email and send to myself. I just can't seem to get it all put together. I've been trying to use ConvertTo-Html but it just seems to return data via email like "pageFooterEntry" and "Microsoft.PowerShell.Commands.Internal.Format.AutosizeInfo" versus the actual data. I can get it to send me the right data if i don't tell it to ConvertTo-Html, just have it pipe the data to a text file and pull from it, but it's all ran together with no formatting. I don't need to save the file, i'd just like to run the command, get the data, put it in HTML and mail it to myself. Here's what I have currently: #Connects to Database and returns information on all users, organized by Total Item Size, User $body = Get-MailboxStatistics -database "Mailbox Database 0846468905" | where {$_.ObjectClass -eq “Mailbox”} | Sort-Object TotalItemSize -Descending | ft @{label=”User”;expression={$_.DisplayName}},@{label=”Total Size (MB)”;expression={$_.TotalItemSize.Value.ToMB()}} -auto | ConvertTo-Html #Pause for 5 seconds for Exchange write-host -foregroundcolor Green "Pausing for 5 seconds for Exchange" Start-Sleep -s 5 $toemail = "[email protected]" # Emails report to this address. $fromemail = "[email protected]" #Emails from this address. $server = "Exchange.company.com" #Exchange server - SMTP. #Email the report. $email = New-Object System.Net.Mail.MailMessage $email.IsBodyHtml = $True $email.To.Add($toemail) $email.From = $fromemail $email.Subject = "Exchange Mailbox Sizes" $email.Body = $body $client = New-Object System.Net.Mail.SmtpClient $server $client.UseDefaultCredentials = $true $client.Send($email) Any thoughts would be helpful, thanks!

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  • MS DPM 2007: Testing the Recovery for a Production Domain

    - by NewToDPM
    Hi everybody! MS DPM 2007 is a new technology in my company, and so am I to the product. We have a classic Microsoft domain with two DCs, Exchange 2007 and a couple Web/MS SQL servers. I have deployed DPM one month ago on the domain, and after fixing the various issues I got with the replicas inconsistence and adapting the schedule and retention range to the server storage pool size, I can say the backup system is working correctly (no errors) as of today. However, there is one problem: we did not attempt to restore from the backups yet, which is a big no-no of course. I'm not sure about the way I should handle this, my main concern being Exchange and the System State of the DCs. From my understanding, DPM can only protect AND restore data on a server which is part of the same domain as the backup server. If I restore the System State (containing Active Directory) and the Exchange Storage Groups on a testing server, I am afraid it would completely disturb the domain functioning (for example, having two primary DCs on the domain). I am thinking about building a second DPM server on a testing separate domain which would mirror the replicas and then restore it on testing servers from this new domain. Is it the right way to handle the data recovery testing? How did you do on your domain when you first deployed DPM? I'd be grateful for any link/documentation or advice. Thank you in advance for your help! EDIT: Two options seem possible so far: i. Create another DC/Exchange server in the alternate location; ii. Create a separate domain in the alternate location and setup a trust between this domain and the production one. The option i is certainly the best but implies setting up a secondary Exchange server, with a dedicated public IP address so that if Exchange #1 dies, we can still send emails with Exchange #2. I don't know how complex this can be and would need to discuss it with my colleagues. The option ii would only fit the testing purposes. My only question regarding this is: if my production and DPM servers are part of domain A, and there is a trust between domains A and B, can I restore a domain A content to any domain B server?

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  • C#/.NET Little Wonders: Interlocked CompareExchange()

    - by James Michael Hare
    Once again, in this series of posts I look at the parts of the .NET Framework that may seem trivial, but can help improve your code by making it easier to write and maintain. The index of all my past little wonders posts can be found here. Two posts ago, I discussed the Interlocked Add(), Increment(), and Decrement() methods (here) for adding and subtracting values in a thread-safe, lightweight manner.  Then, last post I talked about the Interlocked Read() and Exchange() methods (here) for safely and efficiently reading and setting 32 or 64 bit values (or references).  This week, we’ll round out the discussion by talking about the Interlocked CompareExchange() method and how it can be put to use to exchange a value if the current value is what you expected it to be. Dirty reads can lead to bad results Many of the uses of Interlocked that we’ve explored so far have centered around either reading, setting, or adding values.  But what happens if you want to do something more complex such as setting a value based on the previous value in some manner? Perhaps you were creating an application that reads a current balance, applies a deposit, and then saves the new modified balance, where of course you’d want that to happen atomically.  If you read the balance, then go to save the new balance and between that time the previous balance has already changed, you’ll have an issue!  Think about it, if we read the current balance as $400, and we are applying a new deposit of $50.75, but meanwhile someone else deposits $200 and sets the total to $600, but then we write a total of $450.75 we’ve lost $200! Now, certainly for int and long values we can use Interlocked.Add() to handles these cases, and it works well for that.  But what if we want to work with doubles, for example?  Let’s say we wanted to add the numbers from 0 to 99,999 in parallel.  We could do this by spawning several parallel tasks to continuously add to a total: 1: double total = 0; 2:  3: Parallel.For(0, 10000, next => 4: { 5: total += next; 6: }); Were this run on one thread using a standard for loop, we’d expect an answer of 4,999,950,000 (the sum of all numbers from 0 to 99,999).  But when we run this in parallel as written above, we’ll likely get something far off.  The result of one of my runs, for example, was 1,281,880,740.  That is way off!  If this were banking software we’d be in big trouble with our clients.  So what happened?  The += operator is not atomic, it will read in the current value, add the result, then store it back into the total.  At any point in all of this another thread could read a “dirty” current total and accidentally “skip” our add.   So, to clean this up, we could use a lock to guarantee concurrency: 1: double total = 0.0; 2: object locker = new object(); 3:  4: Parallel.For(0, count, next => 5: { 6: lock (locker) 7: { 8: total += next; 9: } 10: }); Which will give us the correct result of 4,999,950,000.  One thing to note is that locking can be heavy, especially if the operation being locked over is trivial, or the life of the lock is a high percentage of the work being performed concurrently.  In the case above, the lock consumes pretty much all of the time of each parallel task – and the task being locked on is relatively trivial. Now, let me put in a disclaimer here before we go further: For most uses, lock is more than sufficient for your needs, and is often the simplest solution!    So, if lock is sufficient for most needs, why would we ever consider another solution?  The problem with locking is that it can suspend execution of your thread while it waits for the signal that the lock is free.  Moreover, if the operation being locked over is trivial, the lock can add a very high level of overhead.  This is why things like Interlocked.Increment() perform so well, instead of locking just to perform an increment, we perform the increment with an atomic, lockless method. As with all things performance related, it’s important to profile before jumping to the conclusion that you should optimize everything in your path.  If your profiling shows that locking is causing a high level of waiting in your application, then it’s time to consider lighter alternatives such as Interlocked. CompareExchange() – Exchange existing value if equal some value So let’s look at how we could use CompareExchange() to solve our problem above.  The general syntax of CompareExchange() is: T CompareExchange<T>(ref T location, T newValue, T expectedValue) If the value in location == expectedValue, then newValue is exchanged.  Either way, the value in location (before exchange) is returned. Actually, CompareExchange() is not one method, but a family of overloaded methods that can take int, long, float, double, pointers, or references.  It cannot take other value types (that is, can’t CompareExchange() two DateTime instances directly).  Also keep in mind that the version that takes any reference type (the generic overload) only checks for reference equality, it does not call any overridden Equals(). So how does this help us?  Well, we can grab the current total, and exchange the new value if total hasn’t changed.  This would look like this: 1: // grab the snapshot 2: double current = total; 3:  4: // if the total hasn’t changed since I grabbed the snapshot, then 5: // set it to the new total 6: Interlocked.CompareExchange(ref total, current + next, current); So what the code above says is: if the amount in total (1st arg) is the same as the amount in current (3rd arg), then set total to current + next (2nd arg).  This check and exchange pair is atomic (and thus thread-safe). This works if total is the same as our snapshot in current, but the problem, is what happens if they aren’t the same?  Well, we know that in either case we will get the previous value of total (before the exchange), back as a result.  Thus, we can test this against our snapshot to see if it was the value we expected: 1: // if the value returned is != current, then our snapshot must be out of date 2: // which means we didn't (and shouldn't) apply current + next 3: if (Interlocked.CompareExchange(ref total, current + next, current) != current) 4: { 5: // ooops, total was not equal to our snapshot in current, what should we do??? 6: } So what do we do if we fail?  That’s up to you and the problem you are trying to solve.  It’s possible you would decide to abort the whole transaction, or perhaps do a lightweight spin and try again.  Let’s try that: 1: double current = total; 2:  3: // make first attempt... 4: if (Interlocked.CompareExchange(ref total, current + i, current) != current) 5: { 6: // if we fail, go into a spin wait, spin, and try again until succeed 7: var spinner = new SpinWait(); 8:  9: do 10: { 11: spinner.SpinOnce(); 12: current = total; 13: } 14: while (Interlocked.CompareExchange(ref total, current + i, current) != current); 15: } 16:  This is not trivial code, but it illustrates a possible use of CompareExchange().  What we are doing is first checking to see if we succeed on the first try, and if so great!  If not, we create a SpinWait and then repeat the process of SpinOnce(), grab a fresh snapshot, and repeat until CompareExchnage() succeeds.  You may wonder why not a simple do-while here, and the reason it’s more efficient to only create the SpinWait until we absolutely know we need one, for optimal efficiency. Though not as simple (or maintainable) as a simple lock, this will perform better in many situations.  Comparing an unlocked (and wrong) version, a version using lock, and the Interlocked of the code, we get the following average times for multiple iterations of adding the sum of 100,000 numbers: 1: Unlocked money average time: 2.1 ms 2: Locked money average time: 5.1 ms 3: Interlocked money average time: 3 ms So the Interlocked.CompareExchange(), while heavier to code, came in lighter than the lock, offering a good compromise of safety and performance when we need to reduce contention. CompareExchange() - it’s not just for adding stuff… So that was one simple use of CompareExchange() in the context of adding double values -- which meant we couldn’t have used the simpler Interlocked.Add() -- but it has other uses as well. If you think about it, this really works anytime you want to create something new based on a current value without using a full lock.  For example, you could use it to create a simple lazy instantiation implementation.  In this case, we want to set the lazy instance only if the previous value was null: 1: public static class Lazy<T> where T : class, new() 2: { 3: private static T _instance; 4:  5: public static T Instance 6: { 7: get 8: { 9: // if current is null, we need to create new instance 10: if (_instance == null) 11: { 12: // attempt create, it will only set if previous was null 13: Interlocked.CompareExchange(ref _instance, new T(), (T)null); 14: } 15:  16: return _instance; 17: } 18: } 19: } So, if _instance == null, this will create a new T() and attempt to exchange it with _instance.  If _instance is not null, then it does nothing and we discard the new T() we created. This is a way to create lazy instances of a type where we are more concerned about locking overhead than creating an accidental duplicate which is not used.  In fact, the BCL implementation of Lazy<T> offers a similar thread-safety choice for Publication thread safety, where it will not guarantee only one instance was created, but it will guarantee that all readers get the same instance.  Another possible use would be in concurrent collections.  Let’s say, for example, that you are creating your own brand new super stack that uses a linked list paradigm and is “lock free”.  We could use Interlocked.CompareExchange() to be able to do a lockless Push() which could be more efficient in multi-threaded applications where several threads are pushing and popping on the stack concurrently. Yes, there are already concurrent collections in the BCL (in .NET 4.0 as part of the TPL), but it’s a fun exercise!  So let’s assume we have a node like this: 1: public sealed class Node<T> 2: { 3: // the data for this node 4: public T Data { get; set; } 5:  6: // the link to the next instance 7: internal Node<T> Next { get; set; } 8: } Then, perhaps, our stack’s Push() operation might look something like: 1: public sealed class SuperStack<T> 2: { 3: private volatile T _head; 4:  5: public void Push(T value) 6: { 7: var newNode = new Node<int> { Data = value, Next = _head }; 8:  9: if (Interlocked.CompareExchange(ref _head, newNode, newNode.Next) != newNode.Next) 10: { 11: var spinner = new SpinWait(); 12:  13: do 14: { 15: spinner.SpinOnce(); 16: newNode.Next = _head; 17: } 18: while (Interlocked.CompareExchange(ref _head, newNode, newNode.Next) != newNode.Next); 19: } 20: } 21:  22: // ... 23: } Notice a similar paradigm here as with adding our doubles before.  What we are doing is creating the new Node with the data to push, and with a Next value being the original node referenced by _head.  This will create our stack behavior (LIFO – Last In, First Out).  Now, we have to set _head to now refer to the newNode, but we must first make sure it hasn’t changed! So we check to see if _head has the same value we saved in our snapshot as newNode.Next, and if so, we set _head to newNode.  This is all done atomically, and the result is _head’s original value, as long as the original value was what we assumed it was with newNode.Next, then we are good and we set it without a lock!  If not, we SpinWait and try again. Once again, this is much lighter than locking in highly parallelized code with lots of contention.  If I compare the method above with a similar class using lock, I get the following results for pushing 100,000 items: 1: Locked SuperStack average time: 6 ms 2: Interlocked SuperStack average time: 4.5 ms So, once again, we can get more efficient than a lock, though there is the cost of added code complexity.  Fortunately for you, most of the concurrent collection you’d ever need are already created for you in the System.Collections.Concurrent (here) namespace – for more information, see my Little Wonders – The Concurent Collections Part 1 (here), Part 2 (here), and Part 3 (here). Summary We’ve seen before how the Interlocked class can be used to safely and efficiently add, increment, decrement, read, and exchange values in a multi-threaded environment.  In addition to these, Interlocked CompareExchange() can be used to perform more complex logic without the need of a lock when lock contention is a concern. The added efficiency, though, comes at the cost of more complex code.  As such, the standard lock is often sufficient for most thread-safety needs.  But if profiling indicates you spend a lot of time waiting for locks, or if you just need a lock for something simple such as an increment, decrement, read, exchange, etc., then consider using the Interlocked class’s methods to reduce wait. Technorati Tags: C#,CSharp,.NET,Little Wonders,Interlocked,CompareExchange,threading,concurrency

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  • How do I prevent mail from my Exchange server from being blocked?

    - by Mike C
    Recently one of our client machines was infected with a virus and I believe was spamming addresses in the user's contact list. Since then our server has been appearing on blacklists and it has been causing our e-mail to be blocked and returned by many clients. The virus has since been cleared, what can I do to get our server off these blacklists so that we will have more reliable e-mail service? Will I have to change my IP address? Thanks, Mike

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  • If I exchange the CPU, must I reinstall the OSes? (swapping cpus from one *nix-like to another)

    - by dag729
    Hi, as suggested by the title, I want to change CPU: actually I have two computers, one with Ubuntu running on an AMD Athlon 64 dual core 5200+ and the other with FreeBSD running on an AMD Sempron single core LE-1250. I would like to swap (I am not sure that this is the correct term...) the CPUs from one computer to the other one, that is take the dual core from the ubuntu pc and put it inside the freebsd pc and viceversa. The mobo is the same. Do you think I will encounter problems?

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  • How do I get Outlook (via Exchange) to accept Thunderbird/Lightning meeting requests?

    - by user39646
    Lightning/1.0b1 addon to Thunderbird/3.0.4 has no problem accepting Meeting Requests sent from my network Outlook session. However, Meeting Requests sent to an email address hosted on a POP server and to be delivered to my Outlook mailbox never seem to arrive in any fashion. Nothing in my Outlook Inbox or Messages and nothing on my calendar or anything. I was expecting at least a std email, perhaps with an *ics attachment file, to arrive just like regular Thunderbird-originated email does, but no dice. Any ideas on what I'm doing wrong?

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  • Do busy smtp servers use long running tcp connections to exchange lot of mails?

    - by iamrohitbanga
    I had this idea from http://stackoverflow.com/questions/2813326/maximum-number-of-bytes-that-can-be-sent-on-a-tcp-connection is it possible that smtp servers like gmail and yahoo enter into some form of agreement to maintain a tcp connection between them so that lots of mails could be sent on the same tcp connection. it would be efficient as there would be heavy mail traffic between these mail servers.

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  • What would Stack Exchange's yearly expenses be if it were to be using a third party host?

    - by abel
    StackExchange manages it's own servers, as it should, but if SE were to be hosted on a 3rd party "cloud" hosting (like Amazon's), what would it's monthly / yearly expenses be(keeping everything else the same)? A detailed answer comparing it to the bills that Stackexchange boots currently (including power/property/staff) would help. (PS: I know that the blog is a good resource. I also understand that managing your own hosting is almost the same as setting up a hosting company and using it for your own needs. Plus is this a question for meta or does it fit within serverfault's purview?)

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  • Visual Studio Little Wonders: Box Selection

    - by James Michael Hare
    So this week I decided I’d do a Little Wonder of a different kind and focus on an underused IDE improvement: Visual Studio’s Box Selection capability. This is a handy feature that many people still don’t realize was made available in Visual Studio 2010 (and beyond).  True, there have been other editors in the past with this capability, but now that it’s fully part of Visual Studio we can enjoy it’s goodness from within our own IDE. So, for those of you who don’t know what box selection is and what it allows you to do, read on! Sometimes, we want to select beyond the horizontal… The problem with traditional text selection in many editors is that it is horizontally oriented.  Sure, you can select multiple rows, but if you do you will pull in the entire row (at least for the middle rows).  Under the old selection scheme, if you wanted to select a portion of text from each row (a “box” of text) you were out of luck.  Box selection rectifies this by allowing you to select a box of text that bounded by a selection rectangle that you can grow horizontally or vertically.  So let’s think a situation that could occur where this comes in handy. Let’s say, for instance, that we are defining an enum in our code that we want to be able to translate into some string values (possibly to be stored in a database, output to screen, etc.). Perhaps such an enum would look like this: 1: public enum OrderType 2: { 3: Buy, // buy shares of a commodity 4: Sell, // sell shares of a commodity 5: Exchange, // exchange one commodity for another 6: Cancel, // cancel an order for a commodity 7: } 8:  Now, let’s say we are in the process of creating a Dictionary<K,V> to translate our OrderType: 1: var translator = new Dictionary<OrderType, string> 2: { 3: // do I really want to retype all this??? 4: }; Yes the example above is contrived so that we will pull some garbage if we do a multi-line select. I could select the lines above using the traditional multi-line selection: And then paste them into the translator code, which would result in this: 1: var translator = new Dictionary<OrderType, string> 2: { 3: Buy, // buy shares of a commodity 4: Sell, // sell shares of a commodity 5: Exchange, // exchange one commodity for another 6: Cancel, // cancel an order for a commodity 7: }; But I have a lot of junk there, sure I can manually clear it out, or use some search and replace magic, but if this were hundreds of lines instead of just a few that would quickly become cumbersome. The Box Selection Now that we have the ability to create box selections, we can select the box of text to delete!  Most of us are familiar with the fact we can drag the mouse (or hold [Shift] and use the arrow keys) to create a selection that can span multiple rows: Box selection, however, actually allows us to select a box instead of the typical horizontal lines: Then we can press the [delete] key and the pesky comments are all gone! You can do this either by holding down [Alt] while you select with your mouse, or by holding down [Alt+Shift] and using the arrow keys on the keyboard to grow the box horizontally or vertically. So now we have: 1: var translator = new Dictionary<OrderType, string> 2: { 3: Buy, 4: Sell, 5: Exchange, 6: Cancel, 7: }; Which is closer, but we still need an opening curly, the string to translate to, and the closing curly and comma. Fortunately, again, this is easy with box selections due to the fact box selection can even work for a zero-width selection! That is, hold down [Alt] and either drag down with no width, or hold down [Alt+Shift] and arrow down and you will define a selection range with no width, essentially, a vertical line selection: Notice the faint selection line on the right? So why is this useful? Well, just like with any selected range, we can type and it will replace the selection. What does this mean for box selections? It means that we can insert the same text all the way down on each line! If we have the same selection above, and type a curly and a space, we’d get: Imagine doing this over hundreds of lines and think of what a time saver it could be! Now make a zero-width selection on the other side: And type a curly and a comma, and we’d get: So close! Now finally, imagine we’ve already defined these strings somewhere and want to paste them in: 1: const private string BuyText = "Buy Shares"; 2: const private string SellText = "Sell Shares"; 3: const private string ExchangeText = "Exchange"; 4: const private string CancelText = "Cancel"; We can, again, use our box selection to pull out the constant names: And clicking copy (or [CTRL+C]) and then selecting a range to paste into: And finally clicking paste (or [CTRL+V]) to get the final result: 1: var translator = new Dictionary<OrderType, string> 2: { 3: { Buy, BuyText }, 4: { Sell, SellText }, 5: { Exchange, ExchangeText }, 6: { Cancel, CancelText }, 7: };   Sure, this was a contrived example, but I’m sure you’ll agree that it adds myriad possibilities of new ways to copy and paste vertical selections, as well as inserting text across a vertical slice. Summary: While box selection has been around in other editors, we finally get to experience it in VS2010 and beyond. It is extremely handy for selecting columns of information for cutting, copying, and pasting. In addition, it allows you to create a zero-width vertical insertion point that can be used to enter the same text across multiple rows. Imagine the time you can save adding repetitive code across multiple lines!  Try it, the more you use it, the more you’ll love it! Technorati Tags: C#,CSharp,.NET,Visual Studio,Little Wonders,Box Selection

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