Containers & Iterators

STL Containers Container: object that holds other objects Sequence containers in STL: vector: dynamic array deque: double-ended queue list: linear list Associative containers: permit efficient retrieval of values based on keys – elements referenced by key, not position Unique keys – no two elements can have equivalent keys map: key-value pairs, like a dictionary set: no duplicate elements

Iterators Objects that act like pointers increment, decrement, dereference Use to cycle through container's elements Depending on type of iterator, may permit: moving forward and/or backward through elements retrieve elements modify elements Declare using iterator type defined by container Example: vector<int>::iterator iter = ...;

vector Review vector<int> v; // empty vector vector<int> vec(1000); // 1000 elements, all initialized to 0 cout << vec.size() << endl; // 1000 vec.clear(); // remove all elements cout << vec.size() << endl; // 0

vector resize() for vectors: specify size only (elements initialized to 0) Specify size and value for new elements new elements added at end of vector if vector shrinks, elements removed from end vector<int> v; // empty v.resize(10, 0); // Add 10 new elements to end, set all of them to 0 v.resize(5); // remove 5 elements from end v.resize(8, 1); // add three new elements to end, all initialized to 1 pop_back(): remove element from end v.pop_back();

vector iteration begin() iterator points to 1st element of vector end() iterator points one position past end of vector for(vector<int>::iterator it = myVector.begin(); it != myVector.end(); it++) cout << *itr << endl; begin() end() 120 42 3 -9 16

vector front(): return reference to first element back(): return reference to last element insert(): add element at specified position vec.insert(vec.begin() + n, element); // Add element at index n vec.insert(vec.begin() + n; numCopies, element); // add multiple copies at index n pop_back(): delete last element vector<int> v(5, 3); // 5 elements with value 3 v.push_back(7); int num = v.back(); // 7 v.pop_back(); // remove 7 from end v.insert(v.begin(), 9); // add 9 at beginning of vector

vector begin(): returns iterator pointing to first element end(): returns iterator pointing past the end of the vector doesn't point to an element in vector (don't dereference) erase(): remove a single element or range of elements from vector erase(iterator): iterator points to element to remove erase(iterator start, iterator end): remove elements in range [start, end) invalidates iterators pointing to or after removed element(s) returns iterator pointing at element following last element erased vec.erase(vec.begin() + 3); // remove element at index 3 vec.erase(vec.begin(), vec.begin() + 2); // erase first two elements

vector empty(): returns true if vector is empty, false otherwise insert(): add new element(s) at specified position vec.insert(vec.begin() + n, element); // add element at index n vec.insert(vec.begin() + n, numCopies, element); // add numCopies at index n invalidates existing iterators vector<int> vec(3, 50); // [50, 50, 50] vec.insert(vec.begin() + 1, 100); // add 100 at index 1: [50, 100, 50, 50] vec.insert(vec.begin() + 2, 2, 40); // [50, 100, 40, 40, 50, 50]

vector vector<string> v; v.push_back("hello"); v.push_back("bye"); vector<string>::iterator it = v.begin(); cout << itlength() << endl;

Iterating Backwards To traverse container's elements in reverse order, use a reverse_iterator For a reverse_iterator object: ++revIterator moves to the previous element rbegin(), rend() for(vector<int>::reverse_iterator it = vec.rbegin(); it != vec.rend(); it++) cout << *it << end; rend() rbegin() 18 -4 522 17

set set: represents an unordered collection without duplicates efficient insertion and removal, determining if element is in set insert(), erase(), count() count(): returns 1 if element in set, 0 otherwise elements must be comparable using < underlying implementation: binary search tree set<int> set1; set1.insert(120); // {120} set1.insert(32); // {32, 120} set1.insert(4); // {4, 32, 120} set1.insert(32); // {4, 32, 120} (no dupes) if(set1.count(32)) cout << "32 is in the set" << endl; cout << set1.size() << endl; // 3 set1.clear(); // removes all elements

set iterator set<int> mySet = ...; for(set<int>::iterator it = mySet.begin(); it != mySet.end(); it++) cout << *it << endl; The set's elements are traversed in sorted (increasing) order Example: #include<set> #include<cstdlib> set<int> randNums; for(int k = 0; k < 10; k++) randNums.insert(rand() % 100); // print random numbers in sorted order for(set<int>::iterator it = randomNums.begin(); it != randomNums.end(); it++) cout << *it << " ";

set Iterate over elements in a set in a specified range lower_bound(lower): returns iterator to first element >= lower upper_bound(upper): returns iterator to first element > upper To iterate over values in range [lower, upper]: for(set<int>::iterator it = mySet.lower_bound(100); it != mySet.upper_bound(200); it++) { cout << *it << endl; If set contains the odd numbers from 3 to 301, the loop will print 101, 103, ..., 199

pair parameterized helper class pair object holds two values of any type: pair<Type1, Type2> two fields: first (of Type1), second (of Type2) defined in <utility> pair<int, string> p1; p1.first = 312; p1.second = "I love C++"; pair<int, string> p2 = make_pair(422, "I love Java");

set Methods Method Description set<T> (iterator start, iterator stop) Constructs set containing copies of elements in range [start, stop). Duplicates are discarded. Elements are sorted. Accepts iterator from any source. set<int> theSet(vec.begin(), vec.end()); set<T>() Constructs empty set set<T>(const set<T>& other) Copy constructor set<int> set2 = set1; size_type size() const Returns number of elements in the set. int num = set1.size(); bool empty() const Returns true if set is empty, false otherwise iterator begin() Returns an iterator to the start of the set. set<int>::iterator it = set1.begin(); pair<iterator, bool> insert(const T& value) void insert(iterator begin, iterator end) first version: inserts value into set, returning a pair which contains an iterator to the element and true if value was inserted, false if it was already in set. second version: inserts range of elements into set, ignoring duplicates.

set Methods Method Description void clear() Removes all elements iterator end() Returns an iterator iterator find(const T& elt) Returns an iterator to specified element if it exists, and end otherwise size_type erase(const T& elt) void erase(iterator it) void erase(iterator start, iterator stop) if(mySet.erase(0)) {... // 0 erased mySet.erase(mySet.begin()); mySet.erase(mySet.begin(), mySet.end()); Removes an element from set. First version: 1 returned if element removed, 0 otherwise Second: removes element pointed at by it Third: Erases elements in range [start, stop) iterator lower_bound(const T& value) Returns iterator to first element >= value iterator upper_bound(const T& value) Returns iterator to first element > value

map key-value pairs map<string, string> dict; dict["thanks"] = "an expression of gratitude";

map mapping between set of keys and set of values elements are unordered – cannot be retrieved by position, only by key implemented using balanced binary tree keys must be comparable using < keys are unique – no two key-value pairs in map may have same key Useful operations: insert a key-value pair membership: check whether a key is in the map get value associated with a key remove a key-value pair

map: inserting elements myMap[key] = value; // add (key, value) to map map<string, int> myMap; // empty map myMap["apple"] = 2; //insert ("apple", 2) in myMap myMap["elephant"] = 3; square brackets can also be used to retrieve value associated with key: cout << myMap["apple"] << endl; // 2 cout << myMap["laugh"] << endl; // adds ("laugh", 0) and prints 0 find(): takes key and returns iterator that points to pair with that key, or end() if key doesn't exist in map map<string, int>::iterator it = myMap.find("laugh"); if(it == myMap.end()) cout << "key is not in map" << endl;

map iterator Dereferencing a map iterator produces a pair of type pair<const KeyType, valueType> Keys in a map cannot be modified, but they can be removed. Value can be modified. map<string, int>::iterator it = myMap.find("laugh"); if(it == myMap.end()) cout << "key is not in map" << endl; else cout << "Key " << itfirst << " has value " << itsecond << endl; To loop over a map: for(map<string, int>::iterator it = myMap.begin(); it != myMap.end(); it++) cout << itfirst << ": " << itsecond << endl; key-value pairs traversed in increasing sorted order of key (alphabetical order in our example) apple: 2 elephant: 3 laugh: 0 Is key in map?

map To remove an element with a specified key: myMap.erase(key); To remove all elements from map: clear() Number of elements in map: size() How to add a key-value pair: map<string, string> map1; map1["ee312"] = "happy"; map1["ee312"] = "sad"; // map1 is {("ee312", "sad")} map<string, string> map2; map2.insert(make_pair("ee312", "happy")); map2.insert(make_pair("ee312", "sad")); // map2 is {("ee312", "happy")} insert will not insert pair if a pair with the specified key is already in the map.

map: insert() A map stores a unique set of keys insert returns a value of type pair<iterator, bool> bool value: true if key-value pair inserted into map, false if key already existed in map iterator: points to key-value pair in map with specified key, whether or not it was newly added Using brackets to add a key-value pair often preferable pair<map<string, int>::iterator, bool> result = myMap.insert(make_pair("hi", 2)); if(!result.second) // if insertion failed, bool value is false result.firstsecond = 2; // update the value assigned to key update the second entry in the key-value pair to 2 iterator points to key-value pair with key "hi"

map Example Map a string to a set of synonyms // Map of <string, set<string>> map<string, set<string> > pairs; set<string> s1; s1.insert("hello"); s1.insert("hi"); s1.insert("hi ya"); pairs["greeting"] = s1; set<string> s2; s2.insert("bye"); s2.insert("later"); pairs["farewell"] = s2;

map Example Map a string to a set of synonyms for(map<string, set<string> >::iterator it = pairs.begin(); it != pairs.end(); it++) { cout << "Key: " << it->first << endl; for(set<string>::iterator it2 = (it->second).begin(); it2 != (it->second).end(); it2++) { cout << *it2 << " "; } cout << endl; Output: Key: farewell bye later Key: greeting hello hi hi ya Elements in pairs: ("greeting", {"hello", "hi", "hi ya"}) ("farewell", {"bye", "later"})