Week 5- Associative Containers:

sets and maps

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ContentsContents

Container TypesSequence Containers

Adapter Containers

Associative Containers

Vector Stack Set, Multiset

Deque Queue Map, Mutltimap

List Priority Queue

Associative containers

• Designed to be especially efficient in accessing its elements by their key, as opposed to sequence containers which are more efficient in accessing elements by their position.

• A programmer can use the key without concern for how elements are physically stored and how operations make the association between the key and an element.

• Insertion, deletion, and testing whether an element is in it, in logarithmic time - O(log n). Implemented using binary search trees.

Sets

• In a set, the data value is just the key:

C++ Set

• Sets are containers that store unique elements following a specific order.

• The value of the elements in a set cannot be modified once in the container (the elements are always const), but they can be inserted or removed from the container.

• Multisets are containers where multiple elements can have equivalent values.

Iterators

• Set containers keep their elements in ascending order, which follows the container's sorting criterion (By default, this is a less operator<).

• Hence, begin() points at the smallest element in the set, and end() points just past the largest element in the set.

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CLASS set Operations <set>

iterator begin();Return an iterator pointing at the first member in

the set.

iterator end();Return an iterator pointing just past the last

member in the set.

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CLASS set Constructors <set>

set();Create an empty set. This is the Default

Constructor.set(T *first, T *last);

Initialize the set by using the address range [first, last).CLASS set Operations <set>

bool empty() const;Is the set empty?

int size() const;Return the number of elements in the set.

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CLASS set Operations <set>

int count(const T& key) const;Search for key in the set and return 1 if it is in the

set and 0 otherwise.

iterator find(const T& key);Search for key in the set and return an iterator

pointing at it, or end() if it is not found.

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CLASS set Operations <set>

pair<iterator, bool> insert(const T& key);If key is not in the set, insert it and then return a

pair whose first element is an iterator pointing to the new element and whose second element is true. Otherwise, return a pair whose first element is an iterator pointing at the existing element and whose second element is false.

Postcondition: The set size increases by 1 if key is

not in the set.int erase(const T& key);

If key is in the set, erase it and return 1; otherwise, return 0.

Postcondition: The set size decreases by 1 if key is

in the set.

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CLASS set Operations <set>

void erase(iterator pos);Erase the item pointed to by pos.

Preconditions: The set is not empty, and pos points

to a valid set element.Postcondition: The set size decreases by 1.

void erase(iterator first, iterator last);Erase the elements in the range [first, last).

Precondition: The set is not empty.Postcondition: The set size decreases by the

number of elements in the range.

Set Operations

Set Operations

Set Operations

Example

Implementing Set Intersection

Complexity: Up to linear O(N)at most 2*(count1+count2)-1 comparisons(where countX is the distance between firstX and lastX)

C++ implementation

Maps

C++ Map• Maps are associative containers that store elements formed by

a combination of a key value and a mapped value, following a specific order.

• The key values are generally used to sort and uniquely identify the elements, while the mapped values store the content associated to this key.

• The types of key and mapped value may differ, and are grouped together in a pair type.

• Multimaps are associative containers where multiple elements can have equivalent keys.

Maps• A map stores an element as a <key-value> pair,

The index operator [] leads us to refer to a map as an associative array. The index (key) for a map is not limited to integer values, but can be of any type.

If k does not match the key of any element in the container, the function inserts a new element.

Map iterators

insert()

find() and erase()

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Binary search trees

sets, maps

The storage structure should be selected so thatthe container operations can be implementedefficiently

Reading & HW reminder

• Book chapter 4.8

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HW3 is due this Friday. (Submission before Thursday will receive 20

additional points)