Modern C++
Programming
18. Containers, Iterators,
Ranges, and Algorithms
Federico Busato
2024-11-05
Containers and Iterators
Container
A container is a class, a data structure, or an abstract data type, whose instances
are collections of other objects
• Containers store objects following specific access rules
Iterator
An iterator is an object allowing to traverse a container
• Iterators are a generalization of pointers
• A pointer is the simplest iterator, and it supports all its operations
C++ Standard Template Library (STL) is strongly based on containers and
iterators
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Reasons to use Standard Containers
• STL containers eliminate redundancy, and save time avoiding writing your own
code (productivity)
• STL containers are implemented correctly, and they do not need to spend time to
debug (reliability)
• STL containers are well-implemented and fast
• STL containers do not require external libraries
• STL containers share common interfaces, making it simple to utilize different
containers without looking up member function definitions
• STL containers are well-documented and easily understood by other developers,
improving the understandability and maintainability
• STL containers are thread safe. Sharing objects across threads preserve the
consistency of the container
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Container Properties
C++ Standard Template Library (STL) Containers have the following properties:
• Default constructor
• Destructor
• Copy constructor and assignment (deep copy)
• Iterator methods begin() , end()
• Support std::swap
• Content-based and order equality ( ==, != )
• Lexicographic order comparison ( >, >=, <, <= )
• size()
∗
, empty() , and max size() methods
∗
except for std::forward list
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Iterator Concept
STL containers provide the following methods to get iterator objects:
• begin() returns an iterator pointing to the first element
• end() returns an iterator pointing to the end of the container (i.e. the element after the
last element)
There are different categories of iterators and each of them supports a subset of the
following operations:
Operation Example
Read *it
Write *it =
Increment it++
Decrement it--
Comparison it1 < it2
Random access it + 4 , it[2]
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Iterator Categories/Tags
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Iterator Semantic 1/2
Iterator
• Copy Constructible It(const It&)
• Copy Assignable It operator=(const It&)
• Destructible ∼X()
• Dereferenceable It value& operator*()
• Pre-incrementable It& operator++()
Input/Output Iterator
• Satisfy Iterator
• Equality bool operator==(const It&)
• Inequality bool operator!=(const It&)
• Post-incrementable It operator++(int)
Forward Iterator
• Satisfy Input/Output Iterator
• Default constructible It()
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Iterator Semantics 2/2
Bidirectional Iterator
• Satisfy Forward Iterator
• Pre/post-decrementable It& operator--(), It operator--(int)
Random Access Iterator
• Satisfy Bidirectional Iterator
• Addition/Subtraction
void operator+(const It& it) , void operator+=(const It& it) ,
void operator-(const It& it) , void operator-=(const It& it)
• Comparison
bool operator<(const It& it) , bool operator>(const It& it) ,
bool operator<=(const It& it) , bool operator>=(const It& it)
• Subscripting It value& operator[](int index)
anderberg.me/2016/07/04/c-custom-iterators/
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Overview
Sequence containers are data structures storing objects of the same data type in a
linear mean manner
The STL Sequence Container types are:
• std::array provides a fixed-size contiguous array (on stack)
• std::vector provides a dynamic contiguous array ( constexpr in C++20)
•
std::list provides a double-linked list
• std::deque provides a double-ended queue (implemented as array-of-array)
• std::forward list provides a single-linked list
While std::string is not included in most container lists, it actually meets the requirements
of a Sequence Container
embeddedartistry.com
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std::array
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std::vector
Other methods:
• resize() resizes the allocated elements of the container
• capacity() number of allocated elements
• reserve() resizes the allocated memory of the container (not size)
• shrink to fit() reallocate to remove unused capacity
• clear() removes all elements from the container (no reallocation)
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std::deque
Other methods:
• resize() resizes the allocated elements of the container
• shrink to fit() reallocate to remove unused capacity
• clear() removes all elements from the container (no reallocation)
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std::list
Other methods:
• resize() resizes the allocated elements of the container
• shrink to fit() reallocate to remove unused capacity
• clear() removes all elements from the container (no reallocation)
• remove() removes all elements satisfying specific criteria
• reverse() reverses the order of the elements
• unique() removes all consecutive duplicate elements
• sort() sorts the container elements
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std::forward list
Other methods:
• resize() resizes the allocated elements of the container
• shrink to fit() reallocate to remove unused capacity
• clear() removes all elements from the container (no reallocation)
• remove() removes all elements satisfying specific criteria
• reverse() reverses the order of the elements
• unique() removes all consecutive duplicate elements
• sort() sorts the container elements
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Supported Operations and Complexity
CONTAINERS operator[]/at front back
std::array O (1) O (1) O (1)
std::vector O (1) O (1) O (1)
std::list O (1) O (1)
std::deque O (1) O (1) O (1)
std::forward list O (1)
CONTAINERS
push front
pop front
push back
pop back
insert
(it)
erase
(it)
std::array
std::vector O (1)
∗
O (1)
∗
O (n) O (n)
std::list O (1) O (1) O (1) O (1) O (1) O (1)
std::deque O (1)
∗
O (1) O (1) O (1) O (1)
∗
/O (n)
†
O (1)
std::forward list O (1) O (1) O (1) O (1)
∗
Amortized time
†
Worst case (middle insertion)
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std::array example
# include <algorithm> // std::sort
# include <array>
// std::array supports initialization only throw initialization list
std::array<int, 3> arr1 = { 5, 2, 3 };
std::array<int, 4> arr2 = { 1, 2 }; // [3]: 0, [4]: 0
// std::array<int, 3> arr3 = { 1, 2, 3, 4 }; // compiler error
std::array<int, 3> arr4(arr1); // copy constructor
std::array<int, 3> arr5 = arr1; // assign operator
arr5.fill(3); // equal to { 3, 3, 3 }
std::sort(arr1.begin(), arr1.end()); // arr1: 2, 3, 5
cout << (arr1 >= arr5); // true
cout << sizeof(arr1); // 12
cout << arr1.size(); // 3
for (const auto& it : arr1)
cout << it << ", "; // 2, 3, 5
cout << arr1[0]; // 2
cout << arr1.at(0); // 2, throw if the index is not within the range
cout << arr1.data()[0]; // 2 (raw array)
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std::vector example
# include <vector>
std::vector<int> vec1 { 2, 3, 4 };
std::vector<std::string> vec2 = { "abc", "efg" };
std::vector<int> vec3(2); // [0, 0]
std::vector<int> vec4{2}; // [2]
std::vector<int> vec5(5, -1); // [-1, -1, -1, -1, -1]
fill(vec5.begin(), vec5.end(), 3); // equal to { 3, 3, 3, 3, 3 }
cout << sizeof(vec1); // 24
cout << vec1.size(); // 3
for (const auto& it : vec1)
std::cout << it << ", "; // 2, 3, 4
cout << vec1[0]; // 2
cout << vec1.at(0); // 2 (bound check)
cout << vec1.data()[0] // 2 (raw array)
vec1.push_back(5); // [2, 3, 4, 5]
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std::list example
# include <list>
std::list<int> list1 { 2, 3, 2 };
std::list<std::string> list2 = { "abc", "efg" };
std::list<int> list3(2); // [0, 0]
std::list<int> list4{2}; // [2]
std::list<int> list5(2, -1); // [-1, -1]
std::fill(list5.begin(), list5.end(), 3); // [3, 3]
list1.push_back(5); // [2, 3, 2, 5]
list1.sort(); // [2, 2, 3, 5]
list1.merge(list5); // [-1, -1, 2, 2, 3, 5] merge two sorted lists
list1.remove(2); // [-1, -1, 3, 5]
list1.unique(); // [-1, 3, 5]
list1.reverse(); // [5, 3, -1]
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std::deque example
# include <deque>
std::deque<int> queue1 { 2, 3, 2 };
std::deque<std::string> queue2 = { "abc", "efg" };
std::deque<int> queue3(2); // [0, 0]
std::deque<int> queue4{2}; // [2]
std::deque<int> queue5(2, -1); // [-1, -1]
std::fill(queue5.begin(), queue5.end(), 3); // [3, 3]
queue1.push_front(5); // [5, 2, 3, 2]
queue1[0]; // retuns 5
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std::forward list example
# include <forward_list>
std::forward_list<int> flist1 { 2, 3, 2 };
std::forward_list<std::string> flist2 = { "abc", "efg" };
std::forward_list<int> flist3(2); // [0, 0]
std::forward_list<int> flist4{2}; // [2]
std::forward_list<int> flist5(2, -1); // [-1, -1]
std::fill(flist5.begin(), flist5.end(), 4); // [4, 4]
flist1.push_front(5); // [5, 2, 3, 2]
flist1.insert_after(flist1.begin(), 0); // [5, 0, 2, 3, 2]
flist1.erase_after(flist1.begin()); // [5, 2, 3, 2]
flist1.remove(2); // [5, 3, 3]
flist1.unique(); // [5, 3]
flist1.reverse(); // [3, 5]
flist1.sort(); // [3, 5]
flist1.merge(flist5); // [3, 4, 4, 5] merge two sorted lists
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Overview
An associative container is a collection of elements not necessarily indexed with
sequential integers and that supports efficient retrieval of the stored elements through
keys
Keys are unique
• std::set is a collection of sorted unique elements (operator<)
• std::unordered set is a collection of unsorted unique keys
• std::map is a collection of unique <key, value> pairs, sorted by keys
• std::unordered map is a collection of unique <key, value> pairs, unsorted
Multiple entries for the same key are permitted
• std::multiset is a collection of sorted elements (operator<)
• std::unordered multiset is a collection of unsorted elements
• std::multimap is a collection of <key, value> pairs, sorted by keys
• std::unordered multimap is a collection of <key, value> pairs
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Internal Representation
Sorted associative containers are typically implemented using red-black trees, while
unordered associative containers (C++11) are implemented using hash tables
Red-Black Tree
Hash Table
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Supported Operations and Complexity
CONTAINERS
insert
erase
count
find
lower bound
upper bound
Ordered Containers O (log(n)) O (log(n)) O (log(n )) O (log(n )) O (log(n ))
Unordered Containers O (1)
∗
O (1)
∗
O (1)
∗
O (1)
∗
∗
O (n) worst case
• count() returns the number of elements with key equal to a specified argument
• find() returns the element with key equal to a specified argument
• lower bound() returns an iterator pointing to the first element that is not less than key
• upper bound() returns an iterator pointing to the first element that is greater than key
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Other Methods
Ordered/Unordered containers:
• equal range() returns a range containing all elements with the given key
std::map, std::unordered map
• operator[]/at() returns a reference to the element having the specified key in the
container. A new element is generated in the set unless the key is found
Unordered containers:
• bucket count() returns the number of buckets in the container
• reserve() sets the number of buckets to the number needed to accommodate at least
count elements without exceeding maximum load factor and rehashes the container
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std::set example
# include <set>
std::set<int> set1 { 5, 2, 3, 2, 7 };
std::set<int> set2 = { 2, 3, 2 };
std::set<std::string> set3 = { "abc", "efg" };
std::set<int> set4; // empty set
set2.erase(2); // [ 3 ]
set3.insert("hij"); // [ "abc", "efg", "hij" ]
for (const auto& it : set1)
cout << it << " "; // 2, 3, 5, 7 (sorted)
auto search = set1.find(2); // iterator
cout << search != set1.end(); // true
auto it = set1.lower_bound(4);
cout << *it; // 5
set1.count(2); // 1, note: it can only be 0 or 1
auto it_pair = set1.equal_range(2); // iterator between [2, 3)
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std::map example
# include <map>
std::map<std::string, int> map1 { {"bb", 5}, {"aa", 3} };
std::map<double, int> map2; // empty map
cout << map1["aa"]; // prints 3
map1["dd"] = 3; // insert <"dd", 3>
map1["dd"] = 7; // change <"dd", 7>
cout << map1["cc"]; // insert <"cc", 0>
for (const auto& it : map1)
cout << it.second << " "; // 3, 5, 0, 7
map1.insert( {"jj", 1} ); // insert pair
auto search = map1.find("jj"); // iterator
cout << (search != map1.end()); // true
auto it = map1.lower_bound("bb");
cout << (*it).second; // 5
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std::multiset example
# include <set> // std::multiset
std::multiset<int> mset1 {1, 2, 5, 2, 2}; // 1, 2, 2, 2, 5
std::multiset<double> mset2; // empty set
mset1.insert(5);
for (const auto& it : mset1)
cout << it << " "; // 1, 2, 2, 2, 5, 5
cout << mset1.count(2); // 3
auto it = mset1.find(5); // iterator
cout << *it; // 5
it = mset1.lower_bound(4);
cout << *it; // 5
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Overview
Container adaptors are interfaces for reducing the number of functionalities normally
available in a container
The underlying container of a container adaptors can be optionally specified in the
declaration
The STL Container Adaptors are:
• std::stack LIFO data structure
default underlying container: std::deque
• std::queue FIFO data structure
default underlying container: std::deque
• std::priority queue (max) priority queue
default underlying container: std::vector
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Container Adaptors Methods
std::stack interface for a FILO (first-in, last-out) data structure
• top() accesses the top element
• push() inserts element at the top
• pop() removes the top element
std::queue interface for a FIFO (first-in, first-out) data structure
• front() access the first element
• back() access the last element
• push() inserts element at the end
• pop() removes the first element
std::priority queue interface for a priority queue data structure (lookup to the
largest element by default)
• top() accesses the top element
• push() inserts element at the end
• pop() removes the first element
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Container Adaptor Examples
# include <stack> // <--
# include <queue> // <-- also include priority_queue
std::stack<int> stack1;
stack1.push(1); stack1.push(4); // [1, 4]
stack1.top(); // 4
stack1.pop(); // [1]
std::queue<int> queue1;
queue1.push(1); queue1.push(4); // [1, 4]
queue1.front(); // 1
queue1.pop(); // [4]
std::priority_queue<int> pqueue1;
pqueue1.push(1); pqueue1.push(5); pqueue1.push(4); // [5, 4, 1]
pqueue1.top(); // 5
pqueue1.pop(); // [4, 1]
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Implement a Simple Iterator 1/6
Goal: implement a simple iterator to iterate over a List of elements:
# include <iostream>
# include <algorithm>
// !! List implementation here
int main() {
List list;
list.push_back(2);
list.push_back(4);
list.push_back(7);
std::cout << *std::find(list.begin(), list.end(), 4); // print 4
for (const auto& it : list) // range-based loop
std::cout << it << " "; // 2, 4, 7
}
Range-based loops require: begin() , end() , pre-increment ++it , not equal comparison
it != end() , dereferencing *it
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Implement a Simple Iterator (List declaration) 2/6
using value_t = int;
struct List {
struct Node { // Internal Node Structure
value_t _value; // Node value
Node* _next; // Pointer to next node
};
Node* _head { nullptr }; // head of the list
Node* _tail { nullptr }; // tail of the list
void push_back(const value_t& value); // insert a value at the end
// !! here we have to define the List iterator "It"
It begin() { return It{_head}; } // begin of the list
It end() { return It{nullptr}; } // end of the list
};
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Implement a Simple Iterator (List definition) 3/6
void List::push_back(const value_t& value) {
auto new_node = new Node{value, nullptr};
if (_head == nullptr) { // empty list
_head = new_node; // head is updated
_tail = _head;
return;
}
assert(_tail != nullptr);
_tail->_next = new_node; // add new node at the end
_tail = new_node; // tail is updated
}
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Implement a Simple Iterator (Iterator declaration) 4/6
struct It {
Node* _ptr; // internal pointer
It(Node* ptr); // Constructor
value_t& operator*(); // Deferencing
// Not equal -> stop traversing
friend bool operator!=(const It& itA, const It& itB);
It& operator++(); // Pre-increment
It operator++(int); // Post-increment
// !! Type traits here
};
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Implement a Simple Iterator (Iterator definition) 5/6
List::It::It(Node* ptr) :_ptr(ptr) {}
value_t& Lis::It::operator*() { return _ptr->_value; }
bool operator!=(const It& itA, const It& itB) {
return itA._ptr != itB._ptr;
}
List::It& List::It::operator++() {
_ptr = _ptr->_next;
return *this;
}
List::It List::It::operator++(int) {
auto tmp = *this;
++(*this);
return tmp;
}
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Implement a Simple Iterator (Type Traits) 6/6
The type traits of an iterator describe its properties, e.g. the type of the value held,
and they are widely used in the std algorithms
std::iterator class template defines the type traits for an iterator. It has been
deprecated in C++17, so users need to provide the type traits explicitly
# include <iterator>
// !! Type traits
using iterator_category = std::forward_iterator_tag;
using difference_type = std::ptrdiff_t;
using value_type = value_t;
using pointer = value_t*;
using reference = value_t&;
internalpointers.com/post/writing-custom-iterators-modern-cpp
Preparation for std::iterator Being Deprecated
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Common Errors
Modify a container with a “active” iterators
# include <vector>
std::vector<int> vec{1, 2, 3, 4, 5};
for (auto x : vec)
vec.push_back(x); // iterator invalidation!!
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Iterator Operations 1/2
• std::advance(InputIt& it, Distance n)
Increments a given iterator it by n elements
- InputIt must support input iterator requirements
- Modifies the iterator
- Returns void
- More general than adding a value it + 4
- No performance loss if it satisfies random access iterator requirements
• std::next(ForwardIt it, Distance n) C++11
Returns the n-th successor of the iterator
- ForwardIt must support forward iterator requirements
- Does not modify the iterator
- More general than adding a value it + 4
- The compiler should optimize the computation if it satisfies random access iterator
requirements
- Supports negative values if it satisfies bidirectional iterator requirements
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Iterator Operations 2/2
• std::prev(BidirectionalIt it, Distance n) C++11
Returns the n-th predecessor of the iterator
- InputIt must support bidirectional iterator requirements
- Does not modify the iterator
- More general than adding a value it + 4
- The compiler should optimize the computation if it satisfies random access iterator
requirements
• std::distance(InputIt start, InputIt end)
Returns the number of elements from start to last
- InputIt must support input iterator requirements
- Does not modify the iterator
- More general than adding iterator difference it2 - it1
- The compiler should optimize the computation if it satisfies random access iterator
requirements
- C++11 Supports negative values if it satisfies random iterator requirements
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Examples
# include <iterator>
# include <iostream>
# include <vector>
# include <forward_list>
int main() {
std::vector<int> vector { 1, 2, 3 }; // random access iterator
auto it1 = std::next(vector.begin(), 2);
auto it2 = std::prev(vector.end(), 2);
std::cout << *it1; // 3
std::cout << *it2; // 2
std::cout << std::distance(it2, it1); // 1
std::advance(it2, 1);
std::cout << *it2; // 3
//--------------------------------------
std::forward_list<int> list { 1, 2, 3 }; // forward iterator
// std::prev(list.end(), 1); // compile error
}
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Container Access Methods
C++11 provides a generic interface for containers, plain arrays, and std::initializer list
to access to the corresponding iterator.
Standard method .begin() , .end() etc., are not supported by plain array and initializer list
• std::begin begin iterator
• std::cbegin begin const iterator
• std::rbegin begin reverse iterator
• std::crbegin begin const reverse iterator
• std::end end iterator
• std::cend end const iterator
• std::rend end reverse iterator
• std::crend end const reverse iterator
# include <iterator>
# include <iostream>
int main() {
int array[] = { 1, 2, 3 };
for (auto it = std::crbegin(array); it != std::crend(array); it++)
std::cout << *it << ", "; // 3, 2, 1
}
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Iterator Traits 1/2
std::iterator traits allows retrieving iterator properties
• difference type a type that can be used to identify distance between iterators
• value type the type of the values that can be obtained by dereferencing the
iterator. This type is void for output iterators
• pointer defines a pointer to the type iterated over value type
• reference defines a reference to the type iterated over value type
• iterator category the category of the iterator. Must be one of iterator
category tags
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Iterator Traits 2/2
# include <iterator>
template<typename T>
void f(const T& list) {
using D = std::iterator_traits<T>::difference_type; // D is std::ptrdiff_t
// (pointer difference)
// (signed size_t)
using V = std::iterator_traits<T>::value_type; // V is double
using P = std::iterator_traits<T>::pointer; // P is double*
using R = std::iterator_traits<T>::reference; // R is double&
// C is BidirectionalIterator
using C = std::iterator_traits<T>::iterator_category;
}
int main() {
std::list<double> list;
f(list);
}
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STL Algorithms Library
C++ STL Algorithms library
The algorithm library provides functions for a variety of purposes (e.g. searching,
sorting, counting, manipulating) that operate on ranges of elements
• STL Algorithm library allow great flexibility which makes included functions
suitable for solving real-world problem
• The user can adapt and customize the STL through the use of function objects
• Library functions work independently on containers and plain array
• Many of them support constexpr in C++20
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Examples 1/2
# include <algorithm>
# include <vector>
struct Unary {
bool operator()(int value) {
return value <= 6 && value >= 3;
}
};
struct Descending {
bool operator()(int a, int b) {
return a > b;
}
};
int main() {
std::vector<int> vector { 7, 2, 9, 4 };
// returns an iterator pointing to the first element in the range[3, 6]
std::find_if(vector.begin(), vector.end(), Unary());
// sort in descending order : { 9, 7, 4, 2 };
std::sort(vector.begin(), vector.end(), Descending());
}
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Examples 2/2
# include <algorithm> // it includes also std::multiplies
# include <vector>
# include <cstdlib> // std::rand
# include <numeric> // std::accumulate
struct Unary {
bool operator()(int value) { return value > 100; }
};
int main() {
std::vector<int> vector { 7, 2, 9, 4 };
int product = std::accumulate(vector.begin(), vector.end(), // product = 504
1, std::multiplies<int>());
std::generate(vector.begin(), vector.end(), std::rand);
// now vector has 4 random values
// remove all values > 100 using Erase-remove idiom
auto new_end = std::remove_if(vector.begin(), vector.end(), Unary());
// elements are removed, but vector size is still unchanged
vector.erase(new_end, vector.end()); // shrink vector to finish removal
}
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STL Algorithms Library (Possible Implementations)
std::find
template<class InputIt, class T>
InputIt find(InputIt first, InputIt last, const T& value) {
for (; first != last; ++first) {
if (*first == value)
return first;
}
return last;
}
std::generate
template<class ForwardIt, class Generator>
void generate(ForwardIt first, ForwardIt last, Generator g) {
while (first != last)
*first++ = g();
}
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Algorithm Library 1/5
• swap(v1, v2) Swaps the values of two objects
• min(x, y) Finds the minimum value between x and y
• max(x, y) Finds the maximum value between x and y
• min element(begin, end) (returns a pointer)
Finds the minimum element in the range [begin, end)
• max element(begin, end) (returns a pointer)
Finds the maximum element in the range [begin, end)
• minmax element(begin, end) C++11 (returns pointers <min,max>)
Finds the minimum and the maximum element in the range [begin, end)
en.cppreference.com/w/cpp/algorithm
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Algorithm Library 2/5
• equal(begin1, end1, begin2)
Determines if two sets of elements are the same in
[begin1, end1), [begin2, begin2 + end1 - begin1)
• mismatch(begin1, end1, begin2) (returns pointers <pos1,pos2>)
Finds the first position where two ranges differ in
[begin1, end1), [begin2, begin2 + end1 - begin1)
• find(begin, end, value) (returns a pointer)
Finds the first element in the range [begin, end) equal to value
• count(begin, end, value)
Counts the number of elements in the range [begin, end) equal to value
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Algorithm Library 3/5
• sort(begin, end) (in-place)
Sorts the elements in the range [begin, end) in ascending order
• merge(begin1, end1, begin2, end2, output)
Merges two sorted ranges [begin1, end1), [begin2, end2), and store the results in
[output, output + end1 - start1)
• unique(begin, end) (in-place)
Removes consecutive duplicate elements in the range [begin, end)
• binary search(begin, end, value)
Determines if an element value exists in the (sorted) range [begin, end)
• accumulate(begin, end, value)
Sums up the range [begin, end) of elements with initial value (common case equal to
zero)
• partial sum(begin, end) (in-place)
Computes the inclusive prefix-sum of the range [begin, end)
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Algorithm Library 4/5
• fill(begin, end, value)
Fills a range of elements [begin, end) with value
• iota(begin, end, value) C++11
Fills the range [begin, end) with successive increments of the starting value
• copy(begin1, end1, begin2)
Copies the range of elements [begin1, end1) to the new location
[begin2, begin2 + end1 - begin1)
• swap ranges(begin1, end1, begin2)
Swaps two ranges of elements
[begin1, end1), [begin2, begin2 + end1 - begin1)
• remove(begin, end, value) (in-place)
Removes elements equal to value in the range [begin, end)
• includes(begin1, end1, begin2, end2)
Checks if the (sorted) set [begin1, end1) is a subset of [begin2, end2)
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Algorithm Library 5/5
• set difference(begin1, end1, begin2, end2, output)
Computes the difference between two (sorted) sets
• set intersection(begin1, end1, begin2, end2, output)
Computes the intersection of two (sorted) sets
• set symmetric difference(begin1, end1, begin2, end2, output)
Computes the symmetric difference between two (sorted) sets
• set union(begin1, end1, begin2, end2, output)
Computes the union of two (sorted) sets
• make heap(begin, end) Creates a max heap out of the range of elements
• push heap(begin, end) Adds an element to a max heap
• pop heap(begin, end) Remove an element (top) to a max heap
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Algorithm Library - Other Examples
#include <algorithm>
int a = std::max(2, 5); // a = 5
int array1[] = {7, 6, -1, 6, 3};
int array2[] = {8, 2, 0, 3, 7};
int b = *std::max_element(array1, array1 + 5); // b = 7
auto c = std::minmax_element(array1, array1 + 5);
//*c.first = -1, *c.second = 7
bool d = std::equal(array1, array1 + 5, array2); // d = false
std::sort(array1, array1 + 5); // [-1, 3, 6, 6, 7]
std::unique(array1, array1 + 5); // [-1, 3, 6, 7]
int e = std::accumulate(array1, array1 + 4, 0); // 15
std::partial_sum(array1, array1 + 4, array1); // [-1, 2, 8, 15]
std::iota(array1, array1 + 5, 2); // [2, 3, 4, 5, 6]
std::make_heap(array2, array2 + 5); // [8, 7, 0, 3, 2]
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C++20 Ranges
Ranges are an abstraction that allows to operate on elements of data structures
uniformly. They are an extension of the standard iterators
A range is an object that provides begin() and end() methods (an iterator + a
sentinel)
begin() returns an iterator, which can be incremented until it reaches end()
template<typename T>
concept range = requires(T& t) {
ranges::begin(t);
ranges::end(t);
};
• An Overview of Standard Ranges
• Range, Algorithms, Views, and Actions - A Comprehensive Guide
• Eric Nielbler - Range v3
• Range by Example
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Key Concepts
Range View is a range defined on top of another range
Range Adaptors are utilities to transform a range into a view
Range Factory is a view that contains no elements
Range Algorithms are library-provided functions that directly operate on ranges
(corresponding to std iterator algorithm)
Range Action is an object that modifies the underlying data of a range
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Range View 1/2
A range view is a range defined on top of another range that transforms the
underlying way to access internal data
• Views do not own any data
• copy, move, assignment operations perform in constant time
• Views are composable
• Views are lazy evaluated
Syntax:
range/view | view
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Range View 2/2
#include <iostream>
#include <ranges>
#include <vector>
std::vector<int> v{1, 2, 3, 4};
for (int x : v | std::views::reverse)
std::cout << x << " "; // print: "4, 3, 2, 1"
auto rv2 = v | std::views::reverse; // cheap, it does not copy "v"
auto rv3 = v | std::views::drop(2) | // drop the first two elements
std::views::reverse;
for (int x : rv3) // lazy evaluated
std::cout << x << " "; // print: "4, 3"
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Range Adaptor 1/2
Range Adaptors are utilities to transform a range into a view with custom behaviors
• Range adaptors produce lazily evaluated views
• Range adaptors can be chained or composed (pipeline)
Syntax:
adaptor(range/view, args...)
adaptor(args...)(range/view)
range/view | adaptor(args...) // preferred syntax
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Range Adaptor 2/2
#include <iostream>
#include <ranges>
#include <vector>
std::vector<int> v{1, 2, 3, 4};
for (int x : v | std::ranges::reverse_view(v)) // @\textbf{adaptor}@
std::cout << x << " "; // print: "4, 3, 2, 1"
auto rv2 = std::ranges::reverse_view(v); // cheap, it does not copy "v"
auto rv3 = std::ranges::reverse_view(
std::ranges::drop_view(2, v)); // drop the first two elements
for (int x : rv3) // lazy evaluated
std::cout << x << " "; // print: "4, 3"
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Range Factory
Range Factory produces a view that contains no elements
# include <iostream>
# include <ranges>
for (int x : std::ranges::iota_view{1, 4}) // factory (adaptor)
std::cout << x << " "; // print: "1, 2, 3, 4"
for (int x : std::view::repeat('a', 4)) // factory (view)
std::cout << x << " "; // print: "a, a, a, a"
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Range Algorithms
The range algorithms are almost identical to the corresponding iterator-pair
algorithms in the std namespace, except that they have concept-enforced constraints
and accept range arguments
• Range algorithms are immediately evaluated
• Range algorithms can work directly on containers ( begin() , end() are no
more explicitly needed) and view
#include <algorithm>
#include <vector>
std::vector<int> vec{3, 2, 1};
std::ranges::sort(vec); // 1, 2, 3
Std Library - Range Algorithms
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Algorithm Operators and Projections
# include <algorithm>
# include <vector>
struct Data {
char value1;
int value2;
};
std::vector<int> vec{4, 2, 5};
auto cmp = [](auto a, auto b) { return a > b; }; // Unary boolean predicate
std::ranges::sort(vec, cmp); // 5, 4, 2
std::vector<Data> vec2{{'a', 4}, {'b', 2}, {'c', 5}};
std::ranges::sort(vec2, {}, &Data::value2); // Projection: 2, 4, 5
// {'b', 2}, {'a', 4}, {'c', 5}
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Algorithms and Views
// sum of the squares of the first 'count' numbers
auto sum_of_squares(int count) {
auto squares = std::views::iota(1, count) |
std::views::transform([](int x) { return x * x; });
return std::accumulate(squares, 0);
}
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Range Actions 1/2
The range actions mimic std algorithms and range algorithms adding the
composability property
• Range actions are eager evaluated
• Range algorithms work directly on ranges
• Not included in the std library
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Range Actions 2/2
# include <algorithm>
# include <vector>
std::vector<int> vec{3, 5, 6, 3, 5}
// in-place
vec = vec | actions::sort // 3, 3, 5, 5, 6
| actions::unique; // 3, 5, 6
vec |= actions::sort // 3, 3, 5, 5, 6
| actions::unique; // 3, 5, 6
// out-of-place
auto vec2 = std::move(vec) | actions::sort // 3, 3, 5, 5, 6
| actions::unique; // 3, 5, 6
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