Difference between revisions of "Intermediate C++ Game Programming Tutorial 24"

From Chilipedia
Jump to: navigation, search
(→‎Part 1: unordered associative containers)
(→‎Video Timestamp Index)
Line 23: Line 23:
 
* The <code>std::map<KeyType,ValueType></code> class [https://youtu.be/JlPsCoCO99o?t=0m46s 0:46]
 
* The <code>std::map<KeyType,ValueType></code> class [https://youtu.be/JlPsCoCO99o?t=0m46s 0:46]
 
<div class="mw-collapsible-content">
 
<div class="mw-collapsible-content">
−
** Maps consist of keys to lookup (associated with) values
+
** Maps consist of keys to lookup (associated) values
 
** <code>map.insert( {key,value} )</code> to insert (key,value) pairs
 
** <code>map.insert( {key,value} )</code> to insert (key,value) pairs
 
** <code>map[key]</code> returns a reference to the ValueType for a KeyType
 
** <code>map[key]</code> returns a reference to the ValueType for a KeyType

Revision as of 04:50, 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: ordered associative containers

  • 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: unordered associative containers

  • 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: The ordered associative containers


  • The std::map<KeyType,ValueType> class 0:46
    • Maps consist of keys to lookup (associated) 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)
  • Two <<important>> things to know when working with associative containers 16:04
    • std::remove_if does not work with associative containers (will come with C++20).
    - 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.
    • Keys are const. You're not allowed to modify the keys 18:38
    - Makes sense: the keys define the structure of the binary tree.
    - If you modify the key you invalidate this structure (it would require a deletion and insertion to do it properly)
  • The std::set<KeyType> class 20:00
    • With a set, you only have keys, and a unique entry for each unique key
    • Use case: ensure that there are no duplicates in a set
  • The std::multimap and std::multiset classes 21:28
    • Map has unique keys, with multimap you can insert multiple elements with the same key
    • This enables operations like std::multimap::equal_range that returns a pair of iterators (begin and end) of the range where these elements have that same key
    • std::multimap::count will return the number of elements with specific key
  • Practical example of a multimap use case 22:30
    • Implementation example of a custom Comparison functor for the Vei2 class (2D coordinate vector).
    - Chili's choice for ordering (used in the body of the functor):
    - return (lhs.x == rhs.x) ? lhs.y < rhs.y : lhs.x < rhs.x;
    • Example of how to find and print multiple elements in a multimap using equal_range()
  • Lookup in multimaps 25:21
    • Note: the multimap class does not have an index operator []
    • When you do a lookup on a multimap, you should use equal_range()
    • The problem with find() on a multimap, is that if there are several elements with key in the ccontainer, any of them may be returned

Tutorial 24.2: The unordered associative containers

  • [WORK-IN-PROGRESS]

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

Retrieved from "https://wiki.planetchili.net/index.php?title=Intermediate_C%2B%2B_Game_Programming_Tutorial_24&oldid=2882"