C++ Primer, Quinta Edizione: Oltre la Ricerca Lineare: Il Potere dei Contenitori Associativi

Immagina una biblioteca in cui i libri non sono ordinati per data di arrivo, ma da una Chiave Universale. Questo rappresenta il cambiamento di paradigma dalla sequenzialità ai Contenitori Associativi. Invece di scansionare un vettore in modo lineare ($O(N)$), utilizziamo un map o set per ottenere ricerche in tempo logaritmico ($O(\log n)$).

1. La Natura dell'Associazione

In un map, memorizziamo coppie chiave-valore. La chiave agisce come indice che può essere una stringa, un oggetto personalizzato o qualsiasi tipo che supporti Ordinamento Debole Stretto. Un set, al contrario, memorizza solo chiavi uniche, rendendolo lo strumento ideale per testare l'appartenenza o filtrare.

2. Ordinati vs. Non Ordinati

Contenitori standard (map, set) mantengono le chiavi ordinate. Lo Standard C++11 ha introdotto varianti non ordinate (unordered_map) che usano una funzione hash per prestazioni medie $O(1)$. Cerca il Logo C++11 quando utilizzi questi contenitori ad alte prestazioni.

TERMINALbash — 80x24

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QUESTION 1

What happens when you use the subscript operator m[k] on a map if the key k does not exist?

It throws an out_of_range exception.

It returns a null pointer.

It adds a new element with key k and a value-initialized value.

The program fails to compile.

❌ Incorrect

Unlike at(), the [] operator is an insertion-on-demand mechanism.

QUESTION 2

Which of the following calls is ILLEGAL for a multiset c and a vector v?

copy(v.begin(), v.end(), inserter(c, c.end()));

copy(v.begin(), v.end(), back_inserter(c));

copy(c.begin(), c.end(), back_inserter(v));

copy(c.begin(), c.end(), inserter(v, v.end()));

✅ Correct!

Correct. back_inserter requires push_back, which associative containers like multiset do not support.

❌ Incorrect

Associative containers lack push_back, so back_inserter cannot be used with them.

QUESTION 3

Identify the correct way to define a variable to hold the result of calling find on a map<string, vector>>.

vector<int> res = m.find("key");

map<string, vector>::iterator it = m.find("key");>

pair<string, vector> p = m.find("key");>

bool found = m.find("key");

✅ Correct!

find returns an iterator to the element if found, or m.end() otherwise.

❌ Incorrect

The find method always returns an iterator, not the value or a boolean.

QUESTION 4

What is the primary advantage of using a set over a vector for storing a list of excluded words?

A set uses less memory than a vector.

A set allows for duplicate excluded words.

A set provides O(log N) lookup instead of O(N) linear search.

A set allows elements to be accessed by index.

✅ Correct!

Using exclude.find(word) on a set is significantly more efficient than std::find on a vector for large datasets.

QUESTION 5

In the expression ++word_count.insert({word, 0}).first->second;, what does first refer to?

The key of the inserted element.

The boolean indicating if insertion was successful.

An iterator to the element with the given key.

The first element in the entire map.

✅ Correct!

map::insert returns a pair where first is an iterator to the element.

❌ Incorrect

The return of insert is a pair<iterator, bool>. first is the iterator.