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• Codeigniter only loads the default controller

  - by fh47331

  I am very new to CodeIgniter, but have been programming PHP for ages. I'm writing some software at the moment and using CI for the first time with it. The default controller is set to the first controller I want to action call 'login' (the controller is login.php, the view is login.php. When the form is submitted it calls the 'authenticate' controller. This executes fine, process the login data correctly and then does a redirect command (without any output to the screen prior) to the next page in this case 'newspage'. The problem is that the redirect, never reaches 'newspage' but the default controller runs again. It doesn't matter what I put ... ht tp://domain.name/anything ... (yes im using .htaccess to remove the index.php) the anything never gets called, just the default controller. I have left the standard 'welcome.php' controller and 'welcome_message.php' in the folders and even putting ht tp://domain.name/welcome all I get is the login screen! (Obviously there shouldn't be a space between the http - thats just done so it does not show as a hyperlink!) Can anyone tell me what i've done wrong!

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• What changed in the DataGrid that means it won't work anymore?

  - by Jeff Yates

  I have a Silverlight app with a DataGrid containing some custom columns and all was working well. Then I updated to Silverlight 3 tools for VS 2008 SP1 and rebuilt it. Now it has the following problems: Rows aren't added when the collection is modified. The ItemsSource property is (and always has been) set to an ObservableCollection instance, which notifies when its contents change. This worked fine for Silverlight 2. However, in Silverlight 3 to get this working at all, I now have to null and then re-set ItemsSource - this seems like I'm hiding a bigger issue but I can't work out what that might be. I cannot select a row or a cell anymore. If I'm lucky, I can select one whole row before it stops working. I can't edit anything. I suspect this is related to the previous point. I'll post some source when I am able, but first I have to strip it down to the bare minimum. In the meantime, I was hoping someone might have some idea of what may be going on here. My gut feeling on the second two points is that my bindings are no longer working, but that's just a guess and if it is the case, I have no idea which ones. Thanks for any help anyone might be able to provide. Update So, I finally reduced my problem down to a simple works/doesn't work comparison. The problem seems to occur if I override Equals in my element type. As soon as I do that, something happens strangely in the ObservableCollection that contains that type, it seems, and my application breaks. To make it more interesting, there is a check to make sure that duplicate items don't even get close to being added to the collection. I don't exactly know why ObservableCollection needs to compare equality when inserting items (the stack trace indicates it is using IndexAt) but this seems to cause the issue. So, any thoughts?

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• How to face observable object containing an observable field

  - by iseek

  Hello, I need a hint concerning MVC and Observer-Pattern. For example a model contains the classes "Address" and "Person". The Address class contains the fields street:String, zipcode:String, location:String. Whereas the Person class contains the fields name:String, firstName:String, address:Address. My approach so far looks something like this: Both, Address and Person are observable. If one of their setters is being called, I validate whether the current value and new value differ. Only in this case an update event is fired. The event contains the source, the name of the changed field, the old and the new value. The class for the view contains text fields to display and edit the information of a person: name, firstname, street, zipcode, location. It knows the Person model and is an subscribed observer for the person. So it gets the update events from the person object. My questions concerns the address field from type Address in the person class, since an address is observable on its own. If the view gets an update event from person when a new address has been set, I can update all of the address related fields in the view. But what if a field of the address changes? Should the view also register for update events from the address? Any hints about common design approaches would be appreciated. Greetings.

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• Need help extrapolating Java code

  - by Berlioz

  If anyone familiar with Rebecca Wirfs-Brock, she has a piece of Java code found in her book titled, Object Design: Roles, Responsibilities, and Collaborations. Here is the quote Applying Double Dispatch to a Specific Problem To implement the game Rock, Paper, Scissors we need to write code that determines whether one object “beats” another. The game has nine possible outcomes based on the three kinds of objects. The number of interactions is the cross product of the kinds of objects. Case or switch statements are often governed by the type of data that is being operated on. The object-oriented language equivalent is to base its actions on the class of some other object. In Java, it looks like this Here is the piece of Java code on page 16 ' import java.util.*; import java.lang.*; public class Rock { public static void main(String args[]) { } public static boolean beats(GameObject object) { if (object.getClass.getName().equals("Rock")) { result = false; } else if (object.getClass.getName().equals("Paper")) { result = false; } else if(object.getClass.getName().equals("Scissors")) { result = true; } return result; } }' ===This is not a very good solution. First, the receiver needs to know too much about the argument. Second, there is one of these nested conditional statements in each of the three classes. If new kinds of objects could be added to the game, each of the three classes would have to be modified. Can anyone share with me how to get this "less than optimal" piece of code to work in order to see it 'working'. She proceeds to demonstrate a better way, but I will spare you. Thanks

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• Why does my DIV clip its child DIV when jQuery moves it in IE?

  - by Ben Saufley

  I have two divs, both with position:absolute;, one inside the other. The parent isn't in a place where it can be set as position:relative without an extra layer of complexity (there are a lot of other elements around it that I'd have to account for to put it where it needs to be, which is at the very top of the page, over everything). The child element is made to stick off the bottom of the parent. In Chrome, Safari, Firefox, it all works splendidly. In IE, it works until jQuery moves the parent element - at which point the parent element clips the child, so you can barely see the top of the child. I feel like I've read about this, about IE clipping child elements, but I can't seem to find an answer that applies to my case. It's pretty simple, basically: <div id="parent" style="position:absolute;top:0;left:0;"> [content] <div id="tab" style="position:absolute;bottom:-30px;left:0;width:64px;height:32px;background-image:(...);"></div> </div> <script> $(document).ready( function() { $("#tab").click(function() { $("#parent").animate({"top":"-50px"},300); }); }); </script>

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• How to define default values optional fields in play framework forms?

  - by natalinobusa

  I am implementing a web api using the scala 2.0.2 play framework. I would like to extract and validate a number of get parameters. And for this I am using a play "form" which allows me to define optional fields. Problem: For those optional fields, I need to define a default value if the parameter is not passed. The code is intended to parse correctly these three use cases: /test?top=abc (error, abc is not an integer) /test?top=123 (valid, top is 123) /test (valid, top is 42 (default value)) I have come up with the following code: def test = Action { implicit request => case class CData(top:Int) val p = Form( mapping( "top" -> optional(number) )((top) => CData($top.getOrElse(42))) ((cdata:CData) => Some(Some(cdata.top))) ).bindFromRequest() Ok("all done.") } The code works, but it's definitely not elegant. There is a lot of boiler plate going on just to set up a default value for a missing request parameter. Can anyone suggest a cleaner and more coincise solution?

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• How to handle build rule with unknown targets in OMake when target list generator is built

  - by Michael E

  I have a project which uses OMake for its build system, and I am trying to handle a rather tough corner case. I have some definition files and a tool which can take these definition files and create GraphViz files. There are two problems, though: Each definition file can produce multiple graphs, and the list of graphs it can produce is encoded in the file. My dump tool does have a -list option which lists all the graphs a definition file will produce. This dump tool is built in the source tree. I want this list available in the OMakefile so I can use other rules to convert the DOT files to SVG, and have a phony target depend on all the SVGs (goal: a single build command which builds SVG descriptions of all my graphs). If I only had the first problem, it would be easy - I would run the tool to build a list, and then use that list to build a target which invokes the dumper to output the GraphViz files. However, I am rather stuck on forcing the dump tool to be built before it is needed. If this were make, I would just run make recursively to build the dump tool. OMake does not allow recursive invocation, however, and the build function is only usable from osh. Any suggestions for a good solution to this problem?

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• XPath and XML: Multiple namespaces

  - by emragins

  So I have a document that looks like <a xmlns="uri1" xmlns:pre2="uri2"> <b xmlns:pre3="uri3"> <pre3:c> <stuff></stuff> <goes></goes> <here></here> </pre3:c> <pre3:d xmlns="uri4"> <under></under> <the></the> <tree></tree> </pre3:d> </b> </a> I want an xpath expression that will get me <under>. This has a namespaceURI of uri4. Right now my expression looks like: //ns:a/ns:b/pre3:d/pre4:under I have the namespace manager add 'ns' for the default namespace (uri1 in this case) and I have it defined with pre2, pre3, and pre4 for uri2, uri3, and uri4 respectively. I get the error "Expression must evaluate to a node-set." I know that the node exists. I know that everything up until the pre4:under in my xpath works fine as I use it in the rest of the document with no issues. It's the additional pre4:under that causes the error, and I'm not sure why. Any ideas? Thanks.

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• Viewing recently edited etherpads (note new 'etherpad' tag for open sourced etherpad code!)

  - by dreeves

  Prescript: The amazing etherpad was recently open sourced. Get it here: http://code.google.com/p/etherpad. This is the first question that I know of on StackOverflow about the etherpad code. If you're part of the etherpad open source community, you might want to subscribe to the RSS feed for questions tagged 'etherpad' just in case this catches on! My actual question, which assumes you have etherpad installed on your own server: First, here's a query to view recently edited pads: SELECT id,lastWriteTime,creationTime,headRev FROM PAD_SQLMETA ORDER BY lastWriteTime, headRev; Or, if you want to run it from a unix prompt: mysql -u root -pPASSWD etherpad -e "select id,lastWriteTime,creationTime,headRev from PAD_SQLMETA order by lastWriteTime, headRev" That's handy, however lastWriteTime actually gets updated every time someone so much as views a pad in their browser. I'd rather sort the pads by when they were actually last edited. There's probably a fancy SQL query involving a join with another table that would show actual last edit time. Does anyone know what that is? Alternatively, you could have a script that notices when headRev changes but that doesn't seem like the cleanest way to do it.

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• waveInProc / Windows audio question...

  - by BTR

  I'm using the Windows API to get audio input. I've followed all the steps on MSDN and managed to record audio to a WAV file. No problem. I'm using multiple buffers and all that. I'd like to do more with the buffers than simply write to a file, so now I've got a callback set up. It works great and I'm getting the data, but I'm not sure what to do with it once I have it. Here's my callback... everything here works: // Media API callback void CALLBACK AudioRecorder::waveInProc(HWAVEIN hWaveIn, UINT uMsg, DWORD dwInstance, DWORD dwParam1, DWORD dwParam2) { // Data received if (uMsg == WIM_DATA) { // Get wav header LPWAVEHDR mBuffer = (WAVEHDR *)dwParam1; // Now what? for (unsigned i = 0; i != mBuffer->dwBytesRecorded; ++i) { // I can see the char, how do get them into my file and audio buffers? cout << mBuffer->lpData[i] << "\n"; } // Re-use buffer mResultHnd = waveInAddBuffer(hWaveIn, mBuffer, sizeof(mInputBuffer[0])); // mInputBuffer is a const WAVEHDR * } } // waveInOpen cannot use an instance method as its callback, // so we create a static method which calls the instance version void CALLBACK AudioRecorder::staticWaveInProc(HWAVEIN hWaveIn, UINT uMsg, DWORD_PTR dwInstance, DWORD_PTR dwParam1, DWORD_PTR dwParam2) { // Call instance version of method reinterpret_cast<AudioRecorder *>(dwParam1)->waveInProc(hWaveIn, uMsg, dwInstance, dwParam1, dwParam2); } Like I said, it works great, but I'm trying to do the following: Convert the data to short and copy into an array Convert the data to float and copy into an array Copy the data to a larger char array which I'll write into a WAV Relay the data to an arbitrary output device I've worked with FMOD a lot and I'm familiar with interleaving and all that. But FMOD dishes everything out as floats. In this case, I'm going the other way. I guess I'm basically just looking for resources on how to go from LPSTR to short, float, and unsigned char. Thanks much in advance!

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• Java: fastest way to do random reads on huge disk file(s)

  - by cocotwo

  I've got a moderately big set of data, about 800 MB or so, that is basically some big precomputed table that I need to speed some computation by several orders of magnitude (creating that file took several mutlicores computers days to produce using an optimized and multi-threaded algo... I do really need that file). Now that it has been computed once, that 800MB of data is read only. I cannot hold it in memory. As of now it is one big huge 800MB file but splitting in into smaller files ain't a problem if it can help. I need to read about 32 bits of data here and there in that file a lot of time. I don't know before hand where I'll need to read these data: the reads are uniformly distributed. What would be the fastest way in Java to do my random reads in such a file or files? Ideally I should be doing these reads from several unrelated threads (but I could queue the reads in a single thread if needed). Is Java NIO the way to go? I'm not familiar with 'memory mapped file': I think I don't want to map the 800 MB in memory. All I want is the fastest random reads I can get to access these 800MB of disk-based data. btw in case people wonder this is not at all the same as the question I asked not long ago: http://stackoverflow.com/questions/2346722/java-fast-disk-based-hash-set

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• How to do an additional search on archive in rails if record not found, by extending model?

  - by Nick Gorbikoff

  Hello, I was wondering if somebody knows an elegant solution to the following: Suppose I have a table that holds orders, with a bunch of data. So I'm at 1M records, and searches begin to take time. So I want to speed it up by archiving some data that is more than 3 years old - saving it into a table called orders-archive, and then purging them from the orders table. So if we need to research something or customer wants to pull older information - they still can, but 99% of the lookups are done on the orders no older than a year and a half - so there is no reason to keep looking through older data all the time. These move & purge operations can be then croned to be done on a weekly basis. I already did some tests and I know that I will slash my search times by about 4 times. So far so good, right? However I was thinking about how to implement older archival lookups and the only reasonable thing I can think of is some sort of if-else If not found in orders, do a search in orders-archive. However - I have about 20 tables that I want to archive and god knows how many searches / finds are done through out the code, that I don't want to modify. So I was wondering if there is an elegant rails-way solution to this problem, by extending a model somehow? Has anyone dealt with similar case before? Thank you.

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• Entity framework with Linq to Entities performance

  - by mare

  If I have a static method like this public static string GetTicClassificationTitle(string classI, string classII, string classIII) { using (TicDatabaseEntities ticdb = new TicDatabaseEntities()) { var result = from classes in ticdb.Classifications where classes.ClassI == classI where classes.ClassII == classII where classes.ClassIII == classIII select classes.Description; return result.FirstOrDefault(); } } and use this method in various places in foreach loops or just plain calling it numerous times, does it create and open new connection every time? If so, how can I tackle this? Should I cache the results somewhere, like in this case, I would cache the entire Classifications table in Memory Cache? And then do queries vs this cached object? Or should I make TicDatabaseEntities variable static and initialize it at class level? Should my class be static if it contains only static methods? Because right now it is not.. Also I've noticed that if I return result.First() instead of FirstOrDefault() and the query does not find a match, it will issue an exception (with FirstOrDefault() there is no exception, it returns null). Thank you for clarification.

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• Returning JSON object from an ASP.NET page

  - by Emin

  Hi, In my particular situation, I have couple of solutions to my problem. I want to find out which one is more feasible. In this case, I can also achieve my goal by returning a JSON object from my server side code, however, I do not know how it is done and what is the best way of doing it. First off, I don't need a full aspx page as I only need a response returned from code. So, do I use web services? handler? or is there any other specific way you people do this? Is this solution feasible? do I build the JSON string using the StringBuilder class and inject that string into the target aspx page? are there any precautions? or things that I should be aware of? I appreciate your ideas. Regards, Kemal ------------UPDATE !------------ Suppose I have a JSON object in my userlist.aspx page. which then I use it with jQuery... {"menu": { "id": "color1", "value": "color", "popup": { "menuitem": [ {"value": "Red"}, {"value": "Green"}, {"value": "Yellow"} ] } }} // example taken from the json.org/example page now when I want to add a new menuitem from my aspx page, what do I do... I think this way my question is more specific... Lets assume I create a new string in my aspx code, as such "{"value": "Blue"}. Now how do I inject this into the already existing itemlist in the target page? or is this not the correct approach to this kind of situation? if not how else can it be achieved? Also if I wanted to fire a jquery event when a new item is added to this list, how is this achieved?

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• Matlab fft function

  - by CTZStef

  The code below is from the Matlab 2011a help about fft function. I think there is a problem here : why do they multiply t(1:50) by Fs, and then say it's time in millisecond ? Certainly, it happens to be true in this very particular case, but change the value of Fs to, say, 2000, and it won't work anymore, obviously because of this factor of 2. Right ? Quite misleading, isn't it ? What do I miss ? Fs = 1000; % Sampling frequency T = 1/Fs; % Sample time L = 1000; % Length of signal t = (0:L-1)*T; % Time vector % Sum of a 50 Hz sinusoid and a 120 Hz sinusoid x = 0.7*sin(2*pi*50*t) + sin(2*pi*120*t); y = x + 2*randn(size(t)); % Sinusoids plus noise plot(Fs*t(1:50),y(1:50)) title('Signal Corrupted with Zero-Mean Random Noise') xlabel('time (milliseconds)') Clearer with this : fs = 2000; % Sampling frequency T = 1 / fs; % Sample time L = 1000; % Length of signal t2 = (0:L-1)*T; % Time vector f = 50; % signal frequency s2 = sin(2*pi*f*t2); figure, plot(fs*t2(1:50),s2(1:50)); % NOT good figure, plot(t2(1:50),s2(1:50)); % good

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• Prawn image position

  - by John

  I'm trying to layout 6 images per page with prawn in Ruby: case (idx % 6) # ugly when 0 : (pdf.start_new_page; pdf.image img, :position => :left, :vposition => :top, :width => 270) when 1 : pdf.image img, :position => :right, :vposition => :top, :width => 270 when 2 : pdf.image img, :position => :left, :vposition => :center, :width => 270 when 3 : pdf.image img, :position => :right, :vposition => :center, :width => 270 when 4 : pdf.image img, :position => :left, :vposition => :bottom, :width => 270 when 5 : pdf.image img, :position => :right, :vposition => :bottom, :width => 270 end Not sure what I'm doing wrong, but it prints the first 3 images to the PDF, then creates a new page and prints the last three: Page 1: <img> <img> <blank> <blank> <blank> <blank> Page 2: <blank> <blank> <blank> <img> <img> <img> Any suggestions would help.

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• JDBC programms running long time performance issue

  - by phyerbarte

  My program has an issue with Oracle query performance, I believe the SQL have good performance, because it returns quickly in SQLPlus. But when my program has been running for a long time, like 1 week, the SQL query (using JDBC) becomes slower (In my logs, the query time is much longer than when I originally started the program). When I restart my program, the query performance comes back to normal. I think it is could be something wrong with the way I use the preparedStatement, because the SQL I'm using does not use placeholders "?" at all. Just a complex select query. The query process is done by a util class. Here is the pertinent code building the query: public List<String[]> query(String sql, String[] args) { Connection conn = null; conn = openConnection(); conn.setAutocommit(true); .... PreparedStatement preStatm = null; ResultSet rs = null; ....//set preparedstatment arg code rs = preStatm.executeQuery(); .... finally{ //close rs //close prestatm //close connection } } In my case, the args is always null, so it just passes a query sql to this query method. Is that possible this way could slow down the DB query after program long time running? Or I should use statement instead, or just pass args with "?" in the SQL? How can I find out the root cause for my issue? Thanks.

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• PHP Object Creation and Memory Usage

  - by JohnO

  A basic dummy class: class foo { var $bar = 0; function foo() {} function boo() {} } echo memory_get_usage(); echo "\n"; $foo = new foo(); echo memory_get_usage(); echo "\n"; unset($foo); echo memory_get_usage(); echo "\n"; $foo = null; echo memory_get_usage(); echo "\n"; Outputs: $ php test.php 353672 353792 353792 353792 Now, I know that PHP docs say that memory won't be freed until it is needed (hitting the ceiling). However, I wrote this up as a small test, because I've got a much longer task, using a much bigger object, with many instances of that object. And the memory just climbs, eventually running out and stopping execution. Even though these large objects do take up memory, since I destroy them after I'm done with each one (serially), it should not run out of memory (unless a single object exhausts the entire space for memory, which is not the case). Thoughts?

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• What are salesforce.com and Apex like as an application development platform?

  - by mhollers

  I have recently discovered that salesforce.com is much more than an online CRM after coming across a Morrison's Case Study in which they develop a works management application. I've been trying it out with a view to recreating our own Works Management system on the platform. My background is in Microsoft and .Net, and the obvious 1st choice would be asp.net. However, there's only really myself with .net experience and my manager with a more legacy Synergy programming background, and I am self taught and am looking at evaluating other RAD options (eg Ironspeed). the nature of the business is in the main 2-5 concurrent construction type contracts that run for 3-5 yrs each, each requiring 15-50 system users. Traditionally we have used our character based Works Mangement system for everything and tweaked it for each contract. The Salesforce licensing model on the face of it suits this sort of flexibilty, but I'm worried about the development flexibilty/learning curve and all the issues that surround lock-in. There doesn't seem to be much neutral sober analysis of the platform on the web that isn't salesforce's own material/blogs Has anyone any experience of developing an application on salesforce as compared to the more 'traditional' .Net route?

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• How long is the time frame between context switches on Windows?

  - by mattcodes

  Reading CLR via C# 2.0 (I dont have 3.0 with me at the moment) Is this still the case: If there is only one CPU in a computer, only one thread can run at any one time. Windows has to keep track of the thread objects, and every so often, Windows has to decide which thread to schedule next to go to the CPU. This is additional code that has to execute once every 20 milliseconds or so. When Windows makes a CPU stop executing one thread's code and start executing another thread's code, we call this a context switch. A context switch is fairly expensive because the operating system has to: So circa CLR via C# 2.0 lets say we are on Pentium 4 2.4ghz 1 core non-HT, XP. Every 20 milliseconds? Where a CLR thread or Java thread is mapped to an OS thread only a maximum of 50 threads per second may get a chance to to run? I've read that context switching is very fast in mircoseconds here on SO, but how often roughly (magnitude style guesses) will say a modest 5 year old server Windows 2003 Pentium Xeon single core give the OS the opportunity to context switch? 20ms in the right area? I dont need exact figures I just want to be sure that's in the right area, seems rather long to me.

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• Is there any point in using a volatile long?

  - by Adamski

  I occasionally use a volatile instance variable in cases where I have two threads reading from / writing to it and don't want the overhead (or potential deadlock risk) of taking out a lock; for example a timer thread periodically updating an int ID that is exposed as a getter on some class: public class MyClass { private volatile int id; public MyClass() { ScheduledExecutorService execService = Executors.newScheduledThreadPool(1); execService.scheduleAtFixedRate(new Runnable() { public void run() { ++id; } }, 0L, 30L, TimeUnit.SECONDS); } public int getId() { return id; } } My question: Given that the JLS only guarantees that 32-bit reads will be atomic is there any point in ever using a volatile long? (i.e. 64-bit). Caveat: Please do not reply saying that using volatile over synchronized is a case of pre-optimisation; I am well aware of how / when to use synchronized but there are cases where volatile is preferable. For example, when defining a Spring bean for use in a single-threaded application I tend to favour volatile instance variables, as there is no guarantee that the Spring context will initialise each bean's properties in the main thread.

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• "Function object is unsubscriptable" in basic integer to string mapping function

  - by IanWhalen

  I'm trying to write a function to return the word string of any number less than 1000. Everytime I run my code at the interactive prompt it appears to work without issue but when I try to import wordify and run it with a test number higher than 20 it fails as "TypeError: 'function' object is unsubscriptable". Based on the error message, it seems the issue is when it tries to index numString (for example trying to extract the number 4 out of the test case of n = 24) and the compiler thinks numString is a function instead of a string. since the first line of the function is me defining numString as a string of the variable n, I'm not really sure why that is. Any help in getting around this error, or even just help in explaining why I'm seeing it, would be awesome. def wordify(n): # Convert n to a string to parse out ones, tens and hundreds later. numString = str(n) # N less than 20 is hard-coded. if n < 21: return numToWordMap(n) # N between 21 and 99 parses ones and tens then concatenates. elif n < 100: onesNum = numString[-1] ones = numToWordMap(int(onesNum)) tensNum = numString[-2] tens = numToWordMap(int(tensNum)*10) return tens+ones else: # TODO pass def numToWordMap(num): mapping = { 0:"", 1:"one", 2:"two", 3:"three", 4:"four", 5:"five", 6:"six", 7:"seven", 8:"eight", 9:"nine", 10:"ten", 11:"eleven", 12:"twelve", 13:"thirteen", 14:"fourteen", 15:"fifteen", 16:"sixteen", 17:"seventeen", 18:"eighteen", 19:"nineteen", 20:"twenty", 30:"thirty", 40:"fourty", 50:"fifty", 60:"sixty", 70:"seventy", 80:"eighty", 90:"ninety", 100:"onehundred", 200:"twohundred", 300:"threehundred", 400:"fourhundred", 500:"fivehundred", 600:"sixhundred", 700:"sevenhundred", 800:"eighthundred", 900:"ninehundred", } return mapping[num] if __name__ == '__main__': pass

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• "Invalid use of Null" when using Str() with a Null Recordset field, but Str(Null) works fine

  - by Mike Spross

  I'm banging my head against the wall on this one. I was looking at some old database reporting code written in VB6 and case across this line (the code is moving data from a "source" database into a reporting database): rsTarget!VehYear = Trim(Str(rsSource!VehYear)) When rsSource!VehYear is Null, the above line generates an "Invalid use of Null" run-time error. If I break on the above line and type the following in the Immediate pane: ?rsSource!VehYear It outputs Null. Fine, that makes sense. Next, I try to reproduce the error: ?Str(rsSource!VehYear) I get an "Invalid use of Null" error. However, if I type the following into the Immediate window: ?Str(Null) I don't get an error. It simply outputs Null. If I repeat the same experiment with Trim() instead of Str(), everything works fine. ?Trim(rsSource!VehYear) returns Null, as does ?Trim(Null). No run-time errors. So, my question is, how can Str(rsSource!VehYear) possibly throw an "Invalid use of Null" error when Str(Null) does not, when I know that rsSource!VehYear is equal to Null?

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• What about parallelism across network using multiple PCs?

  - by MainMa

  Parallel computing is used more and more, and new framework features and shortcuts make it easier to use (for example Parallel extensions which are directly available in .NET 4). Now what about the parallelism across network? I mean, an abstraction of everything related to communications, creation of processes on remote machines, etc. Something like, in C#: NetworkParallel.ForEach(myEnumerable, () => { // Computing and/or access to web ressource or local network database here }); I understand that it is very different from the multi-core parallelism. The two most obvious differences would probably be: The fact that such parallel task will be limited to computing, without being able for example to use files stored locally (but why not a database?), or even to use local variables, because it would be rather two distinct applications than two threads of the same application, The very specific implementation, requiring not just a separate thread (which is quite easy), but spanning a process on different machines, then communicating with them over local network. Despite those differences, such parallelism is quite possible, even without speaking about distributed architecture. Do you think it will be implemented in a few years? Do you agree that it enables developers to easily develop extremely powerfull stuff with much less pain? Example: Think about a business application which extracts data from the database, transforms it, and displays statistics. Let's say this application takes ten seconds to load data, twenty seconds to transform data and ten seconds to build charts on a single machine in a company, using all the CPU, whereas ten other machines are used at 5% of CPU most of the time. In a such case, every action may be done in parallel, resulting in probably six to ten seconds for overall process instead of forty.

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• Issue with GCD and too many threads

  - by dariaa

  I have an image loader class which provided with NSURL loads and image from the web and executes completion block. Code is actually quite simple - (void)downloadImageWithURL:(NSString *)URLString completion:(BELoadImageCompletionBlock)completion { dispatch_async(_queue, ^{ // dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ UIImage *image = nil; NSURL *URL = [NSURL URLWithString:URLString]; if (URL) { image = [UIImage imageWithData:[NSData dataWithContentsOfURL:URL]]; } dispatch_async(dispatch_get_main_queue(), ^{ completion(image, URLString); }); }); } When I replace dispatch_async(_queue, ^{ with commented out dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ Images are loading much faster, wich is quite logical (before that images would be loaded one at a time, now a bunch of them are loading simultaneously). My issue is that I have perhaps 50 images and I call downloadImageWithURL:completion: method for all of them and when I use global queue instead of _queue my app eventually crashes and I see there are 85+ threads. Can the problem be that my calling dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0) 50 times in a row makes GCD create too many threads? I thought that gcd handles all the treading and makes sure the number of threads is not huge, but if it's not the case is there any way I can influence number of threads?

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