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std::unordered_map::operator[]

De cppreference.com

 
 
 
std::unordered_map
Las funciones miembro
Original:
Member functions
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Los iteradores
Original:
Iterators
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Capacidad
Original:
Capacity
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Modificadores
Original:
Modifiers
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Buscar
Original:
Lookup
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Bucket interfaz
Original:
Bucket interface
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Política Hash
Original:
Hash policy
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Los observadores
Original:
Observers
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T& operator[]( const Key& key );
(1) (desde C++11)
T& operator[]( Key&& key );
(2) (desde C++11)
Inserta un nuevo elemento en el contenedor utilizando key como la clave y el valor por defecto asignado construido y devuelve una referencia al valor de nueva construcción asignada. Si un elemento con key clave ya existe, no se lleva a cabo la inserción y una referencia a su valor asignado se devuelve .
Original:
Inserts a new element to the container using key as the key and default constructed mapped value and returns a reference to the newly constructed mapped value. If an element with key key already exists, no insertion is performed and a reference to its mapped value is returned.
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1)
Esencialmente realiza (insert(std::make_pair(key, T())).first)->second .
Original:
Essentially performs (insert(std::make_pair(key, T())).first)->second.
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2)
Esencialmente realiza (insert(std::make_pair(std::move(key), T())).first)->second .
Original:
Essentially performs (insert(std::make_pair(std::move(key), T())).first)->second.
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If an insertion occurs and results in a rehashing of the container, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is higher than max_load_factor()*bucket_count().

Contenido

[editar] Parámetros

key -
la clave del elemento de encontrar
Original:
the key of the element to find
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[editar] Valor de retorno

La referencia al valor asignado del nuevo elemento si no hay ningún elemento con key clave existido. De lo contrario una referencia al valor mapeado del elemento existente se devuelve .
Original:
Reference to the mapped value of the new element if no element with key key existed. Otherwise a reference to the mapped value of the existing element is returned.
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[editar] Complejidad

Average case: constant, worst case: linear in size.

[editar] Ejemplo

[editar] Ver también

acceder al elemento especificado con comprobación de límites
Original:
access specified element with bounds checking
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(función miembro público) [editar]

[editar] Ejemplo

Cuenta el número de apariciones de cada palabra en un vector de cadenas .

Original:

Counts the occurrences of each word in a vector of strings.

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#include <string>
#include <iostream>
#include <vector>
#include <unordered_map>
 
int main()
{
    std::vector<std::string> words = { 
        "this", "sentence", "is", "not", "a", "sentence",
	"this", "sentence", "is", "a", "hoax" 
    };
 
    std::unordered_map<std::string,size_t>  word_map;
    for (auto w : words) {
        ++word_map[w];
    }
 
    for (auto elem : word_map) {
        std::cout << elem.second
	          << " occurrences of word '"
	          << elem.first << "'\n";
    }
}

Salida:

1 occurrences of word 'hoax'
2 occurrences of word 'this'
2 occurrences of word 'a'
2 occurrences of word 'is'
1 occurrences of word 'not'
3 occurrences of word 'sentence'