Collection

Collections are ordered lists of values of similar or mixed types (text, number, date, object, boolean, collection, or null).

Collection type variables are managed using object notation.

To access a collection element, you need to pass the element number inside square brackets:

collectionRef[expression]

You can pass any valid 4D expression which returns a positive integer in expression. Examples:

 myCollection[5]  //access to 6th element of the collection
 myCollection[$var]

Warning: Collection elements are numbered from 0.

You can assign a value to a collection element or get a collection element value:

 myCol[10]:="My new element"
 $myVar:=myCol[0]

If you assign an element's index that surpasses the last existing element of the collection, the collection is automatically resized and all new intermediary elements are assigned a null value:

 var myCol : Collection
 myCol:=New collection("A";"B")
 myCol[5]:="Z"
  //myCol[2]=null
  //myCol[3]=null
  //myCol[4]=null

Instantiation

Collections must have been instantiated, otherwise trying to read or modify their elements will generate a syntax error.

Collection instantiation can be done in one of the following ways:

• using the New collection command,
• using the [] operator.

info

Several 4D commands and functions return collections, for example Monitored activity or collection.copy. In this case, it is not necessary to instantiate explicitely the collection, the 4D language does it for you.

New collection command

The New collection command creates a new empty or prefilled collection and returns its reference.

Examples:

 var $colVar : Collection //declaration of a collection type 4D variable
 $colVar:=New collection //instantiation of the collection and assignment to the 4D variable
 
 var $colFilled : Collection
 $colFilled:=New collection("a";"b";1;42;{}) //instantiation and assignment of a prefilled collection

[] operator

The [] operator allows you to create a collection literal. A collection literal is a list of zero or more expressions, each of which represents a collection element, enclosed in square brackets ([]). When you create a collection using a collection literal, it is instantiated with the specified values as its elements, and its length is set to the number of arguments specified.

Since any element is considered an expression, you can create sub-collections using [] in elements. You can also create and reference object literals.

If an element is undefined, it will appear as Null in the collection.

Examples:

var $col1; $col2; $users : Collection
$col1:=[] //empty collection
$col2:=[1;2;3;4;5;6] //collection of numbers
//collection of objects
$users:=[{name: "Alice"; \
	height: 183; \
	eyecolor: "hazel"; \
	id: $col2[5]\
	}; \
	{name: "Bob"; \
	height: 172; \
	eyecolor: "blue"\
	}]

note

If you create a collection literal containing a single element, make sure you do not use a name corresponding to an existing table name, otherwise the table syntax [tableName] will take priority.

Regular or shared collection

You can create two types of collections:

• regular (non-shared) collections, using the New collection command or collection literal syntax ([]). These collections can be edited without any specific access control but cannot be shared between processes.
• shared collections, using the New shared collection command. These collections can be shared between processes, including preemptive threads. Access to these collections is controlled by Use...End use structures.

For more information, refer to the Shared objects and collections section.

Collection functions

4D collection references benefit from special class functions (sometimes named member functions). Collection functions are listed in the Class API Reference section.

For example:

$newCol:=$col.copy() //deep copy of $col to $newCol
$col.push(10;100) //add 10 and 100 to the collection

Some functions return the original collection after modification, so that you can run the calls in a sequence:

 $col:=New collection(5;20)
 $col2:=$col.push(10;100).sort() //$col2=[5,10,20,100]

propertyPath parameter

Several functions accept a propertyPath as parameter. This parameter stands for:

• either an object property name, for example "lastName"
• or an object property path, i.e. a hierarchical sequence of sub-properties linked with dot characters, for example "employee.children.firstName".

Warning: When using functions and propertyPath parameters, you cannot use ".", "[ ]", or spaces in property names since it will prevent 4D from correctly parsing the path:

 $vmin:=$col.min("My.special.property") //undefined
 $vmin:=$col.min(["My.special.property"]) //error

Collection operators

You can use comparison operators with collection references, which means that you can evaluate if two or more references point to the same instance of a collection.

var $c1:=["a";42] //reference to an instance
var $c2:=["a";42] //reference to a different instance
var $c3:=$c1 //reference to the same instance

Based upon the code above, the comparison table is:

Operation	Syntax	Returns	Expression	Value
Equality	collectionRef = collectionRef	Boolean	$c1 = $c3	True
			$c1 = $c2	False
Inequality	collectionRef # collectionRef	Boolean	$c1 # $c3	False
			$c1 # $c2	True

Undefined

Reading the length property of an undefined collection produces 0:

     var $c : Collection //variable created but no collection is defined
     $size:=$c.length //$size = 0