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• How to maintain 2 lists of object

  - by michael

  Hi, My c++ program needs to maintain 2 list of objects. list inuse; list free; So objects A can even in 'inuse' list or in 'free' list, but not both. http://www.cplusplus.com/reference/stl/list/ I am think of using 'list' as the data structure for my lists. My questions are 1. why i can't randomly access an elmenet in the list, I look at the above api, I don't see a way to get inuse[2]; 2. How can I remove an element in the list? There is an erase(), but how can I use it to remove element #2? And after I remove element 2? will STL list fill the erased spot automatically? e.g. #3 will become #2, #4 will become #3 and so on? Thank you.

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• More nest Python nested dictionaries.

  - by clutch

  After reading http://stackoverflow.com/questions/635483/what-is-the-best-way-to-implement-nested-dictionaries-in-python why is it wrong to do: c = collections.defaultdict(collections.defaultdict(int)) in python? I would think this would work to produce {key:{key:1}} or am I thinking about it wrong?

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• JSON.NET and nHibernate Lazy Loading of Collections

  - by grahamscott

  Hi there Is anybody using JSON.NET with nHibernate? I notice that I'm getting errors when i try to load a class with child collections. Thanks Graham

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• Iterators for mutable collections in Scala?

  - by Lukasz Lew

  Does Scala have iterators with 'set' and 'remove' methods for iterating (and modifying) mutable collections like array? If there is no such iterator then is there a good reason for that?

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• What Are Collections Implemented As In VB6?

  - by Tom Tresansky

  So a collection in VB6 keeps track of a key for each object, and you can look up the object by its key. Does that mean collections are implemented as some sort of hashtable under the hood? I realize you can have multiple items with the same key in a collection, hence the SOME SORT. Anybody know what type data structure a VB6 collection is supposed to represent?

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• Javacing code in terminal havivng a Jar in CLASSPATH

  - by Masi

  How can you javac the code in terminal by using google-collections in CLASSPATH? Example of code trying to javac in terminal (works in Eclipse) import com.google.common.collect.BiMap; import com.google.common.collect.HashBiMap; public class Locate { ... BiMap<MyFile, Integer> rankingToResult = HashBiMap.create(); ... } Javacing in terminaling src 288 % javac Locate.java Locate.java:14: package com.google.common.collect does not exist import com.google.common.collect.BiMap; ^ Locate.java:15: package com.google.common.collect does not exist import com.google.common.collect.HashBiMap; ^ Locate.java:153: cannot find symbol symbol : class BiMap location: class Locate BiMap<MyFile, Integer> rankingToResult = HashBiMap.create(); ^ Locate.java:153: cannot find symbol symbol : variable HashBiMap location: class Locate BiMap<MyFile, Integer> rankingToResult = HashBiMap.create(); ^ 4 errors My CLASSPATH src 289 % echo $CLASSPATH /u/1/bin/javaLibraries/google-collect-1.0.jar

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• How do I make my ArrayList Thread-Safe? Another approach to problem in Java?

  - by thechiman

  I have an ArrayList that I want to use to hold RaceCar objects that extend the Thread class as soon as they are finished executing. A class, called Race, handles this ArrayList using a callback method that the RaceCar object calls when it is finished executing. The callback method, addFinisher(RaceCar finisher), adds the RaceCar object to the ArrayList. This is supposed to give the order in which the Threads finish executing. I know that ArrayList isn't synchronized and thus isn't thread-safe. I tried using the Collections.synchronizedCollection(c Collection) method by passing in a new ArrayList and assigning the returned Collection to an ArrayList. However, this gives me a compiler error: Race.java:41: incompatible types found : java.util.Collection required: java.util.ArrayList finishingOrder = Collections.synchronizedCollection(new ArrayList(numberOfRaceCars)); Here is the relevant code: public class Race implements RaceListener { private Thread[] racers; private ArrayList finishingOrder; //Make an ArrayList to hold RaceCar objects to determine winners finishingOrder = Collections.synchronizedCollection(new ArrayList(numberOfRaceCars)); //Fill array with RaceCar objects for(int i=0; i<numberOfRaceCars; i++) { racers[i] = new RaceCar(laps, inputs[i]); //Add this as a RaceListener to each RaceCar ((RaceCar) racers[i]).addRaceListener(this); } //Implement the one method in the RaceListener interface public void addFinisher(RaceCar finisher) { finishingOrder.add(finisher); } What I need to know is, am I using a correct approach and if not, what should I use to make my code thread-safe? Thanks for the help!

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• Is it safe to silently catch ClassCastException when searching for a specific value?

  - by finnw

  Suppose I am implementing a sorted collection (simple example - a Set based on a sorted array.) Consider this (incomplete) implementation: import java.util.*; public class SortedArraySet<E> extends AbstractSet<E> { @SuppressWarnings("unchecked") public SortedArraySet(Collection<E> source, Comparator<E> comparator) { this.comparator = (Comparator<Object>) comparator; this.array = source.toArray(); Collections.sort(Arrays.asList(array), this.comparator); } @Override public boolean contains(Object key) { return Collections.binarySearch(Arrays.asList(array), key, comparator) >= 0; } private final Object[] array; private final Comparator<Object> comparator; } Now let's create a set of integers Set<Integer> s = new SortedArraySet<Integer>(Arrays.asList(1, 2, 3), null); And test whether it contains some specific values: System.out.println(s.contains(2)); System.out.println(s.contains(42)); System.out.println(s.contains("42")); The third line above will throw a ClassCastException. Not what I want. I would prefer it to return false (as HashSet does.) I can get this behaviour by catching the exception and returning false: @Override public boolean contains(Object key) { try { return Collections.binarySearch(Arrays.asList(array), key, comparator) >= 0; } catch (ClassCastException e) { return false; } } Assuming the source collection is correctly typed, what could go wrong if I do this?

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• Compiling Java code in terminal having a Jar in CLASSPATH

  - by Masi

  How can you compile the code using javac in a terminal by using google-collections in CLASSPATH? Example of code trying to compile using javac in a terminal (works in Eclipse) import com.google.common.collect.BiMap; import com.google.common.collect.HashBiMap; public class Locate { ... BiMap<MyFile, Integer> rankingToResult = HashBiMap.create(); ... } Compiling in terminal src 288 % javac Locate.java Locate.java:14: package com.google.common.collect does not exist import com.google.common.collect.BiMap; ^ Locate.java:15: package com.google.common.collect does not exist import com.google.common.collect.HashBiMap; ^ Locate.java:153: cannot find symbol symbol : class BiMap location: class Locate BiMap<MyFile, Integer> rankingToResult = HashBiMap.create(); ^ Locate.java:153: cannot find symbol symbol : variable HashBiMap location: class Locate BiMap<MyFile, Integer> rankingToResult = HashBiMap.create(); ^ 4 errors My CLASSPATH src 289 % echo $CLASSPATH /u/1/bin/javaLibraries/google-collect-1.0.jar

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• WPF: IEditableCollectionView and CanAddNew and empty collections

  - by Aran Mulholland

  We were having some issues with the wpf datagrid and IEditableCollectionView (although this question applies to using IEditableCollectionView and ItemsControl) When you have a collection with no items in it, the IEditableCollectionView cannot determine what items should be inserted so it sets CanAddNew=false we found a solution here (buried deep in the comments) that goes like so : If you derive from ObservableCollection like this public class PersonsList : ObservableCollection<Person> { } you will find out that if the initial collection is empty, there won't be a NewItemPlaceHolder showing up on the view. That's because PersonsList cannot resolve type T at design time. A workaround that works for me is to pass type T as a parameter into the class like this PersonsList<T> : ObservableCollection<T> where T : Person { } This approach will place the NewItemPlaceHolder even if the collection is empty. I'm wondering if there is an interface i can implement on my collections that inform the IEditableCollectionView which type to create should i get an AddNew request.

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• silverlight with WCF(get data through collections)

  - by Piyush

  in my silverlight page I am fetching the data through WCF WCF is returning an BusinessEntityCollection that is the collection of rows SqlParameter[] sqlParameter = new SqlParameter[]{new SqlParameter("@recordType",recordType)}; MenuEntity menuEntity; MenuEntityCollection menuEntityCollection = new MenuEntityCollection(); using (SqlDataReader sqlDataReader = SqlHelper.ExecuteReader(_ConnectionString,CommandType.StoredProcedure, StoredProcedures.GetMenus, sqlParameter)) { if (sqlDataReader.Read()) { menuEntity = new MenuEntity(); DataAccessHelper.GetEntity(sqlDataReader, menuEntity); menuEntityCollection.Add(menuEntity); } } return menuEntityCollection; -- in silverlight page when I am calling WCF there I am getting an error MenuEntity menuList = new MenuEntity(); menuList = e.Result; <-----error line error: Cannot implicitly convert type 'System.Collections.ObjectModel.ObservableCollection' to 'FastTrackSLUI.AdminServiceReference.MenuEntity'

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• how is push_back implemented in STL vector?

  - by skydoor

  I was asked this question in an interview. The points I answered are like this 1) an index pointing to the current position; 2) resize if neccessary. Can anybody elaborate more?

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• How to include simple collections in ConfigurationSection

  - by mikemanne

  Is there a way for me to include a simple array of strings, or List<string> on my custom subclass of ConfigurationSection? (Or an array or generic list of simple data objects, for that matter?) I'm becoming familiar with the new (and VERY verbose) ConfigurationSection, ConfigurationElement, and ConfigurationElementCollection classes, but I'm by no means an expert yet. It seems like ConfigurationSection should handle simple collections/lists on its own, without me having to create a custom ConfigurationElementCollection subclass for each and every one. But I haven't found any reference to this ability online.

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• Visual Studio XSD Tool: Generate Collections Rather Than Arrays

  - by senfo

  I generated some C# classes from an XSD using the Visual Studio XSD utility and it generated arrays for storing a collection of elements, rather than one of the built-in generic Collection<T> (or related) classes. None of the command line parameters mentioned in xsd /? mention anything about generating collections rather than arrays, but I know that this can be done with web service proxy classes that Visual Studio generates, so I figured it must be possible. Does anybody know how to have the XSD utility generate collection classes rather than arrays?

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• Signature of Collections.min/max method

  - by Marco

  In Java, the Collections class contains the following method: public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> c) Its signature is well-known for its advanced use of generics, so much that it is mentioned in the Java in a Nutshell book and in the official Sun Generics Tutorial. However, I could not find a convincing answer to the following question: Why is the formal parameter of type Collection<? extends T>, rather than Collection<T>? What's the added benefit?

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• STL Static-Const Member Definitions

  - by javery

  How does the following work? #include <limits> int main() { const int* const foo = &std::numeric_limits<int> ::digits; } I was under the impression that in order to take an address of a static const-ant member we had to physically define it in some translation unit in order to please the linker. That said, after looking at the preprocessed code for this TU, I couldn't find an external definition for the digits member (or any other relevant members). I tested this on two compilers (VC++ 10 and g++ 4.2.4) and got identical results (i.e., it works). Does the linker auto-magically link against an object file where this stuff is defined, or am I missing something obvious here?

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• Declaring STL Data Structures such as Vector in the .h

  - by sc_ray

  I am trying to declare a private Data Structure such as the Vector in my C++ header file which I want to eventually use within the method implementation of my .cpp. An example would be my header "SomeClass.h" where I have: class SomeClass { private: Vector<T> myVector; public: void AddTtoMyVector(T add); } And in my .cpp which is "SomeClass.cpp", I have the following: #include "SomeClass.h" SomeClass::AddTtoMyVector(T add) { myVector.Push_back(add); } Would the syntax here work? Or is there a different way of declaring and populating such structures?

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• [C++][OpenMP] Proper use of "atomic directive" to lock STL container

  - by conradlee

  I have a large number of sets of integers, which I have, in turn, put into a vector of pointers. I need to be able to update these sets of integers in parallel without causing a race condition. More specifically. I am using OpenMP's "parallel for" construct. For dealing with shared resources, OpenMP offers a handy "atomic directive," which allows one to avoid a race condition on a specific piece of memory without using locks. It would be convenient if I could use the "atomic directive" to prevent simultaneous updating to my integer sets, however, I'm not sure whether this is possible. Basically, I want to know whether the following code could lead to a race condition vector< set<int>* > membershipDirectory(numSets, new set<int>); #pragma omp for schedule(guided,expandChunksize) for(int i=0; i<100; i++) { set<int>* sp = membershipDirectory[5]; #pragma omp atomic sp->insert(45); } (Apologies for any syntax errors in the code---I hope you get the point) I have seen a similar example of this for incrementing an integer, but I'm not sure whether it works when working with a pointer to a container as in my case.

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• Two collections manyToOne to same primary key

  - by Ethiel

  Hi, guys, I'm coding a web page in Hibernate-JPA and Oracle. I need the following: I have two classes: Place and Home. I need two collections of type Place in every Home: I do the following: Home: @ManyToOne @JoinColumn(name="ID_PLACES") private List<Places>places1; @ManyToOne @JoinColumn(name="ID_PLACES") private List<Places>Places2; However, hibernate got an exception (repeated column) and forces to me to mapping with insert and update to false. How Can I get Two ManyToOne relationship to same primary key with insert a true?.

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• Performing time consuming operation on STL container within a lock

  - by Ashley

  I have an unordered_map of an unordered_map which stores a pointer of objects. The unordered map is being shared by multiple threads. I need to iterate through each object and perform some time consuming operation (like sending it through network etc) . How could I lock the multiple unordered_map so that it won't blocked for too long? typedef std::unordered_map<string, classA*>MAP1; typedef std::unordered_map<int, MAP1*>MAP2; MAP2 map2; pthread_mutex_lock(&mutexA) //how could I lock the maps? Could I reduce the lock granularity? for(MAP2::iterator it2 = map2.begin; it2 != map2.end; it2++) { for(MAP1::iterator it1 = *(it2->second).begin(); it1 != *(it2->second).end(); it1++) { //perform some time consuming operation on it1->second eg sendToNetwork(*(it1->second)); } } pthread_mutex_unlock(&mutexA)

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• pointer delegate in STL set.

  - by ananth

  hi. Im kinda stuck with using a set with a pointer delegate. my code is as follows: void Graph::addNodes (NodeSet& nodes) { for (NodeSet::iterator pos = nodes.begin(); pos != nodes.end(); ++pos) { addNode(*pos); } } Here NodeSet is defined as: typedef std::set NodeSet; The above piece of code works perfectly on my windows machine, but when i run the same piece of code on a MAC, it gives me the following error: no matching function for call to 'Graph::addNode(const boost::shared_ptr&)' FYI, Node_ptr is of type: typedef boost::shared_ptr Node_ptr; can somebody plz tell me why this is happening?

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• Does gcc's STL support rvalue references now?

  - by Bear

  I know Visual Studio 2010's standard library has been rewritten to support rvalue references, which boosts its performance considerably. Does the standard library implementation of gcc 4.4 (and above) support rvalue references?

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• Does acos, atan functions in stl uses lots of cpu cycles

  - by jan

  Hi all, I wanted to calculate the angle between two vectors but I have seen these inverse trig operations such as acos and atan uses lots of cpu cycles. Is there a way where I can get this calculation done without using these functions? Also, does these really hit you when you in your optimization?

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• Multithreading with STL container

  - by Steven

  I have an unordered map which stores a pointer of objects. I am not sure whether I am doing the correct thing to maintain the thread safety. typedef std::unordered_map<string, classA*>MAP1; MAP1 map1; pthread_mutex_lock(&mutexA) if(map1.find(id) != map1.end()) { pthread_mutex_unlock(&mutexA); //already exist, not adding items } else { classA* obj1 = new classA; map1[id] = obj1; obj1->obtainMutex(); //Should I create a mutex for each object so that I could obtain mutex when I am going to update fields for obj1? pthread_mutex_unlock(&mutexA); //release mutex for unordered_map so that other threads could access other object obj1->field1 = 1; performOperation(obj1); //takes some time obj1->releaseMutex(); //release mutex after updating obj1 }

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• DTOs Collections mapping Problem

  - by the_knight5000

  I'm working now on a multi-tier project which has layers as following : DAL BLL GUI Layer and Shared DTOs between BLL and GUI layers. I'm facing a problem in mapping the Objects from DAO To DTO, No problem in the simple objects. The problem is in the Objects who have child collections of another objects. ex: Author Category --Categories --Authors the execution goes in an infinite loop of mapping and it get more complex when I want model Self-join tables ex: Safe Safe --TransferSafe(Collection<Safe>) --TransferSafe(Collection<Safe>) the execution goes in an infinite loop of mapping any suggestions about a good solution or a practical mapping pattern?

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