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std::unordered_map::operator[] - cppreference.com

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T& operator[]( const Key& key );

 (1) (desde C++11)

T& operator[]( Key&& key );

 (2) (desde 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().

Parâmetros

key -

a chave do elemento de encontrar

Original:

the key of the element to find

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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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Complexidade

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

Exemplo

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]

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";
    }
}

Saída: 

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'