covariance

问题 I have the following dataframe: A B 0 1 5 1 2 6 2 3 7 3 4 8 I wish to calculate the covariance a = df.iloc[:,0].values b = df.iloc[:,1].values Using numpy for cov as : numpy.cov(a,b) I get: array([[ 1.66666667, 1.66666667], [ 1.66666667, 1.66666667]]) Shouldn't the diagonal elements be 1? How do I get the diagonal elements to 1? 回答1: No they shouldn't. I think you might be confusing it with Correlation. Correlation and Covariance are different. What you see in the diagonals is simply the

问题 I have the following dataframe: A B 0 1 5 1 2 6 2 3 7 3 4 8 I wish to calculate the covariance a = df.iloc[:,0].values b = df.iloc[:,1].values Using numpy for cov as : numpy.cov(a,b) I get: array([[ 1.66666667, 1.66666667], [ 1.66666667, 1.66666667]]) Shouldn't the diagonal elements be 1? How do I get the diagonal elements to 1? 回答1: No they shouldn't. I think you might be confusing it with Correlation. Correlation and Covariance are different. What you see in the diagonals is simply the

问题 I have the following dataframe: A B 0 1 5 1 2 6 2 3 7 3 4 8 I wish to calculate the covariance a = df.iloc[:,0].values b = df.iloc[:,1].values Using numpy for cov as : numpy.cov(a,b) I get: array([[ 1.66666667, 1.66666667], [ 1.66666667, 1.66666667]]) Shouldn't the diagonal elements be 1? How do I get the diagonal elements to 1? 回答1: No they shouldn't. I think you might be confusing it with Correlation. Correlation and Covariance are different. What you see in the diagonals is simply the

问题 I have an extential type of tuple, and I want to pass it to a generic function (I use ClassTag in this example but it is a custom type class with invariant type parameter): type TC = (ClassTag[T], Option[T]) forSome {type T} def foo[T](ct: ClassTag[T], o: Option[T]) = {} val tc: TC = (classTag[String], Option[String](null)) foo(tc._1, tc._2) This gives me error: error: type mismatch; found : scala.reflect.ClassTag[T] required: scala.reflect.ClassTag[Any] Note: T <: Any, but trait ClassTag is

问题 I'm trying to define a custom generic dict, whose keys are of type T_key and values are of type T_val . I also want to put constraints on T_key and T_val , such that T_key can only be of type A or B or their subclass. How do I accomplish this? from typing import TypeVar, Generic class A: ... class B: ... class Asub(A): ... class Bsub(B): ... T_key = TypeVar('T_key', A, B, covariant=True) T_val = TypeVar('T_val', A, B, covariant=True) class MyDict(Generic[T_key, T_val]): ... w: MyDict[ A, B] x

问题 I'm trying to define a custom generic dict, whose keys are of type T_key and values are of type T_val . I also want to put constraints on T_key and T_val , such that T_key can only be of type A or B or their subclass. How do I accomplish this? from typing import TypeVar, Generic class A: ... class B: ... class Asub(A): ... class Bsub(B): ... T_key = TypeVar('T_key', A, B, covariant=True) T_val = TypeVar('T_val', A, B, covariant=True) class MyDict(Generic[T_key, T_val]): ... w: MyDict[ A, B] x

问题 I know that Java's type system is unsound (it fails to type check constructs that are semantically legal) and undecidable (it fails to type check some construct). For instance, if you copy/paste the following snippet in a class and compile it, the compiler will crash with a StackOverflowException (how apt). This is undecidability. static class ListX<T> {} static class C<P> extends ListX<ListX<? super C<C<P>>>> {} ListX<? super C<Byte>> crash = new C<Byte>(); Java uses wildcards with type

问题 I know that Java's type system is unsound (it fails to type check constructs that are semantically legal) and undecidable (it fails to type check some construct). For instance, if you copy/paste the following snippet in a class and compile it, the compiler will crash with a StackOverflowException (how apt). This is undecidability. static class ListX<T> {} static class C<P> extends ListX<ListX<? super C<C<P>>>> {} ListX<? super C<Byte>> crash = new C<Byte>(); Java uses wildcards with type

问题 would this be possible? (I don't have vs. 2010, so I can't try it myself, sorry) public interface IComplexList<out TOutput, in TInput> where TOutput : TInput { public IEnumerator<TOutput> GetEnumerator(); public void Add(TInput item); } public interface IList<T> : IComplexList<T, T> { } If I get it right, you could use this to actually implement covariance and contravariance in the same interface. 回答1: No, you can't. In your example IList<T> is invariant. IList<T> would require to declare in

问题 If a class has a convariant type parameter such as Iterable[+A], is there any difference between declaring def foo(bar: Iterable[_]) and def foo(bar: Iterable[Any]) ? If a class has a contravariant type parameter such as Growable[-A], is there any difference between declaring def foo(bar: Growable[_]) and def foo(bar: Growable[Nothing]) ? 回答1: It does make a little difference when generic parameter is bounded. For example, if you had class BoundedIterable[+A <: Something] class