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  • Understanding Node.js and concept of non-blocking I/O

    - by Saif Bechan
    Recently I became interested in using Node.js to tackle some of the parts of my web-application. I love the part that its full JavaScript and its very light weight so no use anymore to call an JavaScript-PHP call but a lighter JavaScript-JavaScript call. I however do not understand all the concepts explained. Basic concepts Now in the presentation for Node.js Ryan Dahl talks about non-blocking IO and why this is the way we need to create our programs. I can understand the theoretical concept. You just don't wait for a response, you go ahead and do other things. You make a callback for the response, and when the response arrives millions of clock-cycles later, you can fire that. If you have not already I recommend to watch this presentation. It is very easy to follow and pretty detailed. There are some nice concepts explained on how to write your code in a good manner. There are also some examples given and I am going to work with the basic example given. Examples The way we do thing now: puts("Enter your name: "); var name = gets(); puts("Name: " + name); Now the problem with this is that the code is halted at line 1. It blocks your code. The way we need to do things according to node puts("Enter your name: "); gets(function (name) { puts("Name: " + name); }); Now with this your program does not halt, because the input is a function within the output. So the programs continues to work without halting. Questions Now the basic question I have is how does this work in real-life situations. I am talking here for the use in web-applications. The application I am writing does I/O, bit is still does it in am blocking matter. I think that most of the time, if not all, you need to block, because you have to wait on what the response is you have to work with. When you need to get some information from the database, most of the time this data needs to be verified before you can further with the code. Example 1 If you take a login for example. You have to wait for the database to response to return, because you can not do anything else. I can't see a way around this without blocking. Example 2 Going back to the basic example. The use just request something from a database which does not need any verification. You still have to block because you don't have anything to do more. I can not come up with a single example where you want to do other things while you wait for the response to return. Possible answers I have read that this frees up recourses. When you program like this it takes less CPU or memory usage. So this non-blocking IO is ONLY meant to free up recourses and does not have any other practical use. Not that this is not a huge plus, freeing up recourses is always good. Yet I fail to see this as a good solution. because in both of the above examples, the program has to wait for the response of the user. Whether this is inside a function, or just inline, in my opinion there is a program that wait for input. Resources I looked at I have looked at some recourses before I posted this question. They talk a lot about the theoretical concept, which is quite clear. Yet i fail to see some real-life examples where this is makes a huge difference. Stackoverflow: What is in simple words blocking IO and non-blocking IO? Blocking IO vs non-blocking IO; looking for good articles tidy code for asynchronous IO Other recources: Wikipedia: Asynchronous I/O Introduction to non-blocking I/O The C10K problem

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  • AND is better or using Internal "IF"

    - by BDotA
    In a situation like this:" if ((metadata != null) && (metadata.TypeEnum != VariantInfoMetadata.CellTypeEnum.Status)) do you recommend to keep the code as it is above or do you think it is better to make an internal "if" statement and brake that AND into two sections where "outer if" makes sure metadata is not null and inner if does the rest of the checking. I think this way will take care of possible null refrence exception if value of metadata gets null?

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  • Using macro to check null values [migrated]

    - by poliron
    My C code contains many functions with pointers to different structs as parameteres which shouldn't be NULL pointers. To make my code more readable, I decided to replace this code: if(arg1==NULL || arg2==NULL || arg3==NULL...) { return SOME_ERROR; } With that macro: NULL_CHECK(arg1,arg2,...) How should I write it, if the number of args is unknown and they can point to different structs?(I work in C99)

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  • Can a conforming C implementation #define NULL to be something wacky

    - by janks
    I'm asking because of the discussion that's been provoked in this thread: http://stackoverflow.com/questions/2597142/when-was-the-null-macro-not-0/2597232 Trying to have a serious back-and-forth discussion using comments under other people's replies is not easy or fun. So I'd like to hear what our C experts think without being restricted to 500 characters at a time. The C standard has precious few words to say about NULL and null pointer constants. There's only two relevant sections that I can find. First: 3.2.2.3 Pointers An integral constant expression with the value 0, or such an expression cast to type void * , is called a null pointer constant. If a null pointer constant is assigned to or compared for equality to a pointer, the constant is converted to a pointer of that type. Such a pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function. and second: 4.1.5 Common definitions <stddef.h> The macros are NULL which expands to an implementation-defined null pointer constant; The question is, can NULL expand to an implementation-defined null pointer constant that is different from the ones enumerated in 3.2.2.3? In particular, could it be defined as: #define NULL __builtin_magic_null_pointer Or even: #define NULL ((void*)-1) My reading of 3.2.2.3 is that it specifies that an integral constant expression of 0, and an integral constant expression of 0 cast to type void* must be among the forms of null pointer constant that the implementation recognizes, but that it isn't meant to be an exhaustive list. I believe that the implementation is free to recognize other source constructs as null pointer constants, so long as no other rules are broken. So for example, it is provable that #define NULL (-1) is not a legal definition, because in if (NULL) do_stuff(); do_stuff() must not be called, whereas with if (-1) do_stuff(); do_stuff() must be called; since they are equivalent, this cannot be a legal definition of NULL. But the standard says that integer-to-pointer conversions (and vice-versa) are implementation-defined, therefore it could define the conversion of -1 to a pointer as a conversion that produces a null pointer. In which case if ((void*)-1) would evaluate to false, and all would be well. So what do other people think? I'd ask for everybody to especially keep in mind the "as-if" rule described in 2.1.2.3 Program execution. It's huge and somewhat roundabout, so I won't paste it here, but it essentially says that an implementation merely has to produce the same observable side-effects as are required of the abstract machine described by the standard. It says that any optimizations, transformations, or whatever else the compiler wants to do to your program are perfectly legal so long as the observable side-effects of the program aren't changed by them. So if you are looking to prove that a particular definition of NULL cannot be legal, you'll need to come up with a program that can prove it. Either one like mine that blatantly breaks other clauses in the standard, or one that can legally detect whatever magic the compiler has to do to make the strange NULL definition work. Steve Jessop found an example of way for a program to detect that NULL isn't defined to be one of the two forms of null pointer constants in 3.2.2.3, which is to stringize the constant: #define stringize_helper(x) #x #define stringize(x) stringize_helper(x) Using this macro, one could puts(stringize(NULL)); and "detect" that NULL does not expand to one of the forms in 3.2.2.3. Is that enough to render other definitions illegal? I just don't know. Thanks!

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  • Create a JSON Array using Java

    - by Ankur
    Hi I want to create a JSON array. I have tried using: JSONArray jArray = new JSONArray(); while(itr.hasNext()){ int objId = itr.next(); jArray.put(objId, odao.getObjectName(objId)); } results = jArray.toString(); Note: odao.getObjectName(objId) retrieves a name based on the "object Id" which is called objId However I get a very funny looking array like [null,null,null,"SomeValue",null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,"AnotherValue",null,null,null,null,null,null,null,null,null,null,"SomethingElse","AnotherOne","LastOne"] With only "LastOne" being displayed when I retrieve it using jQuery. The Array SHould look like {["3":"SomeValue"],["40":"AnotherValue"],["23":"SomethingElse"],["9":"AnotherOne"],["1":"LastOne"]} The numbers aren't showing up at all for some reason in the array that I am getting

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  • Push Notification in non english languages

    - by jmall
    Hello, I have implemented successfully this code: http://stackoverflow.com/questions/1020762/does-anyone-know-how-to-write-an-apple-push-notification-provider-in-c It works great. But I have a question, can anybody help me how to send non english messages like Hebrew Or Arabic? If the string contains any non english characters, it is not sent. Thank you

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  • linq hierarchy problem

    - by Pratik
    I reterive a result from sql server as ProjectDetailID,ProjectID,ParentID,...,C1,C2,C3,... where C1 implies(=) companyOne, C2=CompanyTwo ... etc and dynamically can have 'n' companies For time being lets consider only 3 companies, So I get : ProjectDetailID,ProjectID,ParentID,C1,C2,C3 10,1,0,NULL,NULL,NULL 10,2,1,NULL,NULL,NULL 10,3,2,90,NULL,NULL 10,4,2,NULL,60,NULL 10,10,1,70,NULL,NULL 10,5,10,20,40,NULL 10,13,2,NULL,NULL,NULL I want from this following result using LINQ (C#) ProjectDetailID,ProjectID,ParentID,C1,C2,C3 10,1,0,180,100,NULL 10,2,1,90,60,NULL 10,3,2,90,NULL,NULL 10,4,2,NULL,60,NULL 10,10,1,90,40,NULL 10,5,10,20,40,NULL 10,13,2,NULL,NULL,NULL The problem is that at parent level i have null value for a company but at its child i have some value, which i keep on adding and have placed that in parent corresponding to that company only. I am not getting from where to start. Please share your ideas. And i am looking to do this in LINQ using C#

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  • Can non-IT people learn and take advantage of regular expressions? [closed]

    - by user1598390
    Often times, not-IT people has to deal with massive text data, clean it, filter it, modify it. Often times normal office tools like Excel lack the tools to make complex search and replace operations on text. Could this people benefit from regexps ? Can regexp be taught to them ? Are regular expressions the exclusive domain of programmers and unix/linux technicians ? Can they be learned by non-IT people, given regexps are not a programming language? Is this a valid or achievable goal to make some users regexp-literate through appopriate training ? Have you have any experiences on this issue? and if so, have it been successful ?

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  • How to have multiple tables with multiple joins

    - by williamsdb
    I have three tables that I need to join together and get a combination of results. I have tried using left/right joins but they don't give the desired results. For example: Table 1 - STAFF id name 1 John 2 Fred Table 2 - STAFFMOBILERIGHTS id staffid mobilerightsid rights --this table is empty-- Table 3 - MOBILERIGHTS id rightname 1 Login 2 View and what I need is this as the result... id name id staffid mobilerightsid rights id rightname 1 John null null null null 1 login 1 John null null null null 2 View 2 Fred null null null null 1 login 2 Fred null null null null 2 View I have tried the following : SELECT * FROM STAFFMOBILERIGHTS SMR RIGHT JOIN STAFF STA ON STA.STAFFID = SMR.STAFFID RIGHT JOIN MOBILERIGHTS MRI ON MRI.ID = SMR.MOBILERIGHTSID But this only returns two rows as follows: id name id staffid mobilerightsid rights id rightname null null null null null null 1 login null null null null null null 2 View Can what I am trying to achieve be done and if so how? Thanks

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  • Non modal "status" form

    - by David Jenings
    At the beginning of a section of C# code that could take several seconds to complete, I'd like to display a non modal form with a label that just says, "Please wait..." WaitForm myWaitForm = null; try { // if conditions suggest process will take awhile myWaitForm = new WaitForm(); myWaitForm.Show(); // do stuff } finally { if (myWaitForm != null) { myWaitForm.Hide(); myWaitForm.Dispose(); myWaitForm = null; } } The problem: the WaitForm doesn't completely display before the rest of the code ties up the thread. So I only see the frame of the form. In Delphi (my old stomping ground) I would call Application.ProcessMessages after the Show() Is there an equivalent in C#? Is there a canned "status" form that I can use in situations like this? Is there a better way to approach this? Thanks in advance. David Jennings

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  • using "IS" is better or checking for "NOT NULL"

    - by BDotA
    In C#.NET language: This style of coding is recommended or the one below it? if (sheet.Models.Data is GroupDataModel) { GroupDataModel gdm = (GroupDataModel)sheet.Models.Data; Group group = gdm.GetGroup(sheet.ActiveCell.Row.Index); if (group!=null && controller != null) { controller.CheckApplicationState(); } } or this one: var gdm = sheet.Models.Data as GroupDataModel; if (gdm != null) { Group group = gdm.GetGroup(sheet.ActiveCell.Row.Index); if (@group!=null && controller != null) { controller.CheckApplicationState(); } }

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  • C#.NET Forms programing - Modal and Non-Modal forms problem

    - by Povilas
    Hello, I have a problem with modality of the forms under C#.NET. Let's say I have main form #0 (see the image below). This form represents main application form, where user can perform various operations. However, from time to time, there is a need to open additional non-modal form to perform additional main application functionality supporting tasks. Let's say this is form #1 in the image. On this #1 form there might be opened few additional modal forms on top of each other (#2 form in the image), and at the end, there is a progress dialog showing a long operation progress and status, which might take from few minutes up to few hours. The problem is that the main form #0 is not responsive until you close all modal forms (#2 in the image). I need that the main form #0 would be operational in this situation. However, if you open a non-modal form in form #2, you can operate with both modal #2 form and newly created non modal form. I need the same behavior between the main form #0 and form #1 with all its child forms. Is it possible? Or am I doing something wrong? Maybe there is some kind of workaround, I really would not like to change all ShowDialog calls to ShowDialog... Thanks, Povilas

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  • WinForms programing - Modal and Non-Modal forms problem

    - by Povilas
    I have a problem with modality of the forms under C#.NET. Let's say I have main form #0 (see the image below). This form represents main application form, where user can perform various operations. However, from time to time, there is a need to open additional non-modal form to perform additional main application functionality supporting tasks. Let's say this is form #1 in the image. On this #1 form there might be opened few additional modal forms on top of each other (#2 form in the image), and at the end, there is a progress dialog showing a long operation progress and status, which might take from few minutes up to few hours. The problem is that the main form #0 is not responsive until you close all modal forms (#2 in the image). I need that the main form #0 would be operational in this situation. However, if you open a non-modal form in form #2, you can operate with both modal #2 form and newly created non modal form. I need the same behavior between the main form #0 and form #1 with all its child forms. Is it possible? Or am I doing something wrong? Maybe there is some kind of workaround, I really would not like to change all ShowDialog calls to ShowDialog...

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  • Is it a bad idea if equals(null) throws NullPointerException instead?

    - by polygenelubricants
    The contract of equals with regards to null, is as follows: For any non-null reference value x, x.equals(null) should return false. This is rather peculiar, because if o1 != null and o2 == null, then we have: o1.equals(o2) // returns false o2.equals(o1) // throws NullPointerException The fact that o2.equals(o1) throws NullPointerException is a good thing, because it alerts us of programmer error. And yet, that error would not be catched if for various reasons we just switched it around to o1.equals(o2), which would just "silently fail" instead. So the questions are: Why is it a good idea that o1.equals(o2) should return false instead of throwing NullPointerException? Would it be a bad idea if wherever possible we rewrite the contract so that anyObject.equals(null) always throw NullPointerException instead?

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  • How to index a date column with null values?

    - by Heinz Z.
    How should I index a date column when some rows has null values? We have to select rows between a date range and rows with null dates. We use Oracle 9.2 and higher. Options I found Using a bitmap index on the date column Using an index on date column and an index on a state field which value is 1 when the date is null Using an index on date column and an other granted not null column My thoughts to the options are: to 1: the table have to many different values to use an bitmap index to 2: I have to add an field only for this purpose and to change the query when I want to retrieve the null date rows to 3: locks tricky to add an field to an index which is not really needed What is the best practice for this case? Thanks in advance Some infos I have read: Oracle Date Index When does Oracle index null column values?

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  • How to get the first non-null value in Java?

    - by froadie
    Is there a Java equivalent of SQL's COALESCE function? That is, is there any way to return the first non-null value of several variables? e.g. Double a = null; Double b = 4.4; Double c = null; I want to somehow have a statement that will return the first non-null value of a, b, and c - in this case, it would return b, or 4.4. (Something like the sql method - return COALESCE(a,b,c)). I know that I can do it explicitly with something like: return a != null ? a : (b != null ? b : c) But I wondered if there was any built-in, accepted function to accomplish this.

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  • Find all those columns which have only null values, in a MySQL table

    - by Robin v. G.
    The situation is as follows: I have a substantial number of tables, with each a substantial number of columns. I need to deal with this old and to-be-deprecated database for a new system, and I'm looking for a way to eliminate all columns that have - apparently - never been in use. I wanna do this by filtering out all columns that have a value on any given row, leaving me with a set of columns where the value is NULL in all rows. Of course I could manually sort every column descending, but that'd take too long as I'm dealing with loads of tables and columns. I estimate it to be 400 tables with up to 50 (!) columns per table. Is there any way I can get this information from the information_schema? EDIT: Here's an example: column_a column_b column_c column_d NULL NULL NULL 1 NULL 1 NULL 1 NULL 1 NULL NULL NULL NULL NULL NULL The output should be 'column_a' and 'column_c', for being the only columns without any filled in values.

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  • Non Blocking Keyboard on WinCE accessing the virtual keyboard

    - by Jan H.
    Hello Guys, I am desperately looking for a solution that enables me to read keyboard events in a non blocking way. These Keyboard events are generated by a VIRTUAL KEYBOARD that comes with the WinCE device. I have a console application running in C++, where the user is asked to navigate via 'ESC', 'U' and other characters through the menu. I first tried to use fread and stdin and realised that it is blocking call and waits for a carriage return. Then I tried to hook up to the windows message WM_KEYUP, but I never recieve this windows message. Furthermore I tried to use QtGUI together with the event QKeyEvent, but I never recieve any event. I wonder if it is in general possible to recieve non-blocking keyboard events on a WinCE device. I would be glad if you have any suggestions! Cheers, Jan

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  • Convert non-breaking spaces to spaces in Ruby

    - by CoolAJ86
    I have cases where user-entered data from an html textarea or input is sometimes sent with \u00a0 (non-breaking spaces) instead of spaces when encoded as utf-8 json. I believe that to be a bug in Firefox, as I know that the user isn't intentionally putting in non-breaking spaces instead of spaces. There are also two bugs in Ruby, one of which can be used to combat the other. For whatever reason \s doesn't match \u00a0 However [^[:print:]] (which definitely should not match) and \xC2\xA0 both will match, but I consider those to be less-than-ideal ways to deal with the issue. Are there other recommendations for getting around this issue?

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  • What's the Matlab equivalent of NULL, when it's calling COM/ActiveX methods?

    - by David M
    Hi, I maintain a program which can be automated via COM. Generally customers use VBS to do their scripting, but we have a couple of customers who use Matlab's ActiveX support and are having trouble calling COM object methods with a NULL parameter. They've asked how they do this in Matlab - and I've been scouring Mathworks' COM/ActiveX documentation for a day or so now and can't figure it out. Their example code might look something like this: function do_something() OurAppInstance = actxserver('Foo.Application'); OurAppInstance.Method('Hello', NULL) end where NULL is where in another language, we'd write NULL or nil or Nothing, or, of course, pass in an object. The problem is this is optional (and these are implemented as optional parameters in most, but not all, cases) - these methods expect to get NULL quite often. They tell me they've tried [] (which from my reading seemed the most likely) as well as '', Nothing, 'Nothing', None, Null, and 0. I have no idea how many of those are even valid Matlab keywords - certainly none work in this case. Can anyone help? What's Matlab's syntax for a null pointer / object for use as a COM method parameter? Update: Thanks for all the replies so far! Unfortunately, none of the answers seem to work, not even libpointer. The error is the same in all cases: Error: Type mismatch, argument 2 This parameter in the COM type library is described in RIDL as: HRESULT _stdcall OurMethod([in] BSTR strParamOne, [in, optional] OurCoClass* oParamTwo, [out, retval] VARIANT_BOOL* bResult); The coclass in question implements a single interface descending from IDispatch.

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  • Why can one null return be tested but another throws an exception?

    - by mickey
    I want to test if an xml attribute is present. Given this: XmlAttributeCollection PG_attrColl = SomeNodeorAnother.Attributes; This first test works: if (null != PG_attrColl["SomeAttribute"]) "GetNamedItem" is supposed to return null, but the following test throws an exception complaining about the null it returns. if (null != PG_attrColl.GetNamedItem("SomeAttribute").Value;) Why the difference? Just curious.

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