variance

问题 The C# spec states that an argument type cannot be both covariant and contravariant at the same time. This is apparent when creating a covariant or contravariant interface you decorate your type parameters with "out" or "in" respectively. There is not option that allows both at the same time ("outin"). Is this limitation simply a language specific constraint or are there deeper, more fundamental reasons based in category theory that would make you not want your type to be both covariant and

问题 The C# spec states that an argument type cannot be both covariant and contravariant at the same time. This is apparent when creating a covariant or contravariant interface you decorate your type parameters with "out" or "in" respectively. There is not option that allows both at the same time ("outin"). Is this limitation simply a language specific constraint or are there deeper, more fundamental reasons based in category theory that would make you not want your type to be both covariant and

问题 The C# spec states that an argument type cannot be both covariant and contravariant at the same time. This is apparent when creating a covariant or contravariant interface you decorate your type parameters with "out" or "in" respectively. There is not option that allows both at the same time ("outin"). Is this limitation simply a language specific constraint or are there deeper, more fundamental reasons based in category theory that would make you not want your type to be both covariant and

问题 I can't find out how should I put up this ques so I used this method. I have a latitude-longitude dataset. The image posted below is what I want to produce.. This is my dataset: Latitude Longitude 21.06941667 71.07952778 21.06941667 71.07952778 21.06666667 71.08158333 21.07186111 71.08688889 21.08625 71.07083333 21.08719444 71.07286111 21.08580556 71.07686111 21.07894444 71.08225 .... I have used geom_path() to find the path. Now, As shown in fig. I have highlighted the variance with white

问题 Consider this short snippet: trait Table[+A] { type RowType = Seq[A] } Scala 2.11.7 compiler gives the following error: covariant type A occurs in invariant position in type Seq[A] of type RowType Why is A considered to be in invariant position in Seq[A] while Seq itself is defined as trait Seq[+A] ? Also, could you please provide a use case demonstrating possible issues with this type definition if we ignore the error? 回答1: For any B <: A your Table[B]#RowType will be more concrete than

问题 I have recently started using the statistics module for python. I've noticed that by default the variance() method returns the 'unbiased' variance or sample variance: import statistics as st from random import randint def myVariance(data): # finds the variance of a given set of numbers xbar = st.mean(data) return sum([(x - xbar)**2 for x in data])/len(data) def myUnbiasedVariance(data): # finds the 'unbiased' variance of a given set of numbers (divides by N-1) xbar = st.mean(data) return sum(

问题 I am trying to get some summary statistics (mean, variance and quantiles) from a data vector with tied values. In particular, it is stored in a frequency distribution table: unique data values var and number of ties frequency . I know I could use rep function to first expand the vector to its full format: xx <- rep(mydata$var, mydata$frequency) then do standard mean(xx) var(xx) quantile(xx) But the frequency is really large and I have many unique values, which makes the program really slow.

问题 I've been trying to do something like Func<string, bool> f Func<object, bool> F = f; and the compiler raises the following error: Cannot implicitly convert type 'System.Func<string,bool>' to 'System.Func<object,bool>' However, we obviously can do object x = "123" 回答1: Imagine if you had done this: Func<string, bool> fStr = str => str.Length > 10; Func<object, bool> fObj = fStr; Well, according to the signature of fObj , you should be able to call it with any argument like this: fObj(7); Which

问题 So, let's say I have a class with a contravariant type parameter: trait Storage[-T] { def list(path: String): Seq[String] def getObject[R <: T](path: String): Future[R] } The idea of the type parameter is to constrain the implementation to the upper boundary of types that it can return. So, Storage[Any] can read anything, while Storage[avro.SpecificRecord] can read avro records, but not other classes: def storage: Storage[avro.SpecificRecord] storage.getObject[MyAvroDoc]("foo") // works

问题 I am trying to plot the elbow of k means using the below code: load CSDmat %mydata for k = 2:20 opts = statset('MaxIter', 500, 'Display', 'off'); [IDX1,C1,sumd1,D1] = kmeans(CSDmat,k,'Replicates',5,'options',opts,'distance','correlation');% kmeans matlab [yy,ii] = min(D1'); %% assign points to nearest center distort = 0; distort_across = 0; clear clusts; for nn=1:k I = find(ii==nn); %% indices of points in cluster nn J = find(ii~=nn); %% indices of points not in cluster nn clusts{nn} = I; %%