self-reference

问题 I am doing the homework of CIS 194. The problem is to implement the ruler function by using streamInterleave . The code looks like data Stream a = Cons a (Stream a) streamRepeat :: a -> Stream a streamRepeat x = Cons x (streamRepeat x) streamMap :: (a -> b) -> Stream a -> Stream b streamMap f (Cons x xs) = Cons (f x) (streamMap f xs) streamInterleave :: Stream a -> Stream a -> Stream a streamInterleave (Cons x xs) ys = Cons x (streamInterleave ys xs) ruler :: Stream Integer ruler =

问题 I am starting out with using the EF Code First with MVC and am a bit stumped with something. I have the following db structure (Sorry but I was not allowed to post an image unfortunately): Table - Products Table - RelatedProducts 1-Many on Products.ProductID -> RelatedProducts.ProductID 1-Many on Products.ProductID -> RelatedProducts.RelatedProductID Basically I have a product that can have a series of products that are related to it. These are kept in the RelatedProducts table with the

问题 I want to self-referentiate a model in a RoR app but, I don't know exactly how. I want to save a linked list where the next node has the id of the previous one. how can I do this rails way? It is a one-to-one relation. 回答1: The easiest way: class MyModel < ActiveRecord::Base belongs_to :parent, :class_name => 'MyModel' has_many :children, :class_name => 'MyModel', :foreign_key => 'parent_id' end 回答2: rails 5 add column xxx_id in users table: in migration file: add_reference :users, :xxx,

问题 As in this question, when a local variable not defined is used within its own assignment, it is evaluated to nil . x = x # => nil But when the name of a local variable conflicts with an existing method name, it is more tricky. Why does the last example below return nil ? {}.instance_eval{a = keys} # => [] {}.instance_eval{keys = self.keys} # => [] {}.instance_eval{keys = keys} # => nil 回答1: In Ruby, because methods can be called without an explicit receiver and without parentheses, there is a

问题 As in this question, when a local variable not defined is used within its own assignment, it is evaluated to nil . x = x # => nil But when the name of a local variable conflicts with an existing method name, it is more tricky. Why does the last example below return nil ? {}.instance_eval{a = keys} # => [] {}.instance_eval{keys = self.keys} # => [] {}.instance_eval{keys = keys} # => nil 回答1: In Ruby, because methods can be called without an explicit receiver and without parentheses, there is a

问题 As noted here, functions in packages, as well as static methods in classes, still need to use a packagename.functionname syntax or import packagename.* for each function (since the imports are part of the function workspace and not global). This means that changing the package/class name later on can become a tedious nuisance. Is there any way to do something like import this.* , i.e. a package/class name agnostic method to access all functions/static methods in the same package/class? 回答1:

问题 Just started crash coursing in Matlab OO programing and I would like to write a set method for a object that will set the value then reciprocate by setting itself in the relevant field on the other object. classdef Person properties age; sex; priority; % net priority based on all adjustment values adjustment; % personal adjustment value for each interest family; end methods function obj = set.sex(obj, value) if value == 'm' || value == 'f' obj.sex = value; else error('Sex must be m or f') end

问题 I a trying to make a std::variant that can contain a vector of the same variant: class ScriptParameter; using ScriptParameter = std::variant<bool, int, double, std::string, std::vector<ScriptParameter> >; I am getting ScriptParameter redefinition. It think it is possibly because a template parameter cannot be forward declared? Is there a way to achieve a variant that could also contain an array of same typed variants? 回答1: Since the forward declaration says ScriptParameter is a class, you can

问题 Wrestling my head to model following User has many Contact categories (family/friends) One Contact category has many contacts One contact can be in one or more contact categories. I end up with following, but I still believe, there must be better solution outhere. class User < ActiveRecord::Base has_many :contact_groups def get_all_contacts self.contact_groups.each do |group| contacts << group.users end end class ContactGroup < ActiveRecord::Base has_and_belongs_to_many :users end 回答1:

问题 I wanted to ask that how will we implement a copy constructor of a class which has self pointer to itself as its data member, i want to implement a deep copy, class City { string name; City* parent; public: City(string nam, double dcov); City(string nam, double dcov, City* c); City(const City& obj) { this-> name = obj.name; // how to assign parent parent = new City(??) } ~City(); void setName(string name1); void setDistanceCovered(int dist); string getName(); double getDistanceCovered(); City