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

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

 (1) (seit C++11)

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

 (2) (seit C++11)

Fügt ein neues Element in den Behälter mit key als Schlüssel und Standard gebaut mapped Wert und gibt einen Verweis auf das neu errichtete kartiert Wert. Wenn ein Element mit Schlüssel key bereits existiert, wird keine Insertion durchgeführt und ein Verweis auf seine kartiert Wert zurückgegeben .

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)

Führt im wesentlichen (insert(std::make_pair(key, T())).first)->second .

Original:

Essentially performs (insert(std::make_pair(key, T())).first)->second.

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2)

Führt im wesentlichen (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().

Parameter

key -

der Schlüssel des Elements zu finden

Original:

the key of the element to find

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Rückgabewert

Bezugnahme auf den zugeordneten Wert des neuen Elements, wenn kein Element mit Schlüssel key existierte. Andernfalls wird ein Verweis auf das zugeordnete Wert der bestehenden Element zurückgegeben .

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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Komplexität

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

Beispiel

Siehe auch

Zugriff auf angegebene Element mit Überprüfung von Grenzen

Original:

access specified element with bounds checking

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(öffentliche Elementfunktion) [edit]

Beispiel

Zählt die Vorkommen jedes Wort in einem Vektor von 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'