Expression Examples

Expressions that are used in Event Filter and Alert definitions must produce a Boolean result. This allows Iotellect Server to decide if an event may pass through a filter or if an alert should be raised upon a certain event.

Expression

Evaluation Result

Result Type

5 + 5 > 7

true

Boolean

3 + 2

5

Integer

"Red" + " " + "Line"

"Red Line"

String

{speed} > 5  

true if reference speed resolves to Integer that is greater the 5.

Boolean

{name} == "admin"  

true if reference name resolves to String “admin”.

Boolean

{failed[3]} == false

true if reference failed[3] resolves to Boolean value "false".

Boolean

{env/context} ~= "^abc.*"

true if reference env/context resolves to String that matches regular expression "^abc.*", i.e. starts with "abc".

Boolean

{int[1]} == {int[2]}

true if both references resolve to values that may be compared to each other (i.e. two numbers, or two strings) and these values are equal.

Boolean

({field} + 5) * ({field2} - 1) / 2 == {field[1]} + 1

true or false, depending on the values of resolved references, that must resolve to numbers.

Boolean

substring("smiles", 1, 5)

"mile"

String

max(100, {pressure})

Value of pressure reference if it is greater than 100 or 100 otherwise.

Number

Calculating Distance Between Points

Let's assume a car rental company doesn't want its cars to drive far from a base station. Every car is equipped with a smart controller that reads car location from a GPS receiver and sends it to Iotellect server via cellular network using GPRS. In our system, we have the coordinates of two points: car and base station. We can calculate the distance between them using an expression, or create an expression that triggers an alert when the distance is greater than a certain limit.

If our coordinates are expressed in radians, we can use the following formula to trigger an alert if the distance between the car and the base station is more than 500 kilometers:

acos(sin({car_latitude}) * sin({base_latitude}) + cos({car_latitude}) * cos({base_latitude}) * cos({car_longitude} - {base_longitude})) * 6371  > 500

Where 6371 is Earth's radius in kilometers and 500 is the maximum allowed distance between the car and the base station in kilometers.

If coordinates are expressed in degrees (that is more likely in the case of GPS receiver data), we just convert degrees to radians {car_latitude} / 180 * 2 * 3.1415926.

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