equality

A discussion came up at work recently about Sets, which in Scala support the zip method and how this can lead to bugs, e.g. scala> val words = Set("one", "two", "three") scala> words zip (words map (_.length)) res1: Set[(java.lang.String, Int)] = Set((one,3), (two,5)) I think it's pretty clear that Set s shouldn't support a zip operation, since the elements are not ordered. However, it was suggested that the problem is that Set isn't really a functor, and shouldn't have a map method. Certainly, you can get yourself into trouble by mapping over a set. Switching to Haskell now, data AlwaysEqual

I would like to test the equivalency of a couple variables of enumeration types, like this: enum AnEnumeration { case aSimpleCase case anotherSimpleCase case aMoreComplexCase(String) } let a1 = AnEnumeration.aSimpleCase let b1 = AnEnumeration.aSimpleCase a1 == b1 // Should be true. let a2 = AnEnumeration.aSimpleCase let b2 = AnEnumeration.anotherSimpleCase a2 == b2 // Should be false. let a3 = AnEnumeration.aMoreComplexCase("Hello") let b3 = AnEnumeration.aMoreComplexCase("Hello") a3 == b3 // Should be true. let a4 = AnEnumeration.aMoreComplexCase("Hello") let b4 = AnEnumeration

I was writing an instructive example for a colleague to show him why testing floats for equality is often a bad idea. The example I went with was adding .1 ten times, and comparing against 1.0 (the one I was shown in my introductory numerical class). I was surprised to find that the two results were equal ( code + output ). float @float = 0.0f; for(int @int = 0; @int < 10; @int += 1) { @float += 0.1f; } Console.WriteLine(@float == 1.0f); Some investigation showed that this result could not be relied upon (much like float equality). The one I found most surprising was that adding code after the

class A(object): def __cmp__(self): print '__cmp__' return object.__cmp__(self) def __eq__(self, rhs): print '__eq__' return True a1 = A() a2 = A() print a1 in set([a1]) print a1 in set([a2]) Why does first line prints True, but second prints False? And neither enters operator eq ? I am using Python 2.6 You need to define __hash__ too. For example class A(object): def __hash__(self): print '__hash__' return 42 def __cmp__(self, other): print '__cmp__' return object.__cmp__(self, other) def __eq__(self, rhs): print '__eq__' return True a1 = A() a2 = A() print a1 in set([a1]) print a1 in set([a2

This question already has an answer here: Why isn't [1,2,3] equal to itself in Javascript? 6 answers Why is [] !== [] in JavaScript? I read through https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness but I could not find anything that explains this. Edit: I don't think this question or this question is an exact duplicate of mine. It asks about the == operator which just behaves crazy. The answer is an answer to my question but it's not the same question. That does a reference check on the two array literals to see if they are the same instance. The fact

I think that Entities should implement equality by primary key comparison as default, but the nhibernate documentation recommends using business identity: The most obvious way is to implement Equals()/GetHashCode() by comparing the identifier value of both objects. If the value is the same, both must be the same database row, they are therefore equal (if both are added to an ISet, we will only have one element in the ISet). Unfortunately, we can't use that approach. NHibernate will only assign identifier values to objects that are persistent, a newly created instance will not have any