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std::unordered_map::operator[]

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C++
 
Contenitori libreria
 
std::unordered_map
Membri funzioni
Original:
Member functions
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unordered_map::unordered_map
unordered_map::~unordered_map
unordered_map::operator=
unordered_map::get_allocator
Iteratori
Original:
Iterators
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unordered_map::begin
unordered_map::cbegin
unordered_map::end
unordered_map::cend
Capacità
Original:
Capacity
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unordered_map::erase
unordered_map::size
unordered_map::max_size
Modificatori
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
Lookup
Original:
Lookup
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unordered_map::count
unordered_map::find
unordered_map::equal_range
Benna interfaccia
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
Politica di Hash
Original:
Hash policy
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unordered_map::load_factor
unordered_map::max_load_factor
unordered_map::rehash
unordered_map::reserve
Osservatori
Original:
Observers
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unordered_map::hash_function
unordered_map::key_eq
 
T& operator[]( const Key& key );
 (1) (dal C++11)
T& operator[]( Key&& key );
 (2) (dal C++11)
Inserisce un nuovo elemento al contenitore con key come chiave e predefinito al valore costruito mappato e restituisce un riferimento al valore di nuova costruzione mappati. Se un elemento con key chiave esiste già, non viene eseguita e l'inserimento di un riferimento al suo valore mappato viene restituito.
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)
Esegue sostanza (insert(std::make_pair(key, T())).first)->second.
Original:
Essentially performs (insert(std::make_pair(key, T())).first)->second.
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2)
Esegue sostanza (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().

Indice

[modifica] Parametri

key -
la chiave dell'elemento da trovare
Original:
the key of the element to find
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[modifica] Valore di ritorno

Riferimento al valore mappato del nuovo elemento se nessun elemento con chiave key esistito. Altrimenti un riferimento al valore mappato dell'elemento esistente viene restituito.
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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[modifica] Complessità

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

[modifica] Esempio

This section is incomplete
Reason: no example

[modifica] Vedi anche
accedere elemento specificato con verifica dei limiti
Original:
access specified element with bounds checking
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(metodo pubblico) [modifica]

[modifica] Esempio

Conta le occorrenze di ogni parola in un vettore di stringhe .

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'
Estratto da "https://it.cppreference.com/mwiki/index.php?title=cpp/container/unordered_map/operator_at&oldid=16625"