set

问题 Inspired when answering the question of How do I know if a value of an element inside an array was changed?, The answer was using a Property Observer for checking if an array has been modified. However, How can I determine what is/are the updated element(s) in a collection type in a property observer? For example: class MyClass { var strings: [String] = ["hello", "world", "!"] { didSet(modifiedStrings) { print("strings array has been modified!!:") print(modifiedStrings) } } } let myClass =

问题 I was wondering if there was a clear/concise way to add something to a set and check if it was added without 2x hashes & lookups. this is what you might do, but it has 2x hash's of item if item not in some_set: # <-- hash & lookup some_set.add(item) # <-- hash & lookup, to check the item already is in the set other_task() This works with a single hash and lookup but is a bit ugly. some_set_len = len(some_set) some_set.add(item) if some_set_len != len(some_set): other_task() Is there a better

问题 public class Document extends Model { ... @ManyToMany public Set<User> accessors; ... } I want to select all Documents which accessors contain a certain user. I have just minimal experiences with SQL and no experiences with JPQL. So how to do that? thanks in advance 回答1: SELECT d FROM Document AS d WHERE :user MEMBER OF d.accessors Should be what you need, and it is simpler than joining tables. Just dont forget to use the user as a parameter instead of using its id: query.setParameter("user",

问题 I have implemented a graph. I want to sort a given subset of vertices with respect to their degrees. Therefore, I've written a custom comparator named DegreeComparator . private class DegreeComparator implements Comparator<Integer> { @Override public int compare(Integer arg0, Integer arg1) { if(adj[arg1].size() == adj[arg0].size()) return arg1 - arg0; else return adj[arg1].size() - adj[arg0].size()); } } So, which one of the below is more efficient? Using TreeSet public Collection<Integer>

问题 Is it even possible? Say you have private Set<String> names = new LinkedHashSet<String>(); and Strings are "Mike", "John", "Karen". Is it possible to get "1" in return to "what's the index of "John" without iteration? The following works fine .. with this question i wonder if there is a better way for (String s : names) { ++i; if (s.equals(someRandomInputString)) { break; } } 回答1: The Set interface doesn't have something like as an indexOf() method. You'd really need to iterate over it or to

问题 I'm currently trying to understand the mechanism behind the hash function defined for Python's built-in frozenset data type. The implementation is shown at the bottom for reference. What I'm interested in particular is the rationale for the choice of this scattering operation: lambda h: (h ^ (h << 16) ^ 89869747) * 3644798167 where h is the hash of each element. Does anyone know where these came from? (That is, was there any particular reason to pick these numbers?) Or were they simply chosen

问题 Maybe this is a silly question, but I cannot seem to find an obvious answer. I need a concurrent FIFO queue that contains only unique values. Attempting to add a value that already exists in the queue simply ignores that value. Which, if not for the thread safety would be trivial. Is there a data structure in Java or maybe a code snipit on the interwebs that exhibits this behavior? 回答1: If you want better concurrency than full synchronization, there is one way I know of to do it, using a

问题 This was quite an unplesant surprise: scala> Set(1, 2, 3, 4, 5) res18: scala.collection.immutable.Set[Int] = Set(4, 5, 1, 2, 3) scala> Set(1, 2, 3, 4, 5).toList res25: List[Int] = List(5, 1, 2, 3, 4) The example by itself suggest a "no" answer to my question. Then what about ListSet ? scala> import scala.collection.immutable.ListSet scala> ListSet(1, 2, 3, 4, 5) res21: scala.collection.immutable.ListSet[Int] = Set(1, 2, 3, 4, 5) This one seems to work, but should I rely on this behavior? What

问题 I have the following enum: package ir.raysis.tcs.rule.days; public enum Days { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY; } I tried to map it as a Set<Days> days as follows: @ElementCollection(targetClass = Days.class) @JoinTable(name = "days",joinColumns = @JoinColumn(name = "rule_id")) @Column(name ="daysOfWeek", nullable = false) @Enumerated(EnumType.STRING) private Set<Days> days = new HashSet<>(); However, it throws the following exception: Initial SessionFactory

问题 I stumbled across a blog post detailing how to implement a powerset function in Python. So I went about trying my own way of doing it, and discovered that Python apparently cannot have a set of sets, since set is not hashable. This is irksome, since the definition of a powerset is that it is a set of sets, and I wanted to implement it using actual set operations. >>> set([ set() ]) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'set' Is