Estructuras de datos .NET: ArrayList, List, HashTable, Dictionary, SortedList, SortedDictionary & # 8212; ¿Velocidad, memoria, y cuándo usar cada uno?

StackOverflow https://stackoverflow.com/questions/128636

Pregunta

.NET tiene muchas estructuras de datos complejas. Desafortunadamente, algunos de ellos son muy similares, y no siempre estoy seguro de cuándo usar uno y cuándo usar otro. La mayoría de mis libros de C # y Visual Basic hablan de ellos en cierta medida, pero en realidad nunca entran en ningún detalle real.

¿Cuál es la diferencia entre Array, ArrayList, List, Hashtable, Dictionary, SortedList y SortedDictionary?

¿Cuáles son enumerables (IList - puede hacer bucles 'foreach')? ¿Cuáles usan pares clave / valor (IDict)?

¿Qué pasa con la huella de memoria? ¿Velocidad de inserción? ¿Velocidad de recuperación?

¿Hay alguna otra estructura de datos que valga la pena mencionar?

Todavía estoy buscando más detalles sobre el uso de memoria y la velocidad (notación Big-O).

¿Fue útil?

Solución

Fuera de la parte superior de mi cabeza:

  • Array *: representa una matriz de memoria de la vieja escuela, algo así como un alias para una matriz normal de type [] . Puede enumerar. No puede crecer automáticamente. Asumiría una velocidad de inserción y recuperación muy rápida.

  • ArrayList : matriz que crece automáticamente. Añade más gastos generales. Se puede enumerar, probablemente más lento que un arreglo normal pero aún así bastante rápido. Estos se utilizan mucho en .NET

  • List - uno de mis favoritos - se puede usar con genéricos, por lo que puedes tener una matriz fuertemente tipada, por ejemplo Lista < cadena > . Aparte de eso, actúa muy parecido a ArrayList

  • Hashtable - hashtable antiguo simple. O (1) a O (n) peor de los casos. Puede enumerar el valor y las propiedades de las claves, y hacer pares clave / valor

  • Diccionario : igual que arriba, solo se escribe con fuerza mediante genéricos, como Diccionario < cadena, cadena >

  • SortedList : una lista genérica clasificada. Disminuido en la inserción, ya que tiene que averiguar dónde poner las cosas. Se puede enumerar, probablemente lo mismo en la recuperación ya que no tiene que recurrir, pero la eliminación será más lenta que una lista antigua.

Tiendo a usar List y Dictionary todo el tiempo: una vez que empiezas a usarlos con tipos genéricos, es realmente difícil volver al estándar no genérico. unos.

También hay muchas otras estructuras de datos: hay KeyValuePair que puedes usar para hacer algunas cosas interesantes, hay un SortedDictionary que también puede ser útil.

Otros consejos

Si es posible, use genéricos. Esto incluye:

  • Lista en lugar de ArrayList
  • Diccionario en lugar de HashTable

Primero, todas las colecciones en .NET implementan IEnumerable.

Segundo, muchas de las colecciones son duplicadas porque los genéricos se agregaron en la versión 2.0 del marco.

Entonces, aunque las colecciones genéricas probablemente agreguen características, en su mayor parte:

  • List es una implementación genérica de ArrayList.
  • Diccionario es una implementación genérica de Hashtable

Las matrices son una colección de tamaño fijo en la que puede cambiar el valor almacenado en un índice determinado.

SortedDictionary es un IDictionary que se ordena según las claves. SortedList es un IDictionary que se ordena según un IComparer requerido.

Por lo tanto, las implementaciones de IDictionary (aquellas que soportan KeyValuePairs) son: * Tabla de picadillo * Diccionario * Lista clasificada * SortedDictionary

Otra colección que se agregó en .NET 3.5 es el Hashset. Es una colección que admite operaciones de conjuntos.

Además, LinkedList es una implementación de lista enlazada estándar (la Lista es una lista de matrices para una recuperación más rápida).

Una buena hoja de trucos que menciona las complejidades de las estructuras de datos, algoritmos, etc.

Aquí hay algunos consejos generales para usted:

  • Puede usar foreach en los tipos que implementan IEnumerable . IList es esencialmente un IEnumberable con Count y Item (acceso a elementos mediante un índice basado en cero). IDictionary , por otra parte, significa que puedes acceder a los elementos mediante cualquier índice de hashable.

  • Array , ArrayList y List implementan IList . Dictionary , SortedDictionary y Hashtable implementan IDictionary.

  • Si está utilizando .NET 2.0 o superior, se recomienda que utilice homólogos genéricos de los tipos mencionados.

  • Para la complejidad de tiempo y espacio de varias operaciones en estos tipos, debe consultar su documentación.

  • Las estructuras de datos .NET están en el espacio de nombres System.Collections . Hay bibliotecas de tipos como PowerCollections que ofrecen estructuras de datos adicionales.

  • Para obtener una comprensión completa de las estructuras de datos, consulte recursos como CLRS .

.NET data structures:

More to conversation about why ArrayList and List are actually different

Arrays

As one user states, Arrays are the "old school" collection (yes, arrays are considered a collection though not part of System.Collections). But, what is "old school" about arrays in comparison to other collections, i.e the ones you have listed in your title (here, ArrayList and List(Of T))? Let's start with the basics by looking at Arrays.

To start, Arrays in Microsoft .NET are, "mechanisms that allow you to treat several [logically-related] items as a single collection," (see linked article). What does that mean? Arrays store individual members (elements) sequentially, one after the other in memory with a starting address. By using the array, we can easily access the sequentially stored elements beginning at that address.

Beyond that and contrary to programming 101 common conceptions, Arrays really can be quite complex:

Arrays can be single dimension, multidimensional, or jadded (jagged arrays are worth reading about). Arrays themselves are not dynamic: once initialized, an array of n size reserves enough space to hold n number of objects. The number of elements in the array cannot grow or shrink. Dim _array As Int32() = New Int32(100) reserves enough space on the memory block for the array to contain 100 Int32 primitive type objects (in this case, the array is initialized to contain 0s). The address of this block is returned to _array.

According to the article, Common Language Specification (CLS) requires that all arrays be zero-based. Arrays in .NET support non-zero-based arrays; however, this is less common. As a result of the "common-ness" of zero-based arrays, Microsoft has spent a lot of time optimizing their performance; therefore, single dimension, zero-based (SZs) arrays are "special" - and really the best implementation of an array (as opposed to multidimensional, etc.) - because SZs have specific intermediary language instructions for manipulating them.

Arrays are always passed by reference (as a memory address) - an important piece of the Array puzzle to know. While they do bounds checking (will throw an error), bounds checking can also be disabled on arrays.

Again, the biggest hindrance to arrays is that they are not re-sizable. They have a "fixed" capacity. Introducing ArrayList and List(Of T) to our history:

ArrayList - non-generic list

The ArrayList (along with List(Of T) - though there are some critical differences, here, explained later) - is perhaps best thought of as the next addition to collections (in the broad sense). ArrayList inherit from the IList (a descendant of 'ICollection') interface. ArrayLists, themselves, are bulkier - requiring more overhead - than Lists.

IList does enable the implementation to treat ArrayLists as fixed-sized lists (like Arrays); however, beyond the additional functionallity added by ArrayLists, there are no real advantages to using ArrayLists that are fixed size as ArrayLists (over Arrays) in this case are markedly slower.

From my reading, ArrayLists cannot be jagged: "Using multidimensional arrays as elements... is not supported". Again, another nail in the coffin of ArrayLists. ArrayLists are also not "typed" - meaning that, underneath everything, an ArrayList is simply a dynamic Array of Objects: Object[]. This requires a lot of boxing (implicit) and unboxing (explicit) when implementing ArrayLists, again adding to their overhead.

Unsubstantiated thought: I think I remember either reading or having heard from one of my professors that ArrayLists are sort of the bastard conceptual child of the attempt to move from Arrays to List-type Collections, i.e. while once having been a great improvement to Arrays, they are no longer the best option as further development has been done with respect to collections

List(Of T): What ArrayList became (and hoped to be)

The difference in memory usage is significant enough to where a List(Of Int32) consumed 56% less memory than an ArrayList containing the same primitive type (8 MB vs. 19 MB in the above gentleman's linked demonstration: again, linked here) - though this is a result compounded by the 64-bit machine. This difference really demonstrates two things: first (1), a boxed Int32-type "object" (ArrayList) is much bigger than a pure Int32 primitive type (List); second (2), the difference is exponential as a result of the inner-workings of a 64-bit machine.

So, what's the difference and what is a List(Of T)? MSDN defines a List(Of T) as, "... a strongly typed list of objects that can be accessed by index." The importance here is the "strongly typed" bit: a List(Of T) 'recognizes' types and stores the objects as their type. So, an Int32 is stored as an Int32 and not an Object type. This eliminates the issues caused by boxing and unboxing.

MSDN specifies this difference only comes into play when storing primitive types and not reference types. Too, the difference really occurs on a large scale: over 500 elements. What's more interesting is that the MSDN documentation reads, "It is to your advantage to use the type-specific implementation of the List(Of T) class instead of using the ArrayList class...."

Essentially, List(Of T) is ArrayList, but better. It is the "generic equivalent" of ArrayList. Like ArrayList, it is not guaranteed to be sorted until sorted (go figure). List(Of T) also has some added functionality.

I sympathise with the question - I too found (find?) the choice bewildering, so I set out scientifically to see which data structure is the fastest (I did the test using VB, but I imagine C# would be the same, since both languages do the same thing at the CLR level). You can see some benchmarking results conducted by me here (there's also some discussion of which data type is best to use in which circumstances).

They're spelled out pretty well in intellisense. Just type System.Collections. or System.Collections.Generics (preferred) and you'll get a list and short description of what's available.

Hashtables/Dictionaries are O(1) performance, meaning that performance is not a function of size. That's important to know.

EDIT: In practice, the average time complexity for Hashtable/Dictionary<> lookups is O(1).

The generic collections will perform better than their non-generic counterparts, especially when iterating through many items. This is because boxing and unboxing no longer occurs.

An important note about Hashtable vs Dictionary for high frequency systematic trading engineering: Thread Safety Issue

Hashtable is thread safe for use by multiple threads. Dictionary public static members are thread safe, but any instance members are not guaranteed to be so.

So Hashtable remains the 'standard' choice in this regard.

There are subtle and not-so-subtle differences between generic and non-generic collections. They merely use different underlying data structures. For example, Hashtable guarantees one-writer-many-readers without sync. Dictionary does not.

Actually, I think MSDN helps provide pretty good answers to all these questions. Just look up .NET collections.

Most popular C# Data Structures and Collections

  • Array
  • ArrayList
  • List
  • LinkedList
  • Dictionary
  • HashSet
  • Stack
  • Queue
  • SortedList

C#.NET has a lot of different data structures, for example, one of the most common ones is an Array. However C# comes with many more basic data structures. Choosing the correct data structure to use is part of writing a well structured and efficient program.

In this article I will go over the built-in C# data structures, including the new ones introduces in C#.NET 3.5. Note that many of these data structures apply for other programming languages.

Array

The perhaps simplest and most common data structure is the array. A C# array is basically a list of objects. Its defining traits are that all the objects are the same type (in most cases) and there is a specific number of them. The nature of an array allows for very fast access to elements based on their position within the list (otherwise known as the index). A C# array is defined like this:

[object type][] myArray = new [object type][number of elements]

Some examples:

 int[] myIntArray = new int[5];
 int[] myIntArray2 = { 0, 1, 2, 3, 4 };

As you can see from the example above, an array can be intialized with no elements or from a set of existing values. Inserting values into an array is simple as long as they fit. The operation becomes costly when there are more elements than the size of the array, at which point the array needs to be expanded. This takes longer because all the existing elements must be copied over to the new, bigger array.

ArrayList

The C# data structure, ArrayList, is a dynamic array. What that means is an ArrayList can have any amount of objects and of any type. This data structure was designed to simplify the processes of adding new elements into an array. Under the hood, an ArrayList is an array whose size is doubled every time it runs out of space. Doubling the size of the internal array is a very effective strategy that reduces the amount of element-copying in the long run. We won't get into the proof of that here. The data structure is very simple to use:

    ArrayList myArrayList = new ArrayList();
    myArrayList.Add(56);
    myArrayList.Add("String");
    myArrayList.Add(new Form());

The downside to the ArrayList data structure is one must cast the retrived values back into their original type:

int arrayListValue = (int)myArrayList[0]

Sources and more info you can find here :

Licenciado bajo: CC-BY-SA con atribución
No afiliado a StackOverflow
scroll top