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• SQL latest/top items in category

  - by drozzy

  What is a scalable way to select latest 10 items from each category. I have a schema list this: item category updated so I want to select 10 last update items from each category. The current solution I can come up with is to query for categories first and then issue some sort of union query: query = none for cat in categories: query += select top 10 from table where category=cat order by updated I am not sure how efficient this will be for bigger databases (1 million rows). If there is a way to do this in one go - that would be nice. Any help appreciated.

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• NoSQL and meteorological data

  - by christian studer

  So there's this new cool thing, these NoSQL-databases. And so there's my data: Rows of rows of rows of meteorological data: Values, representing certain measurements at a certain station (Identified by a WMO number, not coordinates), at a certain time. Not every station measures every parameter, not every parameter is measured all the time. I store this data (30 years worth of hourly values, resulting in ~1 billion values) currently in MySQL. The continous growth and the forseeable addition of even more data give me a little headache. Reading about the document based NoSQL systems which seem to scale rather easily, I was wondering if NoSQL is a viable data storage concept for meteorological data too. Do you have any experience with this?

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• MySqli Prepared Statment Fetch Row

  - by pws5068

  I'm looking for an efficient way to select a specific row with a php prepared statement. $iDB = new mysqliDB(); $stmt = $iDB->prepare($sql); $stmt->bind_param('s',$searhStr); $stmt->execute(); $stmt->store_result(); $stmt->bind_result($objID,$typeID,$year,$updated); // Now jump to row $rowNumber echo("Obj ID = {$objID}"); Any suggestions? Thanks!

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• ideas for algorithm? sorting a list randomly with emphasis on variety

  - by Steve Eisner

  I have a table of items with [ID,ATTR1,ATTR2,ATTR3]. I'd like to select about half of the items, but try to get a random result set that is NOT clustered. In other words, there's a fairly even spread of ATTR1 values, ATTR2 values, and ATTR3 values. This does NOT necessarily represent the data as a whole, in other words, the total table may be generally concentrated on certain attribute values, but I'd like to select a subset with more variety. The attributes are not inter-related, so there's not really a correlation between ATTR1 and ATTR2. Any ideas for an efficient algorithm? Thanks! I don't really even know how to search for this :)

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• posmax: like argmax but gives the position(s) of the element x for which f[x] is maximal

  - by dreeves

  Mathematica has a built-in function ArgMax for functions over infinite domains, based on the standard mathematical definition. The analog for finite domains is a handy utility function. Given a function and a list (call it the domain of the function), return the element(s) of the list that maximize the function. Here's an example of finite argmax in action: http://stackoverflow.com/questions/471029/canonicalize-nfl-team-names/472213#472213 And here's my implementation of it (along with argmin for good measure): (* argmax[f, domain] returns the element of domain for which f of that element is maximal -- breaks ties in favor of first occurrence. *) SetAttributes[{argmax, argmin}, HoldFirst]; argmax[f_, dom_List] := Fold[If[f[#1]>=f[#2], #1, #2]&, First[dom], Rest[dom]] argmin[f_, dom_List] := argmax[-f[#]&, dom] First, is that the most efficient way to implement argmax? What if you want the list of all maximal elements instead of just the first one? Second, how about the related function posmax that, instead of returning the maximal element(s), returns the position(s) of the maximal elements?

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• How do I get the position of a result in the list after an order_by?

  - by Bob Bob

  I'm trying to find an efficient way to find the rank of an object in the database related to it's score. My naive solution looks like this: rank = 0 for q in Model.objects.all().order_by('score'): if q.name == 'searching_for_this' return rank rank += 1 It should be possible to get the database to do the filtering, using order_by: Model.objects.all().order_by('score').filter(name='searching_for_this') But there doesn't seem to be a way to retrieve the index for the order_by step after the filter. Is there a better way to do this? (Using python/django and/or raw SQL.) My next thought is to pre-compute ranks on insert but that seems messy.

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• Searching with Linq

  - by Phil

  I have a collection of objects, each with an int Frame property. Given an int, I want to find the object in the collection that has the closest Frame. Here is what I'm doing so far: public static void Search(int frameNumber) { var differences = (from rec in _records select new { FrameDiff = Math.Abs(rec.Frame - frameNumber), Record = rec }).OrderBy(x => x.FrameDiff); var closestRecord = differences.FirstOrDefault().Record; //continue work... } This is great and everything, except there are 200,000 items in my collection and I call this method very frequently. Is there a relatively easy, more efficient way to do this?

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• Will being self-taught limit me?

  - by Isaiah

  I'm 21 and am pretty efficient in html/css, python, and javascript. I also know my way around lisp languages and enjoy programing in them. My problem is that I'm extremely self-taught and not quite confident that I could land a job programing, but I really need a job soon as I've just become a father. I haven't even created a resume yet because I'm not really sure what to put on it except my lone experience. So I wanted to ask, will being primarily self-taught with some experience on small projects I've done for a few clients limit me too much? I mean I know I need some kind of education so I've enrolled part time in a community college to work on a degree in computer science, but it's years till then. And if it will limit me a lot, what kind of skills would be good to work on to make my chances any better? Thank You

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• Why Switch/Case and not If/Else If?

  - by OB OB

  This question in mainly pointed at C/C++, but I guess other languages are relevant as well. I can't understand why is switch/case still being used instead of if/else if. It seems to me much like using goto's, and results in the same sort of messy code, while the same results could be acheived with if/else if's in a much more organized manner. Still, I see these blocks around quite often. A common place to find them is near a message-loop (WndProc...), whereas these are among the places when they raise the heaviest havoc: variables are shared along the entire block, even when not propriate (and can't be initialized inside it). Extra attention has to be put on not dropping break's, and so on... Personally, I avoid using them, and I wonder wether I'm missing something? Are they more efficient than if/else's? Are they carried on by tradition?

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• Minimizing SQL queries using join with one-to-many relationship

  - by Brian

  So let me preface this by saying that I'm not an SQL wizard by any means. What I want to do is simple as a concept, but has presented me with a small challenge when trying to minimize the amount of database queries I'm performing. Let's say I have a table of departments. Within each department is a list of employees. What is the most efficient way of listing all the departments and which employees are in each department. So for example if I have a department table with: id name 1 sales 2 marketing And a people table with: id department_id name 1 1 Tom 2 1 Bill 3 2 Jessica 4 1 Rachel 5 2 John What is the best way list all departments and all employees for each department like so: Sales Tom Bill Rachel Marketing Jessica John Pretend both tables are actually massive. (I want to avoid getting a list of departments, and then looping through the result and doing an individual query for each department). Think similarly of selecting the statuses/comments in a Facebook-like system, when statuses and comments are stored in separate tables.

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• Fastest way to put contents of Set<String> to a single String with words separated by a whitespace?

  - by Lars Andren

  I have a few Set<String>s and want to transform each of these into a single String where each element of the original Set is separated by a whitespace " ". A naive first approach is doing it like this Set<String> set_1; Set<String> set_2; StringBuilder builder = new StringBuilder(); for (String str : set_1) { builder.append(str).append(" "); } this.string_1 = builder.toString(); builder = new StringBuilder(); for (String str : set_2) { builder.append(str).append(" "); } this.string_2 = builder.toString(); Can anyone think of a faster, prettier or more efficient way to do this?

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• Best data-structure to use for two ended sorted list

  - by fmark

  I need a collection data-structure that can do the following: Be sorted Allow me to quickly pop values off the front and back of the list Remain sorted after I insert a new value Allow a user-specified comparison function, as I will be storing tuples and want to sort on a particular value Thread-safety is not required Optionally allow efficient haskey() lookups (I'm happy to maintain a separate hash-table for this though) My thoughts at this stage are that I need a priority queue and a hash table, although I don't know if I can quickly pop values off both ends of a priority queue. I'm interested in performance for a moderate number of items (I would estimate less than 200,000). Another possibility is simply maintaining an OrderedDictionary and doing an insertion sort it every-time I add more data to it. Furthermore, are there any particular implementations in Python. I would really like to avoid writing this code myself.

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• How to implement a good system for login/out into a webapp

  - by Brandon Wang

  I am one of the developers at PassPad, a secure password generator and username storage system. We're still working on it, but I have a few questions on the best way to implement a secure login/out system. Right now, what we plan on doing is to have the login system save a cookie with the username and a session key, and that's all that serves as authentication. The server verifies the two to match. Upon login/out a new key is created. This is a security-related webapp and while we don't actually store any information that might make the user queasy, because it is security-oriented it makes it a necessity for us to at least appear secure in a way that the user would be happy with. Is there a better way to implement a login/out system in PHP? Preferably it won't take too much coding time or server resources. Is there anything else I need to implement, like brute-force protection, etc? How would I go about that?

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• C# Getting Just Date From Timestamp

  - by Soo

  If I have a timestamp in the form: yyyy-mm-dd hh:mm:ss:mmm How can I just extract the date from the timestamp? For instance, if a timestamp reads: "2010-05-18 08:36:52:236" what is the best way to just get 2010-05-18 from it. What I'm trying to do is isolate the date portion of the timestamp, define a custom time for it to create a new time stamp. Is there a more efficient way to define the time of the timestamp without first taking out the date, and then adding a new time?

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• Update tableview instantly as data pushed in core data iphone

  - by user336685

  I need to update the tableview as soon as the content is pushed in core data database. for this AppDelegate.m contains following code NSManagedObjectContext *moc = [self managedObjectContext]; NSFetchRequest *request = [[NSFetchRequest alloc] init]; [request setEntity:[NSEntityDescription entityForName:@"FeedItem" inManagedObjectContext:moc]]; //for loop // push data in code data & then save context [moc save:&error]; ZAssert(error == nil, @"Error saving context: %@", [error localizedDescription]); //for loop ends This code triggers following code from RootviewController.m - (void)controllerWillChangeContent:(NSFetchedResultsController*)controller { [[self tableView] beginUpdates]; } But this updates the tableview only at the end of the for loop ,the table does not get updated after immediate push in db. I tried following code but that didn't work - (void)controllerDidChangeContent:(NSFetchedResultsController *)controller { // In the simplest, most efficient, case, reload the table view. [self.tableView reloadData]; } I have been stuck with this problem for several days.Please help.Thanks in advance for solution.

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• Alternative databases to use when putting IIS Logs into a database using LogParser

  - by Robin Day

  We have run some scripts that use LogParser to dump our IIS logs into a SQL Server database. We can then query this to get simple stats on hits, usage etc. It's also good when linking it to error log databases and performance counter database to compare usage with errors, etc. Having implemented this for just one system and for the last 2-3 weeks we already have a 5GB database with around 10 million records. This is making any queries to this database quite slow and will no doubt cause storage issues if we continue to log as we are. Can anyone suggest any alternative databases that we could use for this data that would be more efficient for such logs? I'd be particularly interested in any experience of Google's BigTable or Amazon's SimbleDB. Are either of these suitable for reporting queries? COUNTs, GROUP BYs, PIVOTs?

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• VS2010's "Public Property <PropertyName> As <DataType> vs. Public var

  - by Velika2

  In VS2008, I used to type Public Property <PropName> As <dataType> and hit the Enter key and the IDE editor would automatically expand it out to a full blown property block. Now, from what I understand, a new feature of 2010 is that the compiler automatically "expands" the short syntax above into the same IL code that you would get with the full property GET AND SET sub methods that were are accustomed to seeing before in the editor. But functionality, how the heck is this any different than just having a Public class level variable? If the only diff is what it compiles to and if otehrwise there is no functional difference, isn't the new way less efficient than the old since it involves more code than just having a class level memory variable? Public as I thought that if you weren't going to have code behind your properties that they were essentially the same. I guess the diffrenece is that they just added the keyword "Property" but functionality, their is no diff, eh?

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• Make xargs execute the command once for each line of input

  - by Readonly

  How can I make xargs execute the command exactly once for each line of input given? It's default behavior is to chunk the lines and execute the command once, passing multiple lines to each instance. From http://en.wikipedia.org/wiki/Xargs: find /path -type f -print0 | xargs -0 rm In this example, find feeds the input of xargs with a long list of file names. xargs then splits this list into sublists and calls rm once for every sublist. This is more efficient than this functionally equivalent version: find /path -type f -exec rm '{}' \; I know that find has the "exec" flag. I am just quoting an illustrative example from another resource.

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• Perform Grouping of Resultset in Code

  - by NinjaBomb

  Stackoverflowers, I have a resultset from a SQL query in the form of: Category Column2 Column3 A 2 3.50 A 3 2 B 3 2 B 1 5 ... I need to group the resultset based on the Category column and sum the values for Column2 and Column3. I have to do it in code because I cannot perform the grouping in the SQL query that gets the data due to the complexity of the query (long story). This grouped data will then be displayed in a table. I have it working for specific set of values in the Category column, but I would like a solution that would handle any possible values that appear in the Category column. I know there has to be a straightforward, efficient way to do it but I cannot wrap my head around it right now. How would you accomplish it?

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• Efficiency of Java code with primitive types

  - by super89

  Hello! I want to ask which piece of code is more efficient in Java? Code 1: void f() { for(int i = 0 ; i < 99999;i++) { for(int j = 0 ; j < 99999;j++) { //Some operations } } } Code 2: void f() { int i,j; for(i = 0 ; i < 99999;i++) { for(j = 0 ; j < 99999;j++) { //Some operations } } } My teacher said that second is better, but I can't agree that opinion.

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• Put an object in Handler message

  - by Tsimmi

  Hi! I need to download an image from the internet, in a different thread, and then send that image object in the handler message, to the UI thread. I already have this: ... Message msg = Message.obtain(); Bundle b = new Bundle(); b.putParcelable("MyObject", (Parcelable) object); msg.setData(b); handler.sendMessage(msg); And when I receive this message, I want to extract the object: ... public void handleMessage(Message msg) { super.handleMessage(msg); MyObject objectRcvd = (MyObject) msg.getData().getParcelable("IpTile"); addToCache(ipTile); mapView.invalidate(); } But this is giving me: ...java.lang.ClassCastException... Can anyone help? And by the way, is this the most efficient way to pass an object to the UI Thread? Thank you all!

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• ASP.NET AJAX or jQuery (UpdatePanel/ScriptManager or UFrame/jQuery.ajax)

  - by Mark Redman

  Hi, We use the asp.net UpdatePanel and the ScriptManager/ScriptManagerProxy for ajax related functionality; reducing full page refreshes and calling WCF Services respectively. we also use jQuery and plugins for some parts of the UI. We have had some issues with javascript library related conflicts, but have come across some posts indicating that there is a lot more overhead using the UpdatePanel. I have found some limited reference to UFrame: http://www.codeproject.com/KB/aspnet/uframe.aspx www.codeplex.com/uframe Is this a commercially viable replacement for the asp.net UpdatePanel? We use a ScriptManagerProxy to reference WCF services and easily create and use a proxy to call the various WCF service methods. Would using jQuery ajax be a more efficient solution here? We have got this working well on various browsers but now seem to be getting some security related issues (as seen in FF Firebug: Access to restricted URI denied" code: "1012) which seem to have started since using jQuery a lot more. Is it possible/viable to not use ASP.NET Ajax at all?

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• cached schwartzian transform

  - by davidk01

  I'm going through "Intermediate Perl" and it's pretty cool. I just finished the section on "The Schwartzian Transform" and after it sunk in I started to wonder why the transform doesn't use a cache. In lists that have several repeated values the transform recomputes the value for each one so I thought why not use a hash to cache results. Here' some code: # a place to keep our results my %cache; # the transformation we are interested in sub foo { # expensive operations } # some data my @unsorted_list = ....; # sorting with the help of the cache my @sorted_list = sort { ($cache{$a} or $cache{$a} = &foo($a)) <=> ($cache{$b} or $cache{$b} = &foo($b)) } @unsorted_list; Am I missing something? Why isn't the cached version of the Schwartzian transform listed in books and in general just better circulated because on first glance I think the cached version should be more efficient?

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• Any tips of how to handle hierarchical trees in relational model?

  - by George

  Hello all. I have a tree structure that can be n-levels deep, without restriction. That means that each node can have another n nodes. What is the best way to retrieve a tree like that without issuing thousands of queries to the database? I looked at a few other models, like flat table model, Preorder Tree Traversal Algorithm, and so. Do you guys have any tips or suggestions of how to implement a efficient tree model? My objective in the real end is to have one or two queries that would spit the whole tree for me. With enough processing i can display the tree in dot net, but that would be in client machine, so, not much of a big deal. Thanks for the attention

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• Does the iPhone compress images saved within my app's documents directory?

  - by Jane Sales

  We are caching images downloaded from our server. We write them to our local storage like this: NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES); NSString *documentsDirectory = [paths objectAtIndex:0] ; NSString* folder = [[documentsDirectory stringByAppendingPathComponent:@"flook.images"] retain]; NSString* fileName = [folder stringByAppendingFormat:@"/%@", aBaseFilename]; BOOL writeSuccess = [anImageData writeToFile:fileName atomically:NO]; The downloaded images are always the expected size, around 45-85KB. Later, we read images from our cache like this: NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES); NSString *documentsDirectory = [paths objectAtIndex:0] ; NSString* folder = [[documentsDirectory stringByAppendingPathComponent:@"flook.images"] retain]; NSString* fileName = [folder stringByAppendingFormat:@"/%@", aBaseFilename]; image = [UIImage imageWithContentsOfFile:fileName]; Occasionally, the images returned from this cache read are much smaller because they are much more compressed - around 5-10KB. Has the OS done this to us?

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