STL Containers Some other containers in the STL

Today’s Questions What is an associative container? Why would I use an associative container over a sequential container? What is an unordered associative container? Why would I use an unordered associative container over an ordered one?

Motivating Associative Containers How can we find a value in a std::vector? Option 1:

Motivating Associative Containers How can we find a value in a std::vector? Option 1: std::find – which has O(n) complexity Option 2: sort the container, then search efficiently Difficult/error prone What data structure is inherently ordered?

Motivating Associative Containers How can we find a value in a std::vector? Option 1: std::find – which has O(n) complexity Option 2: sort the container, then search efficiently Difficult/error prone What data structure is inherently ordered? From ECE244: a binary search tree (BST) STL equivalent: std::map

std::map Benefits std::map uses std::pair Self-balancing BST Guaranteed O(log n): find, insert, and erase std::map uses std::pair First: key Second: value ECE244 Lab 5 – Domain Name Lookup Key: domain name as an std::string Value: IP address as an int std::pair<std::string, int>

std::map Insertion Example #include <map> #include <string> int main() { std::map<std::string, int> dns_lookup; // Option 1 std::pair<std::string, int> google("www.google.com", 215183441); dns_lookup.insert(google); // Option 2 auto apple = std::make_pair("www.apple.com", 398782126); dns_lookup.insert(apple); // Option 3 dns_lookup.insert(std::make_pair("www.utoronto.ca", 918567265)); } Include the map container Create the map, specifying the key and value Create pair first (constructor), then insert Create pair first (helper function), then insert Pass pair directly to insert map_insert.cpp

Searching std::map #include <iostream> #include <map> #include <string> int main() { std::map<std::string, int> dns_lookup; dns_lookup.insert(std::make_pair("www.google.com", 215183441)); dns_lookup.insert(std::make_pair("www.apple.com", 398782126)); dns_lookup.insert(std::make_pair("www.utoronto.ca", 918567265)); auto it = dns_lookup.find("www.apple.com"); if(it != dns_lookup.end()) { std::cout << it->first << ": " << it->second << "\n"; } it = dns_lookup.find("www.microsoft.com"); if(it == dns_lookup.end()) { std::cout << "Could not find www.microsoft.com\n"; What should we use to search, key or value? map_lookup.cpp

Searching std::map #include <iostream> #include <map> #include <string> int main() { std::map<std::string, int> dns_lookup; dns_lookup.insert(std::make_pair("www.google.com", 215183441)); dns_lookup.insert(std::make_pair("www.apple.com", 398782126)); dns_lookup.insert(std::make_pair("www.utoronto.ca", 918567265)); auto it = dns_lookup.find("www.apple.com"); if(it != dns_lookup.end()) { std::cout << it->first << ": " << it->second << "\n"; } it = dns_lookup.find("www.microsoft.com"); if(it == dns_lookup.end()) { std::cout << "Could not find www.microsoft.com\n"; How do we check if the key was found? How do we check if the key was not found? map_lookup.cpp

Searching std::map #include <iostream> #include <map> #include <string> int main() { std::map<std::string, int> dns_lookup; dns_lookup.insert(std::make_pair("www.google.com", 215183441)); dns_lookup.insert(std::make_pair("www.apple.com", 398782126)); dns_lookup.insert(std::make_pair("www.utoronto.ca", 918567265)); auto it = dns_lookup.find("www.apple.com"); if(it != dns_lookup.end()) { std::cout << it->first << ": " << it->second << "\n"; } it = dns_lookup.find("www.microsoft.com"); if(it == dns_lookup.end()) { std::cout << "Could not find www.microsoft.com\n"; www.apple.com: 398782126 The iterator points to the entire node in the map, which is an std::pair! Could not find www.microsft.com map_lookup.cpp

Iterating Over an std::map int main() { std::map<std::string, int> dns_lookup; dns_lookup.insert(std::make_pair("www.google.com", 215183441)); dns_lookup.insert(std::make_pair("www.apple.com", 398782126)); dns_lookup.insert(std::make_pair("www.utoronto.ca", 918567265)); for(auto it = dns_lookup.begin(); it != dns_lookup.end(); ++it) { std::cout << it->first << ": " << it->second << "\n"; } Use iterators and a for loop to print out all the nodes in the std::map. www.apple.com: 398782126 www.google.com: 215183441 www.utoronto.ca: 918567265 map_loop.cpp

Accessing Elements in std::map operator[key] Returns a reference to the value If key does not exist, will insert a new one Helpful as a replacement for insert Dangerous for lookup

What will this output? Google: 215183441 Microsoft: 0 Apple: 398782126 std::map<std::string, int> dns_lookup; dns_lookup.insert(std::make_pair("www.google.com", 215183441)); dns_lookup.insert(std::make_pair("www.apple.com", 398782126)); //dns_lookup.insert(std::make_pair("www.utoronto.ca", 918567265)); dns_lookup["www.utoronto.ca"] = 918567265; std::cout << "Google: " << dns_lookup["www.google.com"] << "\n"; std::cout << "Microsoft: " << dns_lookup["microsoft.com"] << "\n"; std::cout << "Apple: " << dns_lookup["www.apple.com"] << "\n"; std::cout << "Ebay: " << dns_lookup.["ebay.ca"] << "\n"; Google: 215183441 Microsoft: 0 Apple: 398782126 Ebay: 0 Missing keys inserted with default values! map_access.cpp

Duplicate Keys with Map #include <map> #include <iostream> int main() { std::map<std::string, float> salary_database; salary_database.insert(std::make_pair("John Smith", 45000)); salary_database.insert(std::make_pair("Mary Sue", 90000)); salary_database.insert(std::make_pair("Gary Stu", 40000)); salary_database.insert(std::make_pair("John Smith", 30000)); auto it = salary_database.find("John Smith"); std::cout << it->first " earns $" it->second << "\n"; } Key exists. Not inserted! John Smith earns $45000

Handling Duplicate Keys with Multimap #include <map> #include <iostream> int main() { std::multimap<std::string, float> salary_database; salary_database.insert(std::make_pair("John Smith", 45000)); salary_database.insert(std::make_pair("Mary Sue", 90000)); salary_database.insert(std::make_pair("Gary Stu", 40000)); salary_database.insert(std::make_pair("John Smith", 30000)); auto it = salary_database.find("John Smith"); std::cout << it->first " earns $" it->second << "\n"; } An insertion takes place – this is a multimap. Only one entry is (arbitrarily) returned.

Looking Up Multiple Values in MultiMap #include <map> #include <iostream> int main() { std::multimap<std::string, float> salary_database; salary_database.insert(std::make_pair("John Smith", 45000)); salary_database.insert(std::make_pair("Mary Sue", 90000)); salary_database.insert(std::make_pair("Gary Stu", 40000)); salary_database.insert(std::make_pair("John Smith", 30000)); auto range = salary_database.equal_range("John Smith"); for(auto it = range.first; it != range.second; ++it) { std::cout << it->first " earns $" it->second << "\n"; } equal_range returns a std::pair<iterator, iterator> that gives the range of entries matching the key “John Smith” John Smith earns $30000 John Smith earns $45000

Other Containers Container Find Insert Erase Comments std::map O(log n) Use when you need the keys to be ordered std::multimap Use when you have duplicate keys std::unordered_map O(1) Use when the order of keys does not matter std::set Use when you only need keys and not their values Container Push Pop Comments std::queue O(1) Use when you want first-in, first-out (FIFO) semantics std::priority_queue O(log n) Keeps elements ordered. Use when you want fast look-up of the largest (by default) element. std::stack Use when you want last-in, first-out (LIFO) semantics std::deque Similar to a vector, but can be faster if there are frequent insertions/deletions