1. STL Containers
Some other containers in the STL

2. 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?

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

4. 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?

5. 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

6. 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>

7. 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(" );
dns_lookup.insert(google);
// Option 2
auto apple = std::make_pair(" );
dns_lookup.insert(apple);
// Option 3
dns_lookup.insert(std::make_pair(" )); }
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

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

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

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

11. Iterating Over an std::map
int main() {
std::map<std::string, int> dns_lookup;
dns_lookup.insert(std::make_pair(" ));
dns_lookup.insert(std::make_pair(" ));
dns_lookup.insert(std::make_pair(" ));
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.
map_loop.cpp

12. 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

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

14. 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

15. 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.

16. 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

17. 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