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• Generating unique N-valued key

  - by Bar

  Hi, StackOverflow! I want to generate unique random, N-valued key. This key can contain numbers and latin characters, i.e. A-Za-z0-9. The only solution I am thinking about is something like this (pseudocode): key = ""; smb = "ABC…abc…0123456789"; // allowed symbols for (i = 0; i < N; i++) { key += smb[rnd(0, smb.length() - 1)]; // select symbol at random position } Is there any better solution? What can you suggest? TIA, Michael.

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• Order By Rand by Time (HQL)

  - by Felipe

  Hi all, I'm developing a web application using asp.net Mvc 2 and NHibernate, and I'm paging data (products in a category) in my page, but this data are random, so, I'm using a HQL statement link this: string hql = "from Product p where p.Category.Id=:IdCategory order by rand()"; It's working fine, but when I page, sometimes the same product appears in the first, second, etc... pages because it's order by rand(). Is there any way to make a random order by fixed by period (time internal) ? Or any solution ? thanks Cheers

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• C# how to quickly create dummy image directly in code

  - by PaN1C_Showt1Me

  How to quickly create some random image with the icon size? I don't want to use neither Image.FromFile, nor Image.FromStream

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• Linq to NHibernate, Order by Rand() ?

  - by Felipe

  Hi everybody, I'm using Linq To Nhibernate, and with a HQL statement I can do something like this: string hql = "from Entity e order by rand()"; Andi t will be ordered so random, and I'd link to know How can I do the same statement with Linq to Nhibernate ? I try this: var result = from e in Session.Linq<Entity> orderby new Random().Next(0,100) select e; but it throws a exception and doesn't work... is there any other way or solution? Thanks Cheers

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• How to increment counters based on a printed array

  - by Sam Liew

  I managed to developed a simple board of 5x5 using random numbers and array. Big achievement for someone like me! :) Now I have to increment the counters depending on the frequency of the numbers. If the value within 0-49 is printed..then nCounter++ If the value within 50-75 is printed..then pCounter++ something like that. The problem is that I don't know how to increase the counters based on the printed board. Here is the code: #include <stdio.h> #include <stdlib.h> #include <time.h> int main() { //Initialize Variables int randomNumber; int rows; int columns; int hdCounter =0; int hCounter = 0; int cCounter = 0; int pCounter = 0; int nCounter = 0; //Declare board size int board[5][5]; //size of board is 5 x 5 //Create the random number generator seed srand(time(NULL)); //Assign the random numbers from 1 - 25 into variable randomNumber //Create the rows for the board for ( rows = 1; rows <= 5 ; rows++ ) { //Create the colimns for the board for ( columns = 1; columns <= 5 ; columns++ ) { //Assign variable randomNumber into variable board randomNumber = rand() %100 + 1; board[rows][columns] = randomNumber; //print the board printf("%d\t", board[rows][columns]); //calculate the frequency of numbers on the printed board if (randomNumber >= 85 && randomNumber <= 100 ) hdCounter++; else if ( randomNumber >= 75 ) hCounter++; else if ( randomNumber >= 65 ) cCounter++; else if ( randomNumber >= 50 ) pCounter++; else if ( randomNumber >= 1 ) nCounter++; else continue; } //Newline after the end of 5th column. printf("\n\n"); } printf( "N \t P \t C \t H \t HD\n\n" ); printf("%d \t %d \t %d \t %d \t %d \t", nCounter, pCounter, cCounter, hCounter, hdCounter); }//end main I tried replacing randomNumber in the if-statement with board[rows][columns] but I seem to get the same undesired results.

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• how to quickly create dummy image directly in .NET code

  - by PaN1C_Showt1Me

  How to quickly create some random image with the icon size? I don't want to use neither Image.FromFile, nor Image.FromStream

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• How to get unique numbers using randomint python?

  - by user2519572

  I am creating a 'Euromillions Lottery generator' just for fun and I keep getting the same numbers printing out. How can I make it so that I get random numbers and never get the same number popping up: from random import randint numbers = randint(1,50) stars = randint(1,11) print "Your lucky numbers are: ", numbers, numbers, numbers, numbers, numbers print "Your lucky stars are: " , stars, stars The output is just: >>> Your lucky numbers are: 41 41 41 41 41 >>> Your lucky stars are: 8 8 >>> Good bye! How can I fix this? Regards

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• Characteristics of an Initialization Vector

  - by Jamie Chapman

  I'm by no means a cryptography expert, I have been reading a few questions around Stack Overflow and on Wikipedia but nothing is really 'clear cut' in terms of defining an IV and it's usage. Points I have discovered: An IV is pre-pended to a plaintext message in order to strengthen the encryption The IV is truely random Each message has it's own unique IV Timestamps and cryptographic hashes are sometimes used instead of random values, but these are considered to be insecure as timestamps can be predicted One of the weaknesses of WEP (in 802.11) is the fact that the IV will reset after a specific amount of encryptions, thus repeating the IV I'm sure there are many other points to be made, what have I missed? (or misread!)

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• Choose item from a list with bias?

  - by ooboo

  Given a list of items x1 ... xn and associated probabilties p1 ... pn that sum up to 1 there's a well known procedure to select a random item with its associated proabability by sorting the list according to weight, choosing a random value between 1 and 0, and adding up to a culmination sum until it exceeds the value selected and return the item at this point. So if we have x1 - 0.5, x2 - 0.3, x3 - 0.2, if the randomly chosen value is less than 0.5 x1 will be chosen, if between 0.5 and 0.8, x2, and else x3. This requires sorting so it needs O(nlogn) time. Is there anything more efficient than that?

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• How are a session identifiers generated?

  - by Asaf R

  Most web applications depend on some kind of session with the user (for instance, to retain login status). The session id is kept as a cookie in the user's browser and sent with every request. To make it hard to guess the next user's session these session-ids need to be sparse and somewhat random. The also have to be unique. The question is - how to efficiently generate session ids that are sparse and unique? This question has a good answer for unique random numbers, but it seems not scalable for a large range of numbers, simply because the array will end up taking a lot of memory.

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• Mean of Sampleset and powered Sampleset

  - by Milla Well

  I am working on an ICA implementation wich is based on the assumption, that all source signals are independent. So I checked on the basic concepts of Dependence vs. Correlation and tried to show this example on sample data from numpy import * from numpy.random import * k = 1000 s = 10000 mn = 0 mnPow = 0 for i in arange(1,k): a = randn(s) a = a-mean(a) mn = mn + mean(a) mnPow = mnPow + mean(a**3) print "Mean X: ", mn/k print "Mean X^3: ", mnPow/k But I couldn't produce the last step of this example E(X^3) = 0: >> Mean X: -1.11174580826e-18 >> Mean X^3: -0.00125229267144 First value I would consider to be zero, but second value is too large, isn't it? Since I subtract the mean of a, I expected the mean of a^3 to be zero as well. Does the problem lie in the random number generator, the precision of the numerical values in my misunderstanding of the concepts of mean and expected value?

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• How to get some randomized concats based on 2 columns from 1 table?

  - by Nils Riedemann

  Hey folks, i have a large user Database (13k+), and for some reason i need to create random names. The users table has "first_name" and "last_name". Now i want to have 10 concats of full_name and last_name of two completely random rows. Is that even possible with SQL? My other idea was just to create a full_names and last_names table … but that would'nt be as much challenging. Oh, and it should not take up too much performance :) (so order by rand() is not an option ;))

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• How many double numbers are there between 0.0 and 1.0?

  - by polygenelubricants

  This is something that's been on my mind for years, but I never took the time to ask before. Many (pseudo) random number generators generate a random number between 0.0 and 1.0. Mathematically there are infinite numbers in this range, but double is a floating point number, and therefore has a finite precision. So the questions are: Just how many double numbers are there between 0.0 and 1.0? Are there just as many numbers between 1 and 2? Between 100 and 101? Between 10^100 and 10^100+1? Note: if it makes a difference, I'm interested in Java's definition of double in particular.

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• C++ function for picking from a list where each element has a distinct probability

  - by Stuart

  I have an array of structs and one of the fields in the struct is a float. I want to pick one of the structs where the probability of picking it is relative to the value of the float. ie struct s{ float probability; ... } sArray s[50]; What is the fastest way to decide which s to pick? Is there a function for this? If I knew the sum of all the probability fields (Note it will not be 1), then could I iterate through each s and compare probability/total_probability with a random number, changing the random number for each s? ie if( (float) (rand() / RAND_MAX) < probability)...

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• python -> combinations of numbers and letters

  - by tekknolagi

  #!/usr/bin/python import random lower_a = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'] upper_a = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'] num = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'] all = [] all = " ".join("".join(lower_a) + "".join(upper_a) + "".join(num)) all = all.split() x = 1 c = 1 while x < 10: y = [] for i in range(c): a = random.choice(all) y.append(a) print "".join(y) x += 1 c += 1 what i have now outputs something like the following: 5 hE HAy 1kgy Pt6JM 2pFuCb Jv5osaX 5q8PwWAO SvHWRKfI5 how can i make it systematically go through every combination of letters (upper and lowercase) for a given length, then add 1 to that length and repeat the process?

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• remove the spaces...

  - by tekknolagi

  !/usr/bin/python import random lower_a = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'] upper_a = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'] num = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'] all = [] all = " ".join("".join(lower_a) + "".join(upper_a) + "".join(num)) all = all.split() x = 0 while x < 10: for i in range(7): a = random.choice(all) print a, print x += 1 what i want to do is remove the spaces from the output what it gives now is Z 3 a A I K R G B i N 9 c E v g E r A N 8 e B 6 d v H O c a V 8 c x y b g 2 W a T T f 8 H T r 6 E p D K l 5 p u x q 8 P Z 9 T n I W X n B Q

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• Random DNS Client Issue with BIND9/Windows Server 2003 DNS

  - by upkels

  Within our office, we have a local server running DNS, for internal related "domains", (e.g. .internal, .office, .lan, .vpn, etc.). Randomly, only the hosts configured with those extensions will stop resolving on the Windows-based workstations. Sometimes it'll work for a couple weeks without issue on one machine, then suddenly stop working, or it'll happen on another 15 times per day. It's completely random for all workstations. When troubleshooting, I have opened up a command prompt, and issued various nslookup commands for some of these hosts, and they resolve, however I've been told that nslookup uses different "libraries" for name resolution than other applications such as web browsers, email clients, etc. The only solution thus far, is manually restarting the Windows DNS Client on each workstation when this happens. Issuing the ipconfig /flushdns command multiple times helps every now and then, but is not successful enough to even attempt before restarting the DNS Client. I have tried two different DNS servers; BIND9, and Windows Server 2003 R2 DNS, and the behavior is the same. We have a single Netgear JGS524 switch all workstations and servers are connected to within the office, and a Linksys SR224G switch in another department with workstations attached.

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• Random MKAnnotationView is tapped when there are overlapping annotations

  - by Alexandre Gellibert

  I cannot believe this bug/problem doesn't have any solution! In my iphone application, I'm using MapKit with MKMapView and custom MKAnnotationView. The problem is when annotations overlap (in my app, annotations are photos and those photos may overlap) and when you tap on the annotation that appears on front, other annotation (on back) receives the event (seems to be random). I didn't find any way to send the event to the front annotation. Z ordering and Order of overlapping annotations questions on stackoverflow did not help me that much. Please any idea is welcome (even ugly ones)! Here's some of my code (nothing fancy, very common): CustomAnnotation.h @interface CustomAnnotation : NSObject <MKAnnotation> { @private CustomAnnotationView* view; } @property (nonatomic, retain) CustomAnnotationView* view; @end CustomAnnotation.m @implementation CustomAnnotation @synthetize view; CustomAnnotationView.h @interface CustomAnnotationView : MKAnnotationView { } @end CustomAnnotationView.m @implementation CustomAnnotationView - (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{ // Do something related to the annotation tapped } @end Main class ... // Annotations are added and some of them overlaps with others. - (void)addAnnotation:(CustomAnnotation*)annotation { [map addAnnotation:annotation]; } ... - (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation { NSString* identifier = getIdentifierFromAnnotation(annotation); CustomAnnotationView* view; if(!(view = (CustomAnnotationView*)[map dequeueReusableAnnotationViewWithIdentifier:identifier])) { view = [[CustomAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier: identifier]; [(CustomAnnotation*)annotation setView:view]; [view release]; } return view; }

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• ASP.Net MVC: "Random" URLs getting generated by URL.Action

  - by Daniel Magliola

  I have 2 very similar routes, just because i'm trying to generate two different URLs for the same resource (same Controller/Action), and both are very similar. These are the routes definitions: routes.MapRoute("Post2.Mp3", "sites/{siteSlug}/post/{brand}/aconstant-{slug}.mp3", new { controller = "Posts", action = "Mp3" }, new { siteSlug = @"[A-Za-z0-9-_]+", slug = @"[^(aconstant\-)][A-Za-z0-9-_]+", brand = @"[A-Za-z0-9-_]+" }); routes.MapRoute("Post.Mp3", "sites/{siteSlug}/post/{slug}.mp3", new { controller = "Posts", action = "Mp3" }, new { siteSlug = @"[A-Za-z0-9-_]+", slug = @"[A-Za-z0-9-_]+" }); "brand" is going to be my site name, which is the same as "aconstant" in those routes. Now, if I try this: Url.Action("ShowMp3", "Posts", new { siteSlug = post.Site.Slug, slug = post.Slug, origin = "origfeedwithaudio", brand = "aconstant" }) sometimes I get the URL I expect: /sites/site-Name/post/aconstant/aconstant-post-Name.mp3 but sometimes, I get this: /sites/site-Name/post/post-Name.mp3?brand=aconstant By sometimes, I mean that different sets of "slugs" give me one or the other. I haven't seen the same set of slugs give me different URLs. I haven't found any reasonable rule for when i'm getting one or the other, it seems random. What is going on here? How can I be getting different URLs based on esentially the same arguments? Thanks! Daniel

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• CakePHP 1.3 and Uploadifive/Uploadify - Change Upload Filename to a random string

  - by CaboONE

  I have somehow implemented UPLOADIFIVE in my CakePHP application. Everything seems to work great including uploading multiple files and inserting the correct information in the Database. Based on the following code, I would like to UPLOAD AND SAVE EVERY FILE WITH A RANDOM NAME TAKING INTO ACCOUNT THE CURRENT DATE OR SOMETHING SIMILAR. How could I accomplish this? In my Photos Controller I have the following function: // This function is called at every file upload. It uploads the file onto the server // and save the corresponding image name, etc, to the database table `photos`. function upload() { $uploadDir = '/img/uploads/photos/'; if (!empty($_FILES)) { debug($_FILES); $tempFile = $_FILES['Filedata']['tmp_name'][0]; $uploadDir = $_SERVER['DOCUMENT_ROOT'] . $uploadDir; $targetFile = $uploadDir . $_FILES['Filedata']['name'][0]; // Validate the file type $fileTypes = array('jpg', 'jpeg', 'gif', 'png'); // Allowed file extensions $fileParts = pathinfo($_FILES['Filedata']['name'][0]); // Validate the filetype if (in_array($fileParts['extension'], $fileTypes)) { // Save the file move_uploaded_file($tempFile,$targetFile); $_POST['image'] = $_FILES['Filedata']['name'][0]; $this->Photo->create(); if ($this->Photo->save($_POST)) { $this->Session->setFlash($targetFile, 'default', array('class' => 'alert_success')); $this->redirect(array('action' => 'index')); } } else { // The file type wasn't allowed //echo 'Invalid file type.'; $this->Session->setFlash(__('The photo could not be saved. Please, try again.', true)); } } } In my View file - admin_add.ctp I have added the following function $('#file_upload').uploadifive({ 'auto' : false, 'uploadScript' : '/photos/upload', 'buttonText' : 'BROWSE FILES', 'method' : 'post', 'onAddQueueItem' : function(file) { this.data('uploadifive').settings.formData = { 'photocategory_id' : $('#PhotoPhotocategoryId').val() }; } }); <input type="file" name="file_upload" id="file_upload" />

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• Cassandra random read speed

  - by Jody Powlette

  We're still evaluating Cassandra for our data store. As a very simple test, I inserted a value for 4 columns into the Keyspace1/Standard1 column family on my local machine amounting to about 100 bytes of data. Then I read it back as fast as I could by row key. I can read it back at 160,000/second. Great. Then I put in a million similar records all with keys in the form of X.Y where X in (1..10) and Y in (1..100,000) and I queried for a random record. Performance fell to 26,000 queries per second. This is still well above the number of queries we need to support (about 1,500/sec) Finally I put ten million records in from 1.1 up through 10.1000000 and randomly queried for one of the 10 million records. Performance is abysmal at 60 queries per second and my disk is thrashing around like crazy. I also verified that if I ask for a subset of the data, say the 1,000 records between 3,000,000 and 3,001,000, it returns slowly at first and then as they cache, it speeds right up to 20,000 queries per second and my disk stops going crazy. I've read all over that people are storing billions of records in Cassandra and fetching them at 5-6k per second, but I can't get anywhere near that with only 10mil records. Any idea what I'm doing wrong? Is there some setting I need to change from the defaults? I'm on an overclocked Core i7 box with 6gigs of ram so I don't think it's the machine. Here's my code to fetch records which I'm spawning into 8 threads to ask for one value from one column via row key: ColumnPath cp = new ColumnPath(); cp.Column_family = "Standard1"; cp.Column = utf8Encoding.GetBytes("site"); string key = (1+sRand.Next(9)) + "." + (1+sRand.Next(1000000)); ColumnOrSuperColumn logline = client.get("Keyspace1", key, cp, ConsistencyLevel.ONE); Thanks for any insights

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• windows kernel mode IOCTL returns random results

  - by clyfe

  I use the following code to fetch PSTORAGE_HOTPLUG_INFO capabilities from disks via IOCTL in a minifilter driver, but the returning hotplugInfo structure has all the fields set to random nonzero values on subsequent executions. What am I doing wrong? RESULT: 00000014 0.00046322 IOCTL Volume Media Removable, 64 00000015 0.00046451 IOCTL Volume Media Hotplug 154 00000016 0.00046562 IOCTL Volume Device Hotplug 244 00000054 1020.44311523 IOCTL Volume Media Removable, 240 00000055 1020.44311523 IOCTL Volume Media Hotplug 102 00000056 1020.44311523 IOCTL Volume Device Hotplug 244 Sample code: //int SomeFunction(PFLT_VOLUME pFLTVolume) STORAGE_HOTPLUG_INFO storageHotplugInfo; KEVENT event; IO_STATUS_BLOCK ioStatus; PIRP pirp; PDEVICE_OBJECT deviceObject; PSTORAGE_HOTPLUG_INFO hotplugInfo; ASSERT(KeGetCurrentIrql() <= DISPATCH_LEVEL); status = FltGetDiskDeviceObject(pFLTVolume, &deviceObject); if(!NT_SUCCESS(status)){ DbgPrint("No Device for Volume\n"); return 0; } KeInitializeEvent(&event, NotificationEvent, FALSE); ASSERT(KeGetCurrentIrql() <= APC_LEVEL); pirp = IoBuildDeviceIoControlRequest( IOCTL_STORAGE_GET_HOTPLUG_INFO, deviceObject, NULL, 0, &storageHotplugInfo, sizeof(STORAGE_HOTPLUG_INFO), FALSE, &event, &ioStatus ); if(!pirp){ return 0; } ASSERT(KeGetCurrentIrql() <= DISPATCH_LEVEL); status = IoCallDriver(deviceObject, pirp); if (status == STATUS_PENDING) { status = KeWaitForSingleObject( &event, Executive, KernelMode, FALSE, NULL); } else { ioStatus.Status = status; } status = ioStatus.Status; hotplugInfo = (PSTORAGE_HOTPLUG_INFO) &pirp->AssociatedIrp.SystemBuffer; if(hotplugInfo->MediaRemovable){ DbgPrint("IOCTL Volume Media Removable, %d\n", hotplugInfo->MediaRemovable); } if(hotplugInfo->MediaHotplug){ DbgPrint("IOCTL Volume Media Hotplug %d\n", hotplugInfo->MediaHotplug); } if(hotplugInfo->DeviceHotplug){ DbgPrint("IOCTL Volume Device Hotplug %d\n", hotplugInfo->DeviceHotplug); } ObDereferenceObject(deviceObject);

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• Asp.Net Random Error

  - by John Boker

  At random times, twice in the past two weeks, the we application will start to error and not work until I recycle the app pool in IIS. The specific error and stacktrace are: System.Web.HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.InvalidCastException: Unable to cast object of type 'System.Guid' to type 'System.String'. at System.Data.Linq.SqlClient.SqlProvider.Execute(Expression query, QueryInfo queryInfo, IObjectReaderFactory factory, Object[] parentArgs, Object[] userArgs, ICompiledSubQuery[] subQueries, Object lastResult) at System.Data.Linq.SqlClient.SqlProvider.ExecuteAll(Expression query, QueryInfo[] queryInfos, IObjectReaderFactory factory, Object[] userArguments, ICompiledSubQuery[] subQueries) at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query) at System.Data.Linq.DataQuery`1.System.Linq.IQueryProvider.Execute[S](Expression expression) at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source) at DigitalScout.WEDS.Business.Slug.GetTeamPath(String teamID) at DigitalScout.WEDS.WebApp.Code.Navigator.TeamNavigator.Home(String teamID) at ASP.management_default_aspx.__DataBind__control7(Object sender, EventArgs e) at System.Web.UI.Control.OnDataBinding(EventArgs e) at System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) at System.Web.UI.Control.DataBindChildren() at System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) at System.Web.UI.WebControls.Repeater.CreateControlHierarchy(Boolean useDataSource) at System.Web.UI.WebControls.Repeater.OnDataBinding(EventArgs e) at System.Web.UI.Control.DataBindChildren() at System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) at System.Web.UI.WebControls.Repeater.CreateControlHierarchy(Boolean useDataSource) at System.Web.UI.WebControls.Repeater.OnDataBinding(EventArgs e) at DigitalScout.WEDS.WebApp.Management._default.Page_Load(Object sender, EventArgs e) at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) at System.Web.UI.Control.OnLoad(EventArgs e) at DigitalScout.WEDS.WebApp.Code.BaseClass.Pages.ManagementPage.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace --- at System.Web.UI.Page.HandleError(Exception e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest() at System.Web.UI.Page.ProcessRequest(HttpContext context) at ASP.management_default_aspx.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) This error happens for every user of the system until the app pool is recycled. Any help on this would be helpful as we are not able to reproduce the error.

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• Random Pairings that don't Repeat

  - by Andrew Robinson

  This little project / problem came out of left field for me. Hoping someone can help me here. I have some rough ideas but I am sure (or at least I hope) a simple, fairly efficient solution exists. Thanks in advance.... pseudo code is fine. I generally work in .NET / C# if that sheds any light on your solution. Given: A pool of n individuals that will be meeting on a regular basis. I need to form pairs that have not previously meet. The pool of individuals will slowly change over time. For the purposes of pairing, (A & B) and (B & A) constitute the same pair. The history of previous pairings is maintained. For the purpose of the problem, assume an even number of individuals. For each meeting (collection of pairs) and individual will only pair up once. Is there an algorithm that will allow us to form these pairs? Ideally something better than just ordering the pairs in a random order, generating pairings and then checking against the history of previous pairings. In general, randomness within the pairing is ok.

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• Generating lognormally distributed random number from mean, coeff of variation

  - by Richie Cotton

  Most functions for generating lognormally distributed random numbers take the mean and standard deviation of the associated normal distribution as parameters. My problem is that I only know the mean and the coefficient of variation of the lognormal distribution. It is reasonably straight forward to derive the parameters I need for the standard functions from what I have: If mu and sigma are the mean and standard deviation of the associated normal distribution, we know that coeffOfVar^2 = variance / mean^2 = (exp(sigma^2) - 1) * exp(2*mu + sigma^2) / exp(mu + sigma^2/2)^2 = exp(sigma^2) - 1 We can rearrange this to sigma = sqrt(log(coeffOfVar^2 + 1)) We also know that mean = exp(mu + sigma^2/2) This rearranges to mu = log(mean) - sigma^2/2 Here's my R implementation rlnorm0 <- function(mean, coeffOfVar, n = 1e6) { sigma <- sqrt(log(coeffOfVar^2 + 1)) mu <- log(mean) - sigma^2 / 2 rlnorm(n, mu, sigma) } It works okay for small coefficients of variation r1 <- rlnorm0(2, 0.5) mean(r1) # 2.000095 sd(r1) / mean(r1) # 0.4998437 But not for larger values r2 <- rlnorm0(2, 50) mean(r2) # 2.048509 sd(r2) / mean(r2) # 68.55871 To check that it wasn't an R-specific issue, I reimplemented it in MATLAB. (Uses stats toolbox.) function y = lognrnd0(mean, coeffOfVar, sizeOut) if nargin < 3 || isempty(sizeOut) sizeOut = [1e6 1]; end sigma = sqrt(log(coeffOfVar.^2 + 1)); mu = log(mean) - sigma.^2 ./ 2; y = lognrnd(mu, sigma, sizeOut); end r1 = lognrnd0(2, 0.5); mean(r1) % 2.0013 std(r1) ./ mean(r1) % 0.5008 r2 = lognrnd0(2, 50); mean(r2) % 1.9611 std(r2) ./ mean(r2) % 22.61 Same problem. The question is, why is this happening? Is it just that the standard deviation is not robust when the variation is that wide? Or have a screwed up somewhere?

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