Predicate Delegates in C

Question

Can you explain to me    What is a Predicate Delegate  Where should we use predicates  Any best practices when using predicates    Descriptive source code will be appreciated

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If you re in VB 9  VS2008   a predicate can be a complex function   Dim list As New List Of Integer  New Integer    1  2  3   Dim newList   list FindAll AddressOf GreaterThanTwo      Function GreaterThanTwo ByVal item As Integer  As Boolean      do some work      Return item  gt  2 End Function   Or you can write your predicate as a lambda  as long as it s only one expression   Dim list As New List Of Integer  New Integer    1  2  3   Dim newList   list FindAll Function item  item  gt  2

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The predicate-based searching methods allow a method delegate or lambda expression to decide whether a given element is a    match      A predicate is simply a delegate accepting an object and returning true or false  public delegate bool Predicate  T object       static void Main                         string   names      Lukasz    Darek    Milosz                 string match1   Array Find names  delegate string name    return name Contains  L                      or             string match2   Array Find names  delegate string name    return name Contains  L                      or             string match3   Array Find names  x   gt  x Contains  L                   Console WriteLine match1         match2         match3          Lukasz Lukasz Lukasz                   static bool ContainsL string name    return name Contains  L

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A predicate is a function that returns true or false   A predicate delegate is a reference to a predicate   So basically a predicate delegate is a reference to a function that returns true or false   Predicates are very useful for filtering a list of values - here is an example    using System  using System Collections Generic   class Program       static void Main                 List lt int gt  list   new List lt int gt    1  2  3             Predicate lt int gt  predicate   new Predicate lt int gt  greaterThanTwo            List lt int gt  newList   list FindAll predicate              static bool greaterThanTwo int arg                return arg  gt  2            Now if you are using C  3 you can use a lambda to represent the predicate in a cleaner fashion   using System  using System Collections Generic   class Program       static void Main                 List lt int gt  list   new List lt int gt    1  2  3             List lt int gt  newList   list FindAll i   gt  i  gt  2

User · Answer

There s a good article on predicates here  although it s from the  NET2 era  so there s no mention of lambda expressions in there

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Just a delegate that returns a boolean   It is used a lot in filtering lists but can be used wherever you d like   List lt DateRangeClass gt   myList   new List lt DateRangeClass lt GetSomeDateRangeArrayToPopulate   myList FindAll x   gt   x StartTime  lt   minDateToReturn  amp  amp  x EndTime  gt   maxDateToReturn

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Predicate falls under the category of generic delegates in C   This is called with one argument and always return the boolean type  Basically  the predicate is used to test the condition - true false  Many classes support predicate as an argument  For example  list findall expects the parameter predicate  Here is an example of predicate  Imagine a function pointer with the signature   lt modifier gt  bool delegate myDelegate lt in T gt  T match    Here is the example  Node cs  namespace PredicateExample       class Node               public string Ip Address   get  set            public string Node Name   get  set            public uint Node Area   get  set             Main class  using System  using System Threading  using System Collections Generic   namespace PredicateExample       class Program               static void Main string   args                        Predicate lt Node gt  backboneArea   Node   gt   Node Node Area    0               List lt Node gt  Nodes   new List lt Node gt                 Nodes Add new Node   Ip Address    quot 1 1 1 1 quot   Node Area   0  Node Name    quot Node1 quot                  Nodes Add new Node   Ip Address    quot 2 2 2 2 quot   Node Area   1  Node Name    quot Node2 quot                  Nodes Add new Node   Ip Address    quot 3 3 3 3 quot   Node Area   2  Node Name    quot Node3 quot                  Nodes Add new Node   Ip Address    quot 4 4 4 4 quot   Node Area   0  Node Name    quot Node4 quot                  Nodes Add new Node   Ip Address    quot 5 5 5 5 quot   Node Area   1  Node Name    quot Node5 quot                  Nodes Add new Node   Ip Address    quot 6 6 6 6 quot   Node Area   0  Node Name    quot Node6 quot                  Nodes Add new Node   Ip Address    quot 7 7 7 7 quot   Node Area   2  Node Name    quot Node7 quot                   foreach  var item in Nodes FindAll backboneArea                                 Console WriteLine  quot Node Name  quot    item Node Name    quot  Node IP Address  quot    item Ip Address                              Console ReadLine

User · Answer

Leading on from Andrew s answer with regards to c 2 and c 3     you can also do them inline for a one off search function  see below    using System  using System Collections Generic   class Program       static void Main                 List lt int gt  list   new List lt int gt    1  2  3             List lt int gt  newList   list FindAll delegate int arg                                                              return arg gt  2                                           Hope this helps

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Simply -  they provide True False values based on condition mostly used for querying  mostly used with delegates  consider example of list  List lt Program gt  blabla  new List lt Program gt             blabla Add new Program  shubham   1            blabla Add new Program  google   3            blabla Add new Program  world  5            blabla Add new Program  hello   5            blabla Add new Program  bye   2      contains names and ages   Now say we want to find names on condition So I Will use       Predicate lt Program gt  test   delegate  Program p    return p age  gt  3             List lt Program gt  matches   blabla FindAll test           Action lt Program gt  print   Console WriteLine          matches ForEach print     tried to Keep it Simple

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What is Predicate Delegate   1  Predicate is a feature that returns true or false This concept has come in  net 2 0 framework   2  It is being used with lambda expression       It takes generic type as an argument  3  It allows a predicate function to be defined and passed as a parameter to another function  4  It is a special case of a Func  in that it takes only a single parameter and always returns a bool   In C  namespace   namespace System          public delegate bool Predicate lt in T gt  T obj       It is defined in the System namespace   Where should we use Predicate Delegate   We should use Predicate Delegate in the following cases   1  For searching items in a generic collection    e g    var employeeDetails   employees Where o  gt o employeeId    1237  FirstOrDefault      2  Basic example that shortens the code and returns true or false   Predicate lt int gt  isValueOne   x   gt  x    1    now  Call above predicate   Console WriteLine isValueOne Invoke 1       -- returns true    3  An anonymous method can also be assigned to a Predicate delegate type as below   Predicate lt string gt  isUpper   delegate string s    return s Equals s ToUpper           bool result   isUpper  Hello Chap        Any best practices about predicates   Use Func  Lambda Expressions and Delegates instead of Predicates

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A delegate defines a reference type that can be used to encapsulate a method with a specific signature  C  delegate Life cycle  The life cycle of C  delegate is   Declaration  Instantiation   INVACATION   learn more form  http   asp-net-by-parijat blogspot in 2015 08 what-is-delegates-in-c-how-to-declare html

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