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• Which java technology should i use , if i want to build website with thousands of users

  - by Mirage

  Sorry for asking this question but i searched all java realted question but i got more confused. I am still not clear what should i start with 1)My main is build website in Java because someone told me that there are some machine learning or AI libraries available in java which i can use in java. so i decided to use that as it can reduce my work. Now i don't know java at all some people say following are used to build websites like 1)Servelts alone can build webiste 2)JSP alone can also build webiste 3)Struts 4)Spring with Hibernate 5)SEAM 6)JEE also for webistes Now guys i am confused where should i start from. Where does core JAVA fits in here. i was thinking of perhaps learning python as i know i have to learn Python only not its variations SO please guide me one thing which can solve my basic purpose of using ready made libraries of AI If i can do that with JSP then i will start with that. But i need to learn all of them then its better if can start learning python. I have 5 months to finish webiste. I really don't know why there are many braches of java for doing one thing Please help me.

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• Python - wxPython custom button -> unbound method __init__()? what?

  - by Wallter

  After looking at questions like this it doesn't make sense that my __init__(self, parrent, id) would be throwing a unbound error? help? main.py import wx from customButton import customButton from wxPython.wx import * class MyFrame(wx.Frame): def __init__(self, parent, ID, title): wxFrame.__init__(self, parent, ID, title, wxDefaultPosition, wxSize(400, 400)) # Non-important code here... # This is the first declaration of the Button1 # This is also where the ERROR is thrown. # Omitting this line causes the window to execute # flawlessly. self.Button1 = customButton.__init__(self, parent, -1) # ... finishes in a basic wx.program style... customButton.py # I've included all of the code in the file # because have no idea where the bug/error happens import wx from wxPython.wx import * class Custom_Button(wx.PyControl): # The BMP's Over_bmp = None #wxEmptyBitmap(1,1,1) # When the mouse is over Norm_bmp = None #wxEmptyBitmap(1,1,1) # The normal BMP Push_bmp = None #wxEmptyBitmap(1,1,1) # The down BMP def __init__(self, parent, id, **kwargs): wx.PyControl.__init__(self,parent, id, **kwargs) # Set the BMP's to the ones given in the constructor #self.Over_bmp = wx.Bitmap(wx.Image(MOUSE_OVER_BMP, wx.BITMAP_TYPE_ANY).ConvertToBitmap()) #self.Norm_bmp = wx.Bitmap(wx.Image(NORM_BMP, wx.BITMAP_TYPE_ANY).ConvertToBitmap()) #self.Push_bmp = wx.Bitmap(wx.Image(PUSH_BMP, wx.BITMAP_TYPE_ANY).ConvertToBitmap()) #self.Pos_bmp = self.pos self.Bind(wx.EVT_LEFT_DOWN, self._onMouseDown) self.Bind(wx.EVT_LEFT_UP, self._onMouseUp) self.Bind(wx.EVT_LEAVE_WINDOW, self._onMouseLeave) self.Bind(wx.EVT_ENTER_WINDOW, self._onMouseEnter) self.Bind(wx.EVT_ERASE_BACKGROUND,self._onEraseBackground) self.Bind(wx.EVT_PAINT,self._onPaint) self._mouseIn = self._mouseDown = False def _onMouseEnter(self, event): self._mouseIn = True def _onMouseLeave(self, event): self._mouseIn = False def _onMouseDown(self, event): self._mouseDown = True def _onMouseUp(self, event): self._mouseDown = False self.sendButtonEvent() def sendButtonEvent(self): event = wx.CommandEvent(wx.wxEVT_COMMAND_BUTTON_CLICKED, self.GetId()) event.SetInt(0) event.SetEventObject(self) self.GetEventHandler().ProcessEvent(event) def _onEraseBackground(self,event): # reduce flicker pass def _onPaint(self, event): dc = wx.BufferedPaintDC(self) dc.SetFont(self.GetFont()) dc.SetBackground(wx.Brush(self.GetBackgroundColour())) dc.Clear() dc.DrawBitmap(self.Norm_bmp) # draw whatever you want to draw # draw glossy bitmaps e.g. dc.DrawBitmap if self._mouseIn: # If the Mouse is over the button dc.DrawBitmap(self, self.Mouse_over_bmp, self.Pos_bmp, useMask=False) if self._mouseDown: # If the Mouse clicks the button dc.DrawBitmap(self, self.Push_bmp, self.Pos_bmp, useMask=False)

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• File Storage for Web Applications: Filesystem vs DB vs NoSQL engines

  - by El Yobo

  I have a web application that stores a lot of user generated files. Currently these are all stored on the server filesystem, which has several downsides for me. When we move "folders" (as defined by our application) we also have to move the files on disk (although this is more due to strange design decisions on the part of the original developers than a requirement of storing things on the filesystem). It's hard to write tests for file system actions; I have a mock filesystem class that logs actions like move, delete etc, without performing them, which more or less does the job, but I don't have 100% confidence in the tests. I will be adding some other jobs which need to access the files from other service to perform additional tasks (e.g. indexing in Solr, generating thumbnails, movie format conversion), so I need to get at the files remotely. Doing this over network shares seems dodgy... Dealing with permissions on the filesystem as sometimes given us problems in the past, although now that we've moved to a pure Linux environment this should be less of an issue. What are the downsides of storing files as BLOBs in MySQL? I guess that it would massively increase the database size and reduce the effectiveness of caches, but are there other problems? Do the same problems exist with NoSQL systems like Cassandra? Does anyone have any other suggestions that might be appropriate?

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• How to configure Eclipse XML formatting?

  - by tputkonen

  I would like to change the way Eclipse formats XML files. For example, if I have in pom.xml file a section as follows: <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>3.8.1</version> <scope>test</scope> </dependency> After ctrl+shift+f it will look like: <dependency> <groupId> junit </groupId> <artifactId> junit </artifactId> <version> 3.8.1 </version> <scope> test </scope> </dependency> I prefer the first version, so how to configure Eclipse to format XML files to look like that?

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• Reducing Time Complexity in Java

  - by Koeneuze

  Right, this is from an older exam which i'm using to prepare my own exam in january. We are given the following method: public static void Oorspronkelijk() { String bs = "Dit is een boodschap aan de wereld"; int max = -1; char let = '*'; for (int i=0;i<bs.length();i++) { int tel = 1; for (int j=i+1;j<bs.length();j++) { if (bs.charAt(j) == bs.charAt(i)) tel++; } if (tel > max) { max = tel; let = bs.charAt(i); } } System.out.println(max + " keer " + let); } The questions are: what is the output? - Since the code is just an algorithm to determine the most occuring character, the output is "6 keer " (6 times space) What is the time complexity of this code? Fairly sure it's O(n²), unless someone thinks otherwise? Can you reduce the time complexity, and if so, how? Well, you can. I've received some help already and managed to get the following code: public static void Nieuw() { String bs = "Dit is een boodschap aan de wereld"; HashMap<Character, Integer> letters = new HashMap<Character, Integer>(); char max = bs.charAt(0); for (int i=0;i<bs.length();i++) { char let = bs.charAt(i); if(!letters.containsKey(let)) { letters.put(let,0); } int tel = letters.get(let)+1; letters.put(let,tel); if(letters.get(max)<tel) { max = let; } } System.out.println(letters.get(max) + " keer " + max); } However, I'm uncertain of the time complexity of this new code: Is it O(n) because you only use one for-loop, or does the fact we require the use of the HashMap's get methods make it O(n log n) ? And if someone knows an even better way of reducing the time complexity, please do tell! :)

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• Reducing moire when downsampling halftone comic images.

  - by drawnonward

  How can I reduce moire effects when downsampling halftone comic book images during live zoom on an iPhone or iPad? I am writing a comic book viewer. It would be nice to provide higher resolution images and allow the user to zoom in while reading the comic book. However, my client is averse to moire effects and will not allow this feature if there are noticeable moire artifacts while zooming, which of course there are. Modifying the images to be less susceptible to moire would only work if the modifications were not perceptible. Blur was specifically prohibited, as is anything that removes the beloved halftone dots. The images are black and white halftone and line art. The originals are 600 dpi but what we ship with the application will be half that at best, so probably 2500 pixels or less tall. So what are my options? If I write a custom downsampling algorithm would it be fast enough for real time on these devices? Are there other tricks I can do? Would it work to just avoid the size ratios that have the most visual moire effects? As you zoom in an out, there are definitely peaks where the moire effects are worst. Is there a way to calculate what those points are and just zoom to a nearby scale that is not as bad? Any suggestions are welcome. I have very little experience with image and signal processing, but am enjoying the opportunity to learn. I know nothing of wavelets and acutance and other jargon, so please be verbose.

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• Prevent "hg status" from showing everything under untracked directories

  - by Wei Hu

  I find the output of hg status too verbose for untracked directories. Suppose I have an empty repository that's managed by both git and hg. So there would be two directories, .git and .hg. The output of git status is: # Untracked files: # (use "git add <file>..." to include in what will be committed) # # .hg/ The output of hg status is: ? .git/HEAD ? .git/config ? .git/description ? .git/hooks/applypatch-msg.sample ? .git/hooks/commit-msg.sample ? .git/hooks/post-commit.sample ? .git/hooks/post-receive.sample ? .git/hooks/post-update.sample ? .git/hooks/pre-applypatch.sample ? .git/hooks/pre-commit.sample ? .git/hooks/pre-rebase.sample ? .git/hooks/prepare-commit-msg.sample ? .git/hooks/update.sample ? .git/info/exclude Is there a way to reduce its output to something like the following line? ? .git/

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• Opening a Unicode file with Perl

  - by Jaco Pretorius

  I'm using osql to run several sql scripts against a database and then I need to look at the results file to check if any errors occurred. The problem is that perl doesn't seem to like the fact that the results files are unicode. I wrote a little test script to test it and the output comes out all warbled. $file = shift; open OUTPUT, $file or die "Can't open $file: $!\n"; while (<OUTPUT>) { print $_; if (/Invalid|invalid|Cannot|cannot/) { push(@invalids, $file); print "invalid file - $inputfile - schedule for retry\n"; last; } } Any ideas? I've tried decoding using decode_utf8 but it makes no difference. I've also tried to set the encoding when opening the file. I think the problem might be that osql puts the result file in UTF-16 format, but I'm not sure. When I open the file in textpad it just tells me 'Unicode'. Edit: Using perl v5.8.8

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• WPF: Editable ComboBox; how to make search/auto-fill functionality case sensitive?

  - by unforgiven3

  Say I have a ComboBox, like so: <ComboBox IsEditable="True" Height="30"> <ComboBoxItem>robot</ComboBoxItem> <ComboBoxItem>Robot</ComboBoxItem> </ComboBox> If a user comes along and starts by typing a lower-case 'r' into that ComboBox when it is empty, the ComboBox predictably auto-fills itself with the word 'robot'. Great. Now the same user comes along and starts typing an upper-case 'R' into that ComboBox when it is again empty. Unpredictable, the ComboBox auto-fills itself with the lower-case word 'robot'. Not great. I desperately want it to auto-fill itself with 'Robot', but WPF does not seem to want to smile down upon me. No matter what you do (CAPS lock, shift+key), the ComboBox will always auto-fill with the lower case 'robot', provided that the lower case 'robot' precedes the upper case 'Robot' in the ComboBox's items collection. Is there any way to prevent this? This behavior is maddening and makes for an absolutely abysmal user experience.

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• Recommended approach to port to ASP.NET MVC

  - by tshao

  I think many of us used to face the same question, what's the best practices to port existing web forms App to MVC. The situation for me is that we'll support both web forms and MVC at the same time. It means, we create new features in MVC, while maintaining legacy pages in web forms, and they're all in a same project. The point is: we want to keep the DRY (do not repeat yourself) principle and reduce duplicate code as much as possible. The ASPX page is not a problem as we only create new features in MVC, but there're still some shared components we want to re-use the both new / legacy pages: Master page UserControl The question here is: Is that possible to create a common master page / usercontrol that could be used in both web forms and MVC? I know that ViewMasterPage inherits from MasterPage and ViewUserControl inherits from UserControl, so it's maybe OK to let both web forms and MVC ASPX page refer to the MVC version. I did some testing and found sometimes it generates errors during the rendering of usercontrols. Any idea / experience you can share with me? Very appreciate to it.

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• Why this base64 function stop working when increasing max length?

  - by flyout

  I am using this class to encode/decode text to base64. It works fine with MAX_LEN up to 512 but if I increase it to 1024 the decode function returns and empty var. This is the function: char* Base64::encode(char *src) { char* ptr = dst+0; unsigned triad; unsigned int d_len = MAX_LEN; memset(dst,'\0', MAX_LEN); unsigned s_len = strlen(src); for (triad = 0; triad < s_len; triad += 3) { unsigned long int sr = 0; unsigned byte; for (byte = 0; (byte<3)&&(triad+byte<s_len); ++byte) { sr <<= 8; sr |= (*(src+triad+byte) & 0xff); } sr <<= (6-((8*byte)%6))%6; // shift left to next 6bit alignment if (d_len < 4) return NULL; // error - dest too short *(ptr+0) = *(ptr+1) = *(ptr+2) = *(ptr+3) = '='; switch(byte) { case 3: *(ptr+3) = base64[sr&0x3f]; sr >>= 6; case 2: *(ptr+2) = base64[sr&0x3f]; sr >>= 6; case 1: *(ptr+1) = base64[sr&0x3f]; sr >>= 6; *(ptr+0) = base64[sr&0x3f]; } ptr += 4; d_len -= 4; } return dst; } Why could be causing this?

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• How to disable selective keys on the keyboard?

  - by Vilx-

  I'd like to write an application which disables certain keys on the keyboard while it's working. More specifically I'm interested in keys that might make the application loose focus (like ALT+TAB, WinKey, Ctrl+Shift+Esc, etc.) The need for this is has to do with babies/animals bashing wildly at the keyboard. :) My first idea was to use SetWindowsHookEx, however I ran into a problem. Since I need a global hook, the procedure would have to reside in a .DLL which would get injected in all active applications. But a .DLL can be either 64-bit or 32-bit, not both. And on a 64-bit system there are both types of applications. I guess then that I must write two copies of the hook .DLL - one for 32-bit and the other for 64-bit. And then I'd also have to launch two copies of the application as well, because the application first has to load the DLL itself before it can pass it on to SetWindowsHookEx(). Sounds pretty clumsy and awkward. Is there perhaps a better way? Or maybe I've misunderstood something?

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• Decoding a jpg in the background in WP7

  - by Shahar Prish

  I have a bunch of apps in the marketplace, and so far I have been able, by changing my functionality or going the extra mile, to work around the issue of being unable to decode a jpg in the background into a WriteableBitmap. I am finding a situation where I can't think of good ways to "work around" the issue. I need to decode the image I get from MediaLibrary, reduce it's resolution to something managable (800x800), rotate it potentially and save to local storage. By far, the thing that takes the most time (80%) is decoding the bitmap to 800x800 - it takes between 700ms to 1000 ms. A user may add 7-10 images when starting, which translates to ~10 seconds of waiting for the images being added. I tried doing this lazily, but at some point you need to pay the piper and the app essentially stutters for ~1000ms at that point and the experience is not great. Is there an alternative I am missing for loading the image in the background somehow? (Note on why CreateOptions.BackgroundCreation is no good for me: It loads the image into a BitmapImage which is great if you want to just use it, but not so great for what I need to do which is create a copy in Isolated Storage).

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• Extend the bootstrap-typeahead in order to take an object instead of a string

  - by Lorraine Bernard

  _.extend($.fn.typeahead.Constructor.prototype, { render: function (items) { var that = this; items = $(items).map(function (i, item) { i = $(that.options.item) .attr('data-value', item[that.options.display]) .attr('data-id', item.id); i.find('a').html(that.highlighter(item)); return i[0]; }); items.first().addClass('active'); this.$menu.html(items); return this; }, select: function () { var val = this.$menu.find('.active').attr('data-value'), id = this.$menu.find('.active').attr('data-id'); this.$element .val(this.updater(val, id)) .change(); return this.hide() } }); return function (element, options) { var getSource = function () { var users = app.userCollection.filter(function (model) { if (options && options.excludeCurrentUser) { return model.id !== app.currentUser.id; } }); return _.map(users, function (user) { return { id: user.get('id'), full_name: user.get('first_name') + ' ' + user.get('last_name') }; }); }; element.typeahead({ minLength: 3, source: getSource, display: 'full_name', sorter: function (items) { var beginswith = [], caseSensitive = [], caseInsensitive = [], item, itemDisplayed; while (item = items.shift()) { itemDisplayed = item[this.options.display]; if (!itemDisplayed.toLowerCase().indexOf(this.query.toLowerCase())) { beginswith.push(item); } else if (~itemDisplayed.indexOf(this.query)) { caseSensitive.push(item); } else { caseInsensitive.push(item); } } return beginswith.concat(caseSensitive, caseInsensitive); }, highlighter: function (item) { var query = this.query.replace(/[\-\[\]{}()*+?.,\\\^$|#\s]/g, '\\$&'); return item[this.options.display].replace(new RegExp('(' + query + ')', 'ig'), function ($1, match) { return '<strong>' + match + '</strong>'; }); }, matcher: function (item) { var value = item[this.options.display]; return { value: ~value.toLowerCase().indexOf(this.query.toLowerCase()), id: item.id }; }, updater: function (item, userId) { options.hiddenInputElement.val(userId); return item; } }); };

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• Caching Authentication Data

  - by PartlyCloudy

  Hi, I'm currently implementing a REST web service using CouchDB and RESTlet. The RESTlet layer is mainly for authentication and some minor filtering of the JSON data served by CouchDB: Clients <= HTTP = [ RESTlet <= HTTP = CouchDB ] I'm using CouchDB also to store user login data, because I don't want to add an additional database server for that purpose. Thus, each request to my service causes two CouchDB requests conducted by RESTlet (auth data + "real" request). In order to keep the service as efficent as possible, I want to reduce the number of requests, in this case redundant requests for login data. My idea now is to provide a cache (i.e.LRU-Cache via LinkedHashMap) within my RESTlet application that caches login data, because HTTP caching will probabily not be enough. But how do I invalidate the cache data, once a user changes the password, for instance. Thanks to REST, the application might run on several servers in parallel, and I don't want to create a central instance just to cache login data. Currently, I save requested auth data in the cache and try to auth new requests by using them. If a authentication fails or there is now entry available, I'll dispatch a GET request to my CouchDB storage in order to obtain the actual auth data. So in a worst case, users that have changed their data will perhaps still be able to login with their old credentials. How can I deal with that? Or what is a good strategy to keep the cache(s) up-to-date in general? Thanks in advance.

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• Speeding up inner-joins and subqueries while restricting row size and table membership

  - by hiffy

  I'm developing an rss feed reader that uses a bayesian filter to filter out boring blog posts. The Stream table is meant to act as a FIFO buffer from which the webapp will consume 'entries'. I use it to store the temporary relationship between entries, users and bayesian filter classifications. After a user marks an entry as read, it will be added to the metadata table (so that a user isn't presented with material they have already read), and deleted from the stream table. Every three minutes, a background process will repopulate the Stream table with new entries (i.e. whenever the daemon adds new entries after the checks the rss feeds for updates). Problem: The query I came up with is hella slow. More importantly, the Stream table only needs to hold one hundred unread entries at a time; it'll reduce duplication, make processing faster and give me some flexibility with how I display the entries. The query (takes about 9 seconds on 3600 items with no indexes): insert into stream(entry_id, user_id) select entries.id, subscriptions_users.user_id from entries inner join subscriptions_users on subscriptions_users.subscription_id = entries.subscription_id where subscriptions_users.user_id = 1 and entries.id not in (select entry_id from metadata where metadata.user_id = 1) and entries.id not in (select entry_id from stream where user_id = 1); The query explained: insert into stream all of the entries from a user's subscription list (subscriptions_users) that the user has not read (i.e. do not exist in metadata) and which do not already exist in the stream. Attempted solution: adding limit 100 to the end speeds up the query considerably, but upon repeated executions will keep on adding a different set of 100 entries that do not already exist in the table (with each successful query taking longer and longer). This is close but not quite what I wanted to do. Does anyone have any advice (nosql?) or know a more efficient way of composing the query?

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• Condensing repeating JQuery code

  - by Craig Ward

  I have a page that has a large image on it with a number of thumbnails. When you mouse over a thumbnail the main image changes to the image you have just rolled your mouse over. The problem is the more thumbnails I have, the more repeated code I have. How could I reduce it? The Jquery code is as follows. <script type="text/javascript"> $('#thumb1') .mouseover(function(){ $('#big_img').fadeOut('slow', function(){ $('#big_img').attr('src', '0001.jpg'); $('#big_img').fadeIn('slow'); }); }); $('#thumb2') .mouseover(function(){ $('#big_img').fadeOut('slow', function(){ $('#big_img').attr('src', 'p_0002.jpg'); $('#big_img').fadeIn('slow'); }); }); $('#thumb3') .mouseover(function(){ $('#big_img').fadeOut('slow', function(){ $('#big_img').attr('src', '_img/p_0003.jpg'); $('#big_img').fadeIn('slow'); }); }); $('#thumb4') .mouseover(function(){ $('#big_img').fadeOut('slow', function(){ $('#big_img').attr('src', '0004.jpg'); $('#big_img').fadeIn('slow'); }); }); </script> #big_img = the ID of the full size image #thumb1, #thumb2, #thumb3, #thumb4 = The ID's of the thumbnails The main code for the page is PHP if that helps.

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• Wizard style navigation, dismissing a view and showing another one

  - by Robin Jamieson

  I'm making a set of screens similar to a wizard and I'd like to know how to make a view dismiss itself and its parent view and immediately show a 'DoneScreen' without worrying about resource leaks. My views look like the following: Base -> Level1 -> DoneScreen -> Level2 -> DoneScreen The Level1 controller is a navigation controller created with a view.xib and shown with [self presentModalViewController ...] by the Base controller. The Level1 controller is also responsible for creating the 'DoneScreen' which may be shown instead of the Level2 Screen based on a certain criteria. When the user taps a button on the screen, the Level1 controller instantiates the the Level2 controller and it displays it via [self.navigationController pushViewController ..] and Level2 controller's view has a 'Next' button. When the use hits the 'Next' button in the Level2 screen, I need to dismiss the current Level2's view as well as the Level1's view and display the 'DoneScreen', which would have been created and passed in to the Level2 controller from Level1. (partly to reduce code duplication, and partly to separate responsibilities among the controllers) In the Level2 controller, if I show the 'DoneScreen' first and dismiss itself with [self.navigationController popViewControllerAnimated:YES]; then the Level1 controller's modal view is still present above the 'Base' but under the Done screen. What's a good way to clear out all of these views except the Base and then show the 'DoneScreen'? Any good suggestions on how to get this done in a simple but elegant manner?

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• Does Perl auto-vivify variables used as references in subroutine calls?

  - by FM

  I've declared 2010 to be the year of higher-order programming, so I'm learning Haskell. The introduction has a slick quick-sort demo, and I thought, "Hey, that's easy to do in Perl". It turned to be easier than I expected. Note that I don't have to worry about whether my partitions ($less and $more) are defined. Normally you can't use an undefined value as an array reference. use strict; use warnings; use List::MoreUtils qw(part); my @data = (5,6,7,4,2,9,10,9,5,1); my @sorted = qsort(@data); print "@sorted\n"; sub qsort { return unless @_; my $pivot = shift @_; my ($less, $more) = part { $_ < $pivot ? 0 : 1 } @_; # Works, even though $less and $more are sometimes undefined. return qsort(@$less), $pivot, qsort(@$more); } As best I can tell, Perl will auto-vivify a variable that you try to use as a reference -- but only if you are passing it to a subroutine. For example, my call to foo() works, but not the attempted print. use Data::Dumper qw(Dumper); sub foo { print "Running foo(@_)\n" } my ($x); print Dumper($x); # Fatal: Can't use an undefined value as an ARRAY reference. # print @$x, "\n"; # But this works. foo(@$x); # Auto-vivification: $x is now []. print Dumper($x); My questions: Am I understanding this behavior correctly? What is the explanation or reasoning behind why Perl does this? Is this behavior explained anywhere in the docs?

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• hadoop implementing a generic list writable

  - by Guruprasad Venkatesh

  I am working on building a map reduce pipeline of jobs(with one MR job's output feeding to another as input). The values being passed around are fairly complex, in that there are lists of different types and hash maps with values as lists. Hadoop api does not seem to have a ListWritable. Am trying to write a generic one, but it seems i can't instantiate a generic type in my readFields implementation, unless i pass in the class type itself: public class ListWritable<T extends Writable> implements Writable { private List<T> list; private Class<T> clazz; public ListWritable(Class<T> clazz) { this.clazz = clazz; list = new ArrayList<T>(); } @Override public void write(DataOutput out) throws IOException { out.writeInt(list.size()); for (T element : list) { element.write(out); } } @Override public void readFields(DataInput in) throws IOException{ int count = in.readInt(); this.list = new ArrayList<T>(); for (int i = 0; i < count; i++) { try { T obj = clazz.newInstance(); obj.readFields(in); list.add(obj); } catch (InstantiationException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } } } } But hadoop requires all writables to have a no argument constructor to read the values back. Has anybody tried to do the same and solved this problem? TIA.

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• HDFS some datanodes of cluster are suddenly disconnected while reducers are running

  - by user1429825

  I have 8 slave computers and 1 master computer for running Hadoop (ver 0.21) some datanodes of cluster are suddenly disconnected while I was running MapReduce code on 10GB data After all mappers finished and around 80% of reducers was processed, randomly one or more datanode disconned from network. and then the other datanodes start to disappear from network even if I killed the MapReduce job when I found some datanode was disconnected. I've tried to change dfs.datanode.max.xcievers to 4096, turned off fire-walls of all computing node, disabled selinux and increased the number of file open limit to 20000 but they didn't work at all... anyone have a idea to solve this problem? followings are error log from mapreduce 12/06/01 12:31:29 INFO mapreduce.Job: Task Id : attempt_201206011227_0001_r_000006_0, Status : FAILED java.io.IOException: Bad connect ack with firstBadLink as ***.***.***.148:20010 at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.createBlockOutputStream(DFSOutputStream.java:889) at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.nextBlockOutputStream(DFSOutputStream.java:820) at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:427) and followings are logs from datanode 2012-06-01 13:01:01,118 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Receiving block blk_-5549263231281364844_3453 src: /*.*.*.147:56205 dest: /*.*.*.142:20010 2012-06-01 13:01:01,136 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(*.*.*.142:20010, storageID=DS-1534489105-*.*.*.142-20010-1337757934836, infoPort=20075, ipcPort=20020) Starting thread to transfer block blk_-3849519151985279385_5906 to *.*.*.147:20010 2012-06-01 13:01:19,135 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(*.*.*.142:20010, storageID=DS-1534489105-*.*.*.142-20010-1337757934836, infoPort=20075, ipcPort=20020):Failed to transfer blk_-5797481564121417802_3453 to *.*.*.146:20010 got java.net.ConnectException: > Connection timed out at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method) at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:701) at org.apache.hadoop.net.SocketIOWithTimeout.connect(SocketIOWithTimeout.java:206) at org.apache.hadoop.net.NetUtils.connect(NetUtils.java:373) at org.apache.hadoop.hdfs.server.datanode.DataNode$DataTransfer.run(DataNode.java:1257) at java.lang.Thread.run(Thread.java:722) 2012-06-01 13:06:20,342 INFO org.apache.hadoop.hdfs.server.datanode.DataBlockScanner: Verification succeeded for blk_6674438989226364081_3453 2012-06-01 13:09:01,781 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(*.*.*.142:20010, storageID=DS-1534489105-*.*.*.142-20010-1337757934836, infoPort=20075, ipcPort=20020):Failed to transfer blk_-3849519151985279385_5906 to *.*.*.147:20010 got java.net.SocketTimeoutException: 480000 millis timeout while waiting for channel to be ready for write. ch : java.nio.channels.SocketChannel[connected local=/*.*.*.142:60057 remote=/*.*.*.147:20010] at org.apache.hadoop.net.SocketIOWithTimeout.waitForIO(SocketIOWithTimeout.java:246) at org.apache.hadoop.net.SocketOutputStream.waitForWritable(SocketOutputStream.java:164) at org.apache.hadoop.net.SocketOutputStream.transferToFully(SocketOutputStream.java:203) at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendChunks(BlockSender.java:388) at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendBlock(BlockSender.java:476) at org.apache.hadoop.hdfs.server.datanode.DataNode$DataTransfer.run(DataNode.java:1284) at java.lang.Thread.run(Thread.java:722) hdfs-site.xml <configuration> <property> <name>dfs.name.dir</name> <value>/home/hadoop/data/name</value> </property> <property> <name>dfs.data.dir</name> <value>/home/hadoop/data/hdfs1,/home/hadoop/data/hdfs2,/home/hadoop/data/hdfs3,/home/hadoop/data/hdfs4,/home/hadoop/data/hdfs5</value> </property> <property> <name>dfs.replication</name> <value>3</value> </property> <property> <name>dfs.datanode.max.xcievers</name> <value>4096</value> </property> <property> <name>dfs.http.address</name> <value>0.0.0.0:20070</value> <description>50070 The address and the base port where the dfs namenode web ui will listen on. If the port is 0 then the server will start on a free port. </description> </property> <property> <name>dfs.datanode.http.address</name> <value>0.0.0.0:20075</value> <description>50075 The datanode http server address and port. If the port is 0 then the server will start on a free port. </description> </property> <property> <name>dfs.secondary.http.address</name> <value>0.0.0.0:20090</value> <description>50090 The secondary namenode http server address and port. If the port is 0 then the server will start on a free port. </description> </property> <property> <name>dfs.datanode.address</name> <value>0.0.0.0:20010</value> <description>50010 The address where the datanode server will listen to. If the port is 0 then the server will start on a free port. </description> <property> <name>dfs.datanode.ipc.address</name> <value>0.0.0.0:20020</value> <description>50020 The datanode ipc server address and port. If the port is 0 then the server will start on a free port. </description> </property> <property> <name>dfs.datanode.https.address</name> <value>0.0.0.0:20475</value> </property> <property> <name>dfs.https.address</name> <value>0.0.0.0:20470</value> </property> </configuration> mapred-site.xml <configuration> <property> <name>mapred.job.tracker</name> <value>masternode:29001</value> </property> <property> <name>mapred.system.dir</name> <value>/home/hadoop/data/mapreduce/system</value> </property> <property> <name>mapred.local.dir</name> <value>/home/hadoop/data/mapreduce/local</value> </property> <property> <name>mapred.map.tasks</name> <value>32</value> <description> default number of map tasks per job.</description> </property> <property> <name>mapred.tasktracker.map.tasks.maximum</name> <value>4</value> </property> <property> <name>mapred.reduce.tasks</name> <value>8</value> <description> default number of reduce tasks per job.</description> </property> <property> <name>mapred.map.child.java.opts</name> <value>-Xmx2048M</value> </property> <property> <name>io.sort.mb</name> <value>500</value> </property> <property> <name>mapred.task.timeout</name> <value>1800000</value> <!-- 30 minutes --> </property> <property> <name>mapred.job.tracker.http.address</name> <value>0.0.0.0:20030</value> <description> 50030 The job tracker http server address and port the server will listen on. If the port is 0 then the server will start on a free port. </description> </property> <property> <name>mapred.task.tracker.http.address</name> <value>0.0.0.0:20060</value> <description> 50060 </property> </configuration>

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• Reducing the opacity on a div without reducing the opacity of the contents

  - by user352744

  Want to reduce the opacity of page contents container background without reducing the opacity of the contents. <div id="container"> <div id="page contents"> page contents goes here, like amazing articles and all that. </div> </div> Needs to be able to expand with the content, thus can't have a fixed height. Absolute positioning it underneath the content will mean there will be no relationship between the two divs and it wont expand with the contents, so I think this is a dead end, feel free to say otherwise. Can't use Jquery as could be too laggy and not instant. Other options preferred please. May have to use 'png' background images but were hoping not to as it is a template and needs to be able to change colour based on colour schemes. Could generate images on demand but not ideal. Oh and to top it off cant use CSS3 as wont work in IE! of course! Any suggestions?

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• Optimization headers for UITableView?

  - by Pask

  I have an optimization problem for the headers of a table with plain style. If I use the standard view for the table (the classic gray with titles set by titleForHeaderInSection:) everything is ok and the scrolling is smooth and immediate. When, instead, use this code to set my personal view: - (UIView *)tableView:(UITableView *)tableView viewForHeaderInSection:(NSInteger)section { return [self headerPerTitolo:[titoliSezioni objectAtIndex:section]]; } - (UIImageView *)headerPerTitolo:(NSString *)titolo { UIImageView *headerView = [[[UIImageView alloc] initWithFrame:CGRectMake(10.0, 0.0, 320.0, 44.0)] autorelease]; headerView.image = [UIImage imageNamed:kNomeImmagineHeader]; headerView.alpha = kAlphaSezioniTablePlain; UILabel * headerLabel = [[[UILabel alloc] initWithFrame:CGRectZero] autorelease]; headerLabel.backgroundColor = [UIColor clearColor]; headerLabel.opaque = NO; headerLabel.textColor = [UIColor whiteColor]; headerLabel.font = [UIFont boldSystemFontOfSize:16]; headerLabel.frame = CGRectMake(10.0,-11.0, 320.0, 44.0); headerLabel.textAlignment = UITextAlignmentLeft; headerLabel.text = titolo; [headerView addSubview:headerLabel]; return headerView; } scrolling is jerky and not immediate (sliding the finger on the screen does not match an immediate shift of the table). I do not know what caused this problem, maybe the fact that every time the method viewForHeaderInSection: is called, the code runs to create a new UIImageView. I tried many ways to solve the problem, such as creating an array of all the necessary view: apart from more time spent loading at startup, there is a continuing problem of low reactivity of the table. 've Attempted by reducing the size of UIImageView positioned from about 66 KB to 4 KB: not only has a deterioration in quality of colors (which distorts a bit 'original graphics), but ... the problem persists! Perhaps you have suggestions about it, or know me obscure techniques that enable me to optimize this aspect of my application ... I apologize for my English, I used Google for translation.

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• Printing to different printers using mozilla.

  - by Nick-ACNB

  I am currently creating a web application that will be deployed in an intranet environment. I chose firefox to be the browser that will run it. However, in the application I am building, I need to be able to print to different printers quickly since they use different paper size depending on what client is coming. To avoid many time-wasting mistakes that could occur, for instance someone choosing the wrong printer and wasting paper. Also, the time used to find the right printer for the job and then pressing print is considered too long in the current context. Is there any solution to this problem? I understand the potential security flaw behind this, but please be aware that this is solely an intranet project and that I can reduce the browser's security to the lowest since they don't access internet. I know there could be something doable behind IE (ActiveX or VBScript) but I am using firefox. Also, I guess there could also be something rather tricky that when you press print on the browser, it saves what needs to be printed to a DB and then there is an exe app that runs and fetch that DB every set ammount of time and print to the right printer. Any suggestion would be greatly appreciated. I doubt I am the only one to ever face this issue! :) Thank you very much.

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• Recycle Freed Objects

  - by uray

  suppose I need to allocate and delete object on heap frequently (of arbitrary size), is there any performance benefit if instead of deleting those objects, I will return it back to some "pool" to be reused later? would it give benefit by reduce heap allocation/deallocation?, or it will be slower compared to memory allocator performance, since the "pool" need to manage a dynamic collection of pointers. my use case: suppose I create a queue container based on linked list, and each node of that list are allocated on the heap, so every call to push() and pop() will allocate and deallocate that node: ` template <typename T> struct QueueNode { QueueNode<T>* next; T object; } template <typename T> class Queue { void push(T object) { QueueNode<T>* newNode = QueueNodePool<T>::get(); //get recycled node if(!newNode) { newNode = new QueueNode<T>(object); } // push newNode routine here.. } T pop() { //pop routine here... QueueNodePool<T>::store(unusedNode); //recycle node return unusedNode->object; } } `

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