Inspection (Built-in Functions)

Expression Syntax: Exists(property)

Example:

Exists("_text_alignV&")

will only return True for Text, BoxText, Label and LineText items.

Returns:

True - The mnemonic exists on the current expression item.

False - The mnemonic does not exist on the current expression item.

Example of use of Exists():

Applying a filter to a new feature

When a new feature is drawn on an overlay its attributes are said to be null, (i.e. they do not exist until a value is assigned to them.)

When searching or applying a filter on an overlay, special measures need to be followed where a particular attribute is null.

Looking for an empty string (or a string with length 0) or even an integer or float of 0 does not return any results. This is because the property is not empty but is simply non-existent.

Use the Exists() function to check whether a property exists; this can be used within SIS expressions. Exists() will return a value of 0 (false) if a property does not exist and a value of -1(true) if it exists.

Consider the senario of a buildings overlay with an attribute called "building_type$".

There are four buildings (features) on the overlay, the information contained in "building_type$" is as follows:

ID Building_type

1 FLAT

2 HOUSE

3

4

Example Filter 1

To create a filter that displays all buildings that are houses:

building_type$ ="HOUSE"

Here only Feature 2 is displayed and not Features 1, 3 or 4.

Example Filter 2

To create a filter that displays all buildings where the building type attribute has not been set (i.e. where the building_type is null):

Exists("building_type$") = 0.

Here only Features 3 and 4 are displayed and not Features 1 or 2.

Example Filter 3

To create a filter that displays all buildings where the building type attribute has been set to anything OTHER than null (i.e. the building_type attribute contains a value):

Exists("building_type$") = -1

Here only Features 1 and 2 are displayed.

Example Filter 4

To create a filter that displays all buildings that are not flats:

building_type$ <>"FLAT"

Here Feature 2 will be displayed and not features 1, 3 and 4.

Example Filter 5

To create a filter that displays all buildings that are not flats and all features that have null for building_type$:

building_type$ <>"FLAT" or Exists("building_type$") = 0

Here Features 2, 3 and 4 are displayed and not Feature 1.

Expression Syntax: "expression" Is Not Null

Example:

startdate@ Is Not Null

returns -1 if startdate@ exists, 0 if startdate@ does not exist.

Expression Syntax: "expression" Is Null

Example:

Value@ Is Null

returns 0 if startdate@ exists, -1 if startdate@ does not exist.