Null

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4D v18

Null


Null -> Function result

Parameter	Type	Description
Function result	Null	Null value

Description

Null returns the Null type value null.

This function allows you to assign or compare the null value to the following language elements:

Language elements	Comments
object property values	Comparing Null to an object property returns true if the property value is null, and false otherwise. To simplify code, comparing Null also returns true if the property does not exist in the object (i.e. is Undefined), see example 4.
collection elements	When a collection is expanded by adding non-adjacent elements, any intermediary elements get automatically the null value.
object variables (C_OBJECT)	See (*) below
collection variables (C_COLLECTION)	See (*) below
pointer variables (C_POINTER)	See (*) below
picture variables (C_PICTURE)	() Assigning the null* value to such a variable type clears its contents. In this case, it has the same effect as calling the CLEAR VARIABLE command.
variant variables (C_VARIANT)

Note: This command cannot be used with scalar database fields. Null values in database fields are managed by the SQL engine, and are handled through the Is field value Null and SET FIELD VALUE NULL commands,

Example 1

You want to assign and test the null value to an object property:

 C_OBJECT(vEmp)

 vEmp:=New object

 vEmp.name:="Smith"

 vEmp.children:=Null

 

 If(vEmp.children=Null) //true

 End if

 If(vEmp.name=Null) //false

 End if

 If(vEmp.parent=Null) //true

 End if

Note: This example requires that the object notation is activated in the database.

Example 2

You want to assign and compare the null value to a collection element:

 C_COLLECTION(myCol)

 myCol:=New collection(10;20;Null)

 ...

 If(myCol[2]=Null)

  // if the 3rd element is null

    ...

 End if

Example 3

These examples show the various ways to assign or compare the null value to variables:

  //Object variable

 C_OBJECT($o)

 $o:=New object

 $o:=Null //equivalent to CLEAR VARIABLE($o)

 If($o#Null) //equivalent to If (OB Is defined($o))

 End if

  //Collection variable

 C_COLLECTION($c)

 $c:=New collection

 $c:=Null //equivalent to CLEAR VARIABLE($c)

 If($c#Null)

 End if

  //Pointer variable

 C_POINTER($p)

 $p:=->$v

 $p:=Null //equivalent to CLEAR VARIABLE($p)

 If($p=Null) //equivalent to If (Is Nil pointer($p))

 End if

  //Picture variable

 C_PICTURE($i)

 $i:=$vpicture

 $i:=Null //equivalent to CLEAR VARIABLE($i)

 If($i#Null) //equivalent to If (Picture size($i)#0)

 End if

Example 4

Here are the different results of the Undefined command as well as the Null command with object properties, depending on the context:

 C_OBJECT(vEmp)

 vEmp:=New object

 vEmp.name:="Smith"

 vEmp.children:=Null

 

 $undefined:=Undefined(vEmp.name) // False

 $null:=(vEmp.name=Null) //False

 

 $undefined:=Undefined(vEmp.children) // False

 $null:=(vEmp.children=Null) //True

 

 $undefined:=Undefined(vEmp.parent) // True

 $null:=(vEmp.parent=Null) //True