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

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(→‎Tutorial 24.2: The unordered associative containers)
(→‎Tutorial 24.2: The unordered associative containers)
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}
 
}
 
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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].  
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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].  
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** 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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:* 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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<div class="mw-collapsible-content">
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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
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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
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*: <code>namespace std</code>
 
*: <code>namespace std</code>
 
*: <code>{</code>
 
*: <code>{</code>
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*:: <code>};</code>
 
*:: <code>};</code>
 
*: <code>}</code>
 
*: <code>}</code>
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** Now you don't need to pass <code>HashVec2</code> in the map definition
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 +
 
 +
:* 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]
 
<div class="mw-collapsible-content">
 
<div class="mw-collapsible-content">
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** Allows you to get information about the buckets in the hash table and access nodes
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:* Allows you to get information about the buckets in the hash table and access nodes
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** 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 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]
 
<div class="mw-collapsible-content">
 
<div class="mw-collapsible-content">
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** Allows you to tune your hash table (and thus the growth behavior & performance of the map)
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:* Allows you to tune your hash table (and thus the growth behavior & performance of the map)
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** Load Factor = average number of elements per bucket. For performance, you typically want to keep this below 1
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:* Load Factor = average number of elements per bucket. For performance, you typically want to keep this below 1
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** You can set the maximum load factor above which the table gets rehashed
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:* You can set the maximum load factor above which the table gets rehashed
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** When the load factor becomes too high, it will automaticall rehash the table and increase the number of buckets
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:* When the load factor becomes too high, it will automaticall rehash the table and increase the number of buckets
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** You can manually rehash to a number of buckets you define
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:* You can manually rehash to a number of buckets you define
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** You can reserve space for max number of elements, is then derives (and manages) the required number of buckets
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:* 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]
 
<div class="mw-collapsible-content">
 
<div class="mw-collapsible-content">
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** For simplicity and when performance is not a critical issue, no need to define a hash function;
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:* For simplicity and when performance is not a critical issue, no need to define a hash function;
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** If you want to iterate in order;
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:* If you want to iterate in order;
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** 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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:* 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>
 
</div>
 
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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]

Revision as of 22:34, 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

[Expand]
  • Main difference between ordered/unordered: 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
  • Template Specialization 18:43
  • The std::unordered_map<> Bucket interface 20:00
  • The std::unordered_map<> Hash policy interface 21:47
  • When to choose std::map over std::unordered_map? 25:15
  • Homework assignment 26:04

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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