Errors during upgrade from 17.10 to 18.04
sudo apt upgrade error 18.04 (Broken Packages)
How to fix "The system is running in low-graphics mode" error? Proper Way to Combine Feature Selection and Hyperparameter Tuning? Collection was modified; enumeration operation may not execute I can't get to the bottom of this error, because when the debugger is attached, it does not seem to occur.  Collection was modified; enumeration operation may not execute  Below is the code. This is a WCF server in a Windows service. The method NotifySubscribers() is called by the service whenever there is a data event (at random intervals, but not very often - about 800 times per day). When a Windows Forms client subscribes, the subscriber ID is added to the subscribers dictionary, and when the client unsubscribes, it is deleted from the dictionary. The error happens when (or after) a client unsubscribes. It appears that the next time the NotifySubscribers() method is called, the foreach() loop fails with the error in the subject line. The method writes the error into the application log as shown in the code below. When a debugger is attached and a client unsubscribes, the code executes fine. Do you see a problem with this code? Do I need to make the dictionary thread-safe? [ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)] public class SubscriptionServer : ISubscriptionServer {     private static IDictionary&lt;Guid, Subscriber&gt; subscribers;      public SubscriptionServer()     {                     subscribers = new Dictionary&lt;Guid, Subscriber&gt;();     }      public void NotifySubscribers(DataRecord sr)     {         foreach(Subscriber s in subscribers.Values)         {             try             {                 s.Callback.SignalData(sr);             }             catch (Exception e)             {                 DCS.WriteToApplicationLog(e.Message,                    System.Diagnostics.EventLogEntryType.Error);                  UnsubscribeEvent(s.ClientId);             }         }     }          public Guid SubscribeEvent(string clientDescription)     {         Subscriber subscriber = new Subscriber();         subscriber.Callback = OperationContext.Current.                 GetCallbackChannel&lt;IDCSCallback&gt;();          subscribers.Add(subscriber.ClientId, subscriber);                  return subscriber.ClientId;     }      public void UnsubscribeEvent(Guid clientId)     {         try         {             subscribers.Remove(clientId);         }         catch(Exception e)         {             System.Diagnostics.Debug.WriteLine(&quot;Unsubscribe Error &quot; +                      e.Message);         }     } } Algorithms for Time Series Anomaly Detection I'm currently using Twitter's AnomalyDetection in R: .  This algorithm provides time series anomaly detection for data with seasonality.  Question: are there any other algorithms similar to this (controlling for seasonality doesn't matter)?  I'm trying to score as many time series algorithms as possible on my data so that I can pick the best one / ensemble. When I use a multiclass move to choose Animal Companion, do I get Command for free? I came across a reference to it recently on  and (as of now) it is not explained.  I suspect  might be it, but I don't know for sure. Beamer, Babel and Tikz conflict How to blacklist kernel modules? What's a big-vocabulary word for someone with a big vocabulary? What's a big-vocabulary word for someone with a big vocabulary?  I'd like to say I'm looking for a "_____". Why do I have two docks in Ubuntu desktop (with GNOME 3) after installing Dash to Dock? example.com is hosted on IP address c.c.c.c at host1, no access any windows server admin at all blog.example.com is  on IP a.a.a.a at host2, full access to Linux server admin, using WordPress   So basically, I can fiddle with subdomain all I want.  I can't 301 redirect because the content on host 1 is an eCommerce site that I have no server control over. Can't put WordPress on it, can't run scripts, can't do anything on the server period.  Is there a method to "mask" all the URLs on root blog.example.com so they appear as example.com/blog/ folder?  They can't actually be sent to example.com/blog/ as that doesn't exist. It just needs to look to the surfer as if they are there.  Can this be done with some kind of masking or something?  Will this be better for SEO as hopefully search engines will see the blog in a directory on the main domain? When is tension constant in a rope? Consider a massless rope with pulling forces applied at each end.  How do we decide if tension is constant or not in a rope? Consider a few example scenarios:  For example, if there is a knot in the rope the tension is not constant throughout (why?. Similarly, if the rope is hung over a cylindrical pulley of a non-negligible radius, the tension is not constant (why?). If there isn't anything touching the rope, for example, 2 people tugging on the rope at each end, the tension is constant (why?). Executing Java programs through Python How do I do this? I was playing modern with the owner of a shop.  He played a  which he used to bring in 7 lands: 5 basic mountains and 2  cards.  He then claimed that each  triggered 5 times (killing me).  I am doubting this now.    Here is my reason: The two  cards entered at the same time as the other 5 basic lands.  That means when they were on the battlefield and their ability was in effect, the other 5 basic lands were already there too.  That seems too late to trigger a "Enters the Battlefield" effect.  So here is my question.  Am I right or wrong?  If I am wrong, please show me how the enter the battlefield effect can trigger for something that enters the battlefield at the same time as itself.  If I am right, please show me the rules that prove it.  I now have two shop owners that are convinced that this is a legal play.  If it is not legal, I am going to need clear rules citations to convince them.    Side note:  If I am wrong and it is a legal play, then I have a follow up scenario.  Say I have 2  cards and 5 other human cards in my graveyard.  If I play a spell that lets me return all of them to the battlefield at the same time (say ), would the  cards get 6 +1/+1 counters on them?    To me, if the trick that was played on me with  is legal then my  cards should also get the +1/+1 counters. How do I iterate over the words of a string? command not found and syntax error near unexpected token Why multiplying fractions is equal to multiply the tops, multiply the bottoms? $$\frac{a}{b}\times \frac{c}{d}=\frac{a\times c}{b \times d},$$ And why $$\frac{a}{b}\times \frac{c}{c}=\frac{a}{b},$$ Also why $$\frac{a}{b}+\frac{c}{b}=\frac{a+c}{b}.$$ I understand it, but I want a mathematical approach as a math student proves it. Also I want to know the mathematics topic of this question (number theory, logic, etc). A full answer is not necessary. Just a reference. Comments saying "thank you" remind OP to accept answer How do I format a Decimal to a programatically controlled number of decimals in c#? What are the C++ rules for calling the base class constructor from a derived class? For example, I know in Java, you must do it as the first line of the subclass constructor (and if you don't, an implicit call to a no-arg super constructor is assumed - giving you a compile error if that's missing).