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

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* Using an unordered map [https://youtu.be/LsjFAx-dG5I?t=1m38s 1:38]
 
* Using an unordered map [https://youtu.be/LsjFAx-dG5I?t=1m38s 1:38]
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** The interface is pretty much the same as its ordered counterpart
 
** The interface is pretty much the same as its ordered counterpart
 
** Include <code><unordered_map></code>, declare using <code>std::unordered_map<KeyType,ValueType></code>
 
** Include <code><unordered_map></code>, declare using <code>std::unordered_map<KeyType,ValueType></code>
 
** You can initialize your map object with an initializer list if you wanted to using <code>({ {..,..},{..,..},... })</code> inside your declaration
 
** You can initialize your map object with an initializer list if you wanted to using <code>({ {..,..},{..,..},... })</code> inside your declaration
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* The Hash Table data structure [https://youtu.be/LsjFAx-dG5I?t=3m20s 3:20]
 
* The Hash Table data structure [https://youtu.be/LsjFAx-dG5I?t=3m20s 3:20]
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** A hash table allows you to get the quick access to values, comparable to array access using the index, but with efficient memory usage
 
** A hash table allows you to get the quick access to values, comparable to array access using the index, but with efficient memory usage
 
** Buckets are used to group keys; this is done by mapping keys to buckets using a hash function (a.k.a. hashing)
 
** Buckets are used to group keys; this is done by mapping keys to buckets using a hash function (a.k.a. hashing)
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** The Standard Library provides general hashing functions for all the standard types
 
** The Standard Library provides general hashing functions for all the standard types
 
** For general use of unordered maps, we don't have to worry about the technical details of how the hash table works, the STL provides this
 
** For general use of unordered maps, we don't have to worry about the technical details of how the hash table works, the STL provides this
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* Requirements for the KeyType of an <code>unordered_map</code> / a hash table [https://youtu.be/LsjFAx-dG5I?t=11m56s 11:56]
 
* Requirements for the KeyType of an <code>unordered_map</code> / a hash table [https://youtu.be/LsjFAx-dG5I?t=11m56s 11:56]
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** There needs to be a working hash function defined for the KeyType
 
** There needs to be a working hash function defined for the KeyType
 
** There need to be comparison and equality functor definitions for the KeyType
 
** There need to be comparison and equality functor definitions for the KeyType
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* Example: map from <code>Vec2</code> class (2D coordinates) to a string [https://youtu.be/LsjFAx-dG5I?t=12m46s 12:46]
 
* Example: map from <code>Vec2</code> class (2D coordinates) to a string [https://youtu.be/LsjFAx-dG5I?t=12m46s 12:46]
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** In order to make this work, you need to define a hash function and the comparators for <code>Vei2</code>
 
** In order to make this work, you need to define a hash function and the comparators for <code>Vei2</code>
 
** You can implement a comparison/equality functor as a <code>struct</code> that defines a <code>bool operator()( const T& lhs,const T& rhs ) const</code> member function, templated on <code>T</code>
 
** You can implement a comparison/equality functor as a <code>struct</code> that defines a <code>bool operator()( const T& lhs,const T& rhs ) const</code> member function, templated on <code>T</code>
 
** Defining a custom hashing function is an art, it requires knowledge of cryptography, abstract algebra, discrete math, etc.
 
** Defining a custom hashing function is an art, it requires knowledge of cryptography, abstract algebra, discrete math, etc.
 
** Luckily, we don't need this; you can revert to the standard hashing functions for the basic types that make up any custom type
 
** Luckily, we don't need this; you can revert to the standard hashing functions for the basic types that make up any custom type
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* Hash combining [https://youtu.be/LsjFAx-dG5I?t=14m25s 14:25]
 
* Hash combining [https://youtu.be/LsjFAx-dG5I?t=14m25s 14:25]
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** Combining hashes from basic types to create a hash over your custom object  
 
** Combining hashes from basic types to create a hash over your custom object  
 
** Stack Overflow question "How do I combine hash values in C++" give the example as used in the boost library
 
** Stack Overflow question "How do I combine hash values in C++" give the example as used in the boost library
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** You pass these functors when defining the map: <code>std::unordered_map<Vei2,std::string,HashVec2> map;</code> [https://youtu.be/LsjFAx-dG5I?t=17m15s 17:15].  
 
** You pass these functors when defining the map: <code>std::unordered_map<Vei2,std::string,HashVec2> map;</code> [https://youtu.be/LsjFAx-dG5I?t=17m15s 17:15].  
 
** Note that the comparison functor is not needed: we can revert back to the equality operator already defined in the <code>Vec2</code> class definition
 
** Note that the comparison functor is not needed: we can revert back to the equality operator already defined in the <code>Vec2</code> class definition
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* Template Specialization [https://youtu.be/LsjFAx-dG5I?t=18m43s 18:43]
 
* Template Specialization [https://youtu.be/LsjFAx-dG5I?t=18m43s 18:43]
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** Unordered map uses <code>std::hash</code> by default. You can inject Template Specialization for <code>std::hash</code> into the <code>std</code> Namespace for your own custom types only
 
** Unordered map uses <code>std::hash</code> by default. You can inject Template Specialization for <code>std::hash</code> into the <code>std</code> Namespace for your own custom types only
 
*: <code>namespace std</code>
 
*: <code>namespace std</code>
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*: <code>}</code>
 
*: <code>}</code>
 
** Now you don't need to pass <code>HashVec2</code> in the map definition
 
** Now you don't need to pass <code>HashVec2</code> in the map definition
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* The <code>std::unordered_map<></code> Bucket interface [https://youtu.be/LsjFAx-dG5I?t=20m00s 20:00]
 
* The <code>std::unordered_map<></code> Bucket interface [https://youtu.be/LsjFAx-dG5I?t=20m00s 20:00]
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** Allows you to get information about the buckets in the hash table and access nodes
 
** Allows you to get information about the buckets in the hash table and access nodes
 
** The bucket iterator takes an index of the bucket and allows you to iterate over all the elements in that specific bucket
 
** The bucket iterator takes an index of the bucket and allows you to iterate over all the elements in that specific bucket
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* The <code>std::unordered_map<></code> Hash policy interface [https://youtu.be/LsjFAx-dG5I?t=21m47s 21:47]
 
* The <code>std::unordered_map<></code> Hash policy interface [https://youtu.be/LsjFAx-dG5I?t=21m47s 21:47]
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** Allows you to tune your hash table (and thus the growth behavior & performance of the map)
 
** Allows you to tune your hash table (and thus the growth behavior & performance of the map)
 
** Load Factor = average number of elements per bucket. For performance, you typically want to keep this below 1
 
** Load Factor = average number of elements per bucket. For performance, you typically want to keep this below 1
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** You can manually rehash to a number of buckets you define
 
** You can manually rehash to a number of buckets you define
 
** You can reserve space for max number of elements, is then derives (and manages) the required number of buckets
 
** You can reserve space for max number of elements, is then derives (and manages) the required number of buckets
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* When to choose <code>std::map</code> over <code>std::unordered_map</code>? [https://youtu.be/LsjFAx-dG5I?t=24m15s 25:15]
 
* When to choose <code>std::map</code> over <code>std::unordered_map</code>? [https://youtu.be/LsjFAx-dG5I?t=24m15s 25:15]
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** For simplicity and when performance is not a critical issue, no need to define a hash function;
 
** For simplicity and when performance is not a critical issue, no need to define a hash function;
 
** If you want to iterate in order;
 
** If you want to iterate in order;
 
** When you want to be able to find keys that are close to a certain key (with <code>lower_bound</code> and <code>upper_bount</code>
 
** When you want to be able to find keys that are close to a certain key (with <code>lower_bound</code> and <code>upper_bount</code>
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* Homework assignment [https://youtu.be/LsjFAx-dG5I?t=26m04s 26:04]
 
* Homework assignment [https://youtu.be/LsjFAx-dG5I?t=26m04s 26:04]
 
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Revision as of 21:57, 2 February 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

[Expand]
  • The std::map<KeyType,ValueType> class 0:46
  • A Binary Tree data structure is used to manage the order of map elements 2:46
  • A look at the std::map cppreference.com documentation: insert, lookup & find 7:35
  • Requirements on KeyType 14:30
  • std::map cppreference.com documentation continued: erase 15:28
  • Two important things to know when working with associative containers 16:04
  • The std::set<KeyType> class 20:00
  • The std::multimap and std::multiset classes 21:28
  • Practical example of a multimap use case 22:30
  • Lookup in multimaps 25:21

Tutorial 24.2: The unordered associative containers

  • Main difference between ordered/unordered is performance 0:14
  • Using an unordered map 1:38
  • The Hash Table data structure 3:20
  • Requirements for the KeyType of an unordered_map / a hash table 11:56
  • Example: map from Vec2 class (2D coordinates) to a string 12:46

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

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