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| | ** <code>std::remove_if</code> does not work with associative containers. | | ** <code>std::remove_if</code> does not work with associative containers. |
| | *:- You have to iterate over the elements with <code>for( auto i = map.begin(); i != map.end();)</code> | | *:- You have to iterate over the elements with <code>for( auto i = map.begin(); i != map.end();)</code> |
| − | *:- And apply <code>it = map.erase(...)</code> in the body of your <code>if</code> logic, and <code>++i</code> outside the <code>if</code> block. | + | *:- And apply <code>i = map.erase(i);</code> in the body of your <code>if</code> logic, and <code>++i</code> in the <code>else</code> block. |
| | ** Example: removing all elements whose keys have names with more than 2 characters: | | ** Example: removing all elements whose keys have names with more than 2 characters: |
| | *:- <code>std::remove_if( map.begin(),map.end(),[]( auto& el ){ return el.first.size() > 2; } );</code> | | *:- <code>std::remove_if( map.begin(),map.end(),[]( auto& el ){ return el.first.size() > 2; } );</code> |
Revision as of 03:58, 20 January 2020
Associative containers are super useful, both as a convenient fast way to create dictionary or mapping for real-world problems like managing game resources, and as a data structure to help solve more abstract algorithmic computer science problems. And hash tables are fast as balls.
Topics Covered
Part 1
-
std::map container interface
- Binary tree data structure
-
std::map key requirements (comparison)
-
std::map gotchas (std::remove_if and const keys)
-
std::set
-
std::multimap and std::multiset
Part 2
- Hash table performance vs. binary tree performance
- Hash table data structure
-
std::unordered_map key requirements
- Hash combining
-
std::unordered_map bucket interface and hashing policy
- When to choose
std::map over std::unordered_map
Video Timestamp Index
Tutorial 24.1
[Expand]
- The
std::map<KeyType,ValueType> class 0:46
- Maps consist of keys to lookup (associated with) values
-
map.insert( {key,value} ) to insert (key,value) pairs
-
map[key] returns a reference to the ValueType for a KeyType
- The Binary tree data structure 2:46
-
std::map performs lookup in O(log(n)), it uses a Binary tree data structure
- Key properties of a Binary Tree (BT):
- - Nodes can have at most 2 children (hence: binary)
- - Each left child is smaller and each right child is larger than its parent
- - Insertion is done by navigating the tree along a route Left for smaller, Right for larger such that the order property always holds
- The big advantage of the BT properties is that retrieval is very fast
- The beauty of
std::map is that we don't have to implement any of this; it's all there in the STL 7:00
- The STL implementation is further optimized, e.g. it uses a red-black tree for BT rebalancing
- A look at the
std::map cppreference.com documentation: Insert, Lookup, Find 7:35
-
map.insert() takes a pair type std::pair<KeyType,ValueType>, the Map's elements
- C++ can deduce the pair Type, so
map.insert({keyX,valueXYZ}); with curly braces will do the job
- An even better way to insert is through
map.emplace[] operation; it will construct the pair in-place.
- For lookup, you can use square braces,
map[x] will return a reference to the corresponding value
- Note: a lookup with a new key value will create that element in the map with the default constructed ValueType value
-
insert or emplace with a key that already exists will NOT override the existing value: std::map::emplace returns a std::pair<iterator,bool> where the bool inidicates whether an insertion took place
-
map.find("xyz") returns an iterator to the element if it exitst, and an iterator to map.end() if it doesn't exist (useful to check if a key already exists)
-
std::map comes with iterators and because it is a sorted map, when you iterate over its elements with for (auto& el : map), it will be in order (of the keys)
- Requirements on KeyType 14:30
- The KeyType has to be comparable. The third template parameter is a functor for KeyType Comparison that defaults to
std::less<KeyType>
- So by default keys have to implement the "less than" comparison operator or provide your own comparison functor when defining the map
-
std::map cppreference.com documentation continued: Erase 15:28
-
std::map::erase offers three basic ways to erase elements:
- - With an iterator; returns an iterator following the last removed element
- - With an iterator range, idem
- - By key through
map.erase(const KeyType& key); this operation returns the number of elements erased (in size_type)
- Important to know when working with associative containers 16:04
-
std::remove_if does not work with associative containers.
- - You have to iterate over the elements with
for( auto i = map.begin(); i != map.end();)
- - And apply
i = map.erase(i); in the body of your if logic, and ++i in the else block.
- Example: removing all elements whose keys have names with more than 2 characters:
- -
std::remove_if( map.begin(),map.end(),[]( auto& el ){ return el.first.size() > 2; } );
Tutorial 24.2
Homework Assignment
The homework for this video is to enable use of a custom datatype in unordered_map hashing over multiple (4) members of that datatype. The solution video is here.
Supplementary Link
See also
