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std::unordered_map::operator[]

Da cppreference.com

 
 
 
std::unordered_map
Funções de membro
Original:
Member functions
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unordered_map::unordered_map
unordered_map::~unordered_map
unordered_map::operator=
unordered_map::get_allocator
Iteradores
Original:
Iterators
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unordered_map::begin
unordered_map::cbegin
unordered_map::end
unordered_map::cend
Capacidade
Original:
Capacity
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unordered_map::erase
unordered_map::size
unordered_map::max_size
Modificadores
Original:
Modifiers
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unordered_map::clear
unordered_map::insert
unordered_map::emplace
unordered_map::emplace_hint
unordered_map::erase
unordered_map::swap
Pesquisa
Original:
Lookup
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unordered_map::count
unordered_map::find
unordered_map::equal_range
Interface de balde
Original:
Bucket interface
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unordered_map::begin2
unordered_map::end2
unordered_map::bucket_count
unordered_map::max_bucket_count
unordered_map::bucket_size
unordered_map::bucket
Política de hash
Original:
Hash policy
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unordered_map::load_factor
unordered_map::max_load_factor
unordered_map::rehash
unordered_map::reserve
Observadores
Original:
Observers
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unordered_map::hash_function
unordered_map::key_eq
 
T& operator[]( const Key& key );
(1) (a partir do C++ 11)
T& operator[]( Key&& key );
(2) (a partir do C++ 11)
Insere um novo elemento para o recipiente utilizando key como a chave padrão e valor calculado mapeada e retorna uma referência para o valor recém-construído mapeados. Se um elemento com key chave já existe, não é realizada de inserção e uma referência ao seu valor mapeado é devolvido.
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)
Executa essencialmente (insert(std::make_pair(key, T())).first)->second.
Original:
Essentially performs (insert(std::make_pair(key, T())).first)->second.
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2)
Executa essencialmente (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().

Índice

[editar] Parâmetros

key -
a chave do elemento de encontrar
Original:
the key of the element to find
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[editar] Valor de retorno

Referência ao valor mapeada do elemento novo se nenhum elemento com chave key existiu. Caso contrário, uma referência ao valor do elemento mapeada existente é retornado.
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] Complexidade

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

[editar] Exemplo

[editar] Veja também

acessar o elemento especificado com verificação de limites
Original:
access specified element with bounds checking
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(função pública membro) [edit]

[editar] Exemplo

Conta as ocorrências de cada palavra em um vetor de strings .

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";
    }
}

Output:

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'