std::unordered_map::operator[]
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<metanoindex/>
<tbody> </tbody> 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.The text has been machine-translated via Google Translate.
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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.The text has been machine-translated via Google Translate.
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You can help to correct and verify the translation. Click here for instructions.
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.The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions.
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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().
Parametri
| key | - | la chiave dell'elemento da trovare
Original: the key of the element to find The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. |
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.The text has been machine-translated via Google Translate.
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Complessità
Average case: constant, worst case: linear in size.
Esempio
| This section is incomplete Reason: no example |
Vedi anche
accedere elemento specificato con verifica dei limiti Original: access specified element with bounds checking The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (metodo pubblico) | |
Esempio
Conta le occorrenze di ogni parola in un vettore di stringhe .
Original:
Counts the occurrences of each word in a vector of strings.
The text has been machine-translated via Google Translate.
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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'