equality

问题 The python documentation mentions that if you override __eq__ and the object is immutable, you should also override __hash__ in order for the class to be properly hashable. In practice, when I do this I often end up with code like class MyClass(object): def __init__(self, a, b): self.a = a self.b = b def __eq__(self, other): if type(other) is type(self): return (self.a == other.a) and (self.b == other.b) else: return False def __hash__(self): return hash((self.a, self.b)) This is somewhat

问题 This question already has answers here : Comparing two dictionaries and checking how many (key, value) pairs are equal (23 answers) Closed last year . This seems trivial, but I cannot find a built-in or simple way to determine if two dictionaries are equal. what I want is: a = {'foo': 1, 'bar': 2} b = {'foo': 1, 'bar': 2} c = {'bar': 2, 'foo': 1} b = {'foo': 2, 'bar': 1} e = {'foo': 1, 'bar': 2, 'baz':3} f = {'foo': 1} equal(a, b) # True equal(a, c) # True - order does not matter equal(a, d)

问题 Is it possible to define a simple syntactic notion of equality (similar to what GHC might automatically derive as the Eq instance for a Haskell 98 type), without either explicitly proving that each data constructor is injective, or doing something analogous, such as defining the retraction of each constructor and using cong ? In other words, is it possible to exploit the injectivity of data constructors more directly, rather than having to introduce one auxiliary function per constructor? The

问题 I 'm integrating a geographic coordinate class from CodePlex to my personal "toolbox" library. This class uses float fields to store latitude and longitude. Since the class GeoCoordinate implements IEquatable<GeoCoordinate> , I habitually wrote the Equals method like so: public bool Equals(GeoCoordinate other) { if (other == null) { return false; } return this.latitude == other.latitude && this.longitude == other.longitude; } At this point I stopped and considered that I 'm comparing floating

问题 With the following definition of equality, we have refl as constructor data _≡_ {a} {A : Set a} (x : A) : A → Set a where refl : x ≡ x and we can prove that function are congruent on equality cong : ∀ { a b} { A : Set a } { B : Set b } (f : A → B ) {m n} → m ≡ n → f m ≡ f n cong f refl = refl I am not sure I can parse what is going on exactly here. I think we are pattern matching refl on hidden parameters : if we replace the first occurence by refl by another identifier, we get a type error.

问题 System.Collections.IStructuralEquatable and System.Collections.IStructuralComparable were added in .NET 4, but why aren't they generic, like IEquatable<T> and IComparable<T> ? 回答1: The example on MSDN gives part of the answer here; it seems to be useful for heterogeneous equality, rather than homogeneous equality - i.e. for testing whether two objects (/values) of potentially different types should be considered equal. In such scenarios, it is extremely likely that the calling code is dealing

问题 I'm using the JSON library from Stig Brautaset(http://code.google.com/p/json-framework) and I need to serialize an NSDate. I was considering converting it into a string before JSONifying it, however, I ran into this weird behavior: Why aren't these NSDates considered equal? NSDate *d = [[NSDate alloc] init]; NSDate *dd = [NSDate dateWithString:[d description]]; NSLog(@"%@", d); NSLog(@"%@", dd); if( [d isEqualToDate:dd] ){ NSLog(@"Yay!"); } 回答1: When you describe the original date object you

问题 When i am using dictionaries sometimes I have to change the default Equals meaning in order to compare Keys. I see that if I override the Equals and GetHashCode on the key's class or i create a new class which implements IEqualityComparer I have the same result. So what's the difference between using IEqualityComparer and Equals/GethashCode Override? Two Examples: class Customer { public string name; public int age; public Customer(string n, int a) { this.age = a; this.name = n; } public

问题 What is the difference between equivalence and equality in C++? There is a very similar question here. However, this question is tagged with math, while I am interested in the meaning in C++ context. To see the terms in context: Scott Meyers uses them in an example in this video. 回答1: On cppreference.com i found the following quote: For the types that are both EqualityComparable and LessThanComparable, the C++ standard library makes a distinction between equality, which is the value of the

问题 I've never come across this before, but how would you test whether three variables are the same? The following, obviously doesn't work but I can't think of an elegant (and correct) way to write the following: if ($select_above_average === $select_average === $select_below_average) { } 回答1: if ((a == b) && (b == c)) { ... they're all equal ... } by the transitive relation 回答2: $values = array($select_above_average, $select_average, $select_below_average); if(count(array_unique($values)) === 1)