co🔗e: Linked List

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Introduction | Usage | Download | Requirements | Acknowledgements

Introduction

Linked lists are suitable as a flexible and modular way to implement simple and powerful code with ease.
This library will allow you to leverage the simplicity, power and flexibility of linked lists.

A linked list is a data structure that represents a sequence of nodes (Elements). This library implements a so called "double linked list", which simply means that the list gives each Element pointers to both the next Element and the previous Element. Unlike an array, a linked list does not provide constant time access to a particular “index” within the list. This means that if you’d like to find the K-th element in the list, you will need to iterate through K elements. The benefit of a linked list is that you can add and remove Elements from the list during runtime so this means you can very easily extend your code with new FB's without extra coding since itteration with Elements is always done in FOR and/or WHILE loops. This means that by design your code doesn't need to be rewritten as demonstrated in the example below 👇

Usage

• Insert the Library Reference in your program's library manager.
• Declare a new FB named FB_ExtendedElement
• We need to either 1) extend at least one FB_Element since it has been decorated as ABSTRACT.
  or 2) implement interface: LinkedList.IElement.

Pro Tip
Any new FB that will be a part of the List should preferably use method 2) implement interface. This way FB_ExtendElement will act as a baseclass for your Element without being itself of type "FB_ELEMENT". (EXTENDING means "IS OF", while IMPLEMENTS means "MAKES USE OF"). If you use encapsulation and composition it will provide you with more freedom and flexibility but as a penalty it will demand more programming since all methods and properties need to be implemented.

FUNCTION_BLOCK FB_ExtendedElement EXTENDS Element
VAR
    _itfPrev : IElement;
    _itfNext : IElement;
    _itfList : IList;
    _uiCount : UINT := 0;
END_VAR

// Body
(* Empty, we use composition *)


METHOD IsListMemberOf : BOOL // xMember
VAR_INPUT
    itfList : IList;
END_VAR

// Body
IsListMemberOf := (itfList = THIS^._itfList);


METHOD RemoveElem : IElement

// Body
IF _itfList = 0 THEN
    // This element is not a member of a list
    RemoveElem := 0;
    RETURN;
END_IF
RemoveElem := _itfList.RemoveElem(THIS^);


PROPERTY IsLinkedElem : BOOL

// Get 
GetIsLinkedElem := (_itfList <> 0);

// Set is not implemented


PROPERTY List : IList

// Get
List := _itfList;

// Set
_itfList := List;


PROPERTY NextElem : IElement

// Get
List := _itfList;

// Set
_itfList := List;


PROPERTY PrevElem : IElement

// Get
PrevElem := _itfPrev;

// Set
_itfPrev := PrevElem;


METHOD ReturnInstance : POINTER TO FB_ExtendedElement

// Body
ReturnInstance := THIS;


METHOD Count : UINT;

// Body
_uiCount := uiCount + 1;

PROPERTY GetCount : UINT

// Get
GetCount := _uiCount;

• Declare an instance of LIST
• Declare an instance of FB_ExtendedElement;

VAR
    BuildList : Bool;
    List : LinkedList.List;
    Extended1: FB_ExtendedElement;
    Extended2: FB_ExtendedElement;
    Extended3: FB_ExtendedElement;
    Extended4: FB_ExtendedElement;     
    ListSize : UDINT := 0;
    itfCurElement : IElement;   
    HasItfCurElement : BOOL;
END_VAR

// Body
IF BuildList THEN
   List.AppendElement( Extended1 );
   List.AppendElement( Extended2 );
   List.AppendElement( Extended3 );
   List.AppendElement( Extended4 );
   BuildList := FALSE;       
END_IF

ListSize := List.ListSize;            
IF ListSize <> 0 THEN

    itfCurElement := List.HeadElem; // Point itfCurElement to the listhead

    WHILE itfCurElement <> 0 DO

        FOR i := 1 TO 10 DO
        // Count gets tallied up
            itfCurElement.Count();
        END_FOR;
        CurCount := itfCurElement.GetCount;

        // Get the next element by using the pointer stored in the element itself (we can also use PrevElem to get the previous element)
        itfCurElement := itfCurElement.NextElem; 

    END_WHILE

END_IF

Some usage general information and rules about Elements

PRO TIPS
Always obtain access to elements via the functionblock instance of List.
Element require more burnerplate-code when compared to array's but are also more flexible.
Array's are always packed together in memory, while Elements usually are scattered throughout the memory. This is the nature of elements.

Download

By downloading and using our software you abide by the MIT License

Requirements

https://www.linkedin.com/in/hahermsen

Project Members:

• h-hermsen (admin)
• i-campbell (admin)