Expression Overview and Syntax

The expression language is used to define dynamic values for component properties and expression Tags. Expressions often involve one or more other values that are used to calculate a final value. In most cases, expressions only return a value.

The classic example for an expression is to change a temperature that is stored in Celsius to Fahrenheit in order to display it. Suppose you had a Tag,Tank 6/Temp, that was in Celsius. If you wanted to display that Tag in Fahrenheit on a Label, you would use an Expression Binding on the label's text property using the following expression:

1.8 * {Tank 6/Temp} + 32

Every time that the temperature Tag changes, the expression will re-calculate the value and push it into the Label's text property. Now lets say that you wanted to append a "°F" to the end of the label so that the user knew the units of the temperature. You could simply use some string concatenation in your expression, like this:

(1.8 * {Tank 6/Temp} + 32) + " °F"

Lets suppose that you wanted to give the user an option to display the value in Celsius or Fahrenheit, based on checking a checkbox. You could add a Check Box component to the screen called DisplayFahrenheit. Then you could use this expression to dynamically display either unit, based upon the user's selection:

if({Root Container.DisplayFahrenheit.selected}, (1.8 * {Tank 6/Temp} + 32) + " °F", {Tankf/Temp} +  " °C")

Additional Examples

The following headings demonstrate many simple examples that better demonstrate the Expression Language's syntax. 

String Concatenation

You can use an expression binding to concatenate strings resulting in a new string that reflects the concatenation of different strings.

The function starts with CONCAT() and inside the function is a list of strings. They can be manually typed like "42" or they can come from Tags or properties.

Example 1

concat("The answer is: ", "42") //returns "The answer is: 42"

Example 2

You have a Date, and need to extract the year, and concatenate the word "Vintage" to the end for a label display. Bind a label's text property to: 

 dateExtract({Root Container.VintageDate}, 'year') + ' Vintage'

Celsius to Fahrenheit

Use an expression binding to convert a temperature from Celsius to Fahrenheit. This is an example of how an expression binding can handle calculations.

{celsiusTemp} * 9/5 + 32

The reference to the Celsius temperature can come from a property or a Tag. As the property or the Tag changes, so does the expression binding.

Format Date

You can format a date in an expression binding by using the dateFormat and now functions.

To make a label that updates to show the current time:

1. Drag a Label component onto the window.
    
2. Select the label's Text property binding icon and select Expression binding.
    

3. Enter the following code into the expression, and click OK.

   dateFormat(now(1000), "MMM d, yyyy hh:mm:s a") 

   The dateFormat function takes two arguments. The first argument is any date type variable. This can include another function (like now) that returns a date type. The second argument refers to the date format that you want returned. The now function returns the current time and in this case it will update every second. 
   For more information on the date formatting expression, see the Appendix.

Date Manipulations

You can manipulate dates in expression bindings such as a date addition/subtraction with the dateArithmetic function. This is important when you want to use the expression bindings to select a date that is offset by a certain amount.

The following example will return the time 15 minutes ago by using the dateArithmetic expression function:

 dateArithmetic(now(), -15, "minute")

Bit Functions

You can use various bit functions in expression bindings like getBit to return individual bits of a word.

Example 1

Assuming a tag path 'Folder/BitTag', the following would return the binary representation of the Tag's value at the 0 position

getBit({Folder/BitTag}, 0)

For more details on the getBit function, please see the getBit page in the Appendix. 

Example 2

You have 3 bits in a PLC, only one of which will be on at a time. You want to turn these 3 bits into a single integer (0,1,2) to drive a component's Styles. Bind a custom integer property to:

binEnum({MyTags/Bit1}, {MyTags/Bit2}, {MyTags/Bit3})

Switch

You can use the switch function in expression bindings to evaluate conditional statements. This function acts like the switch statement in C-like programming languages. It takes the value argument and compares it to each of the case1 through caseN expressions.

The following example returns the string "Running" when it is given the value of 1. Its options are 0,1, and 2. And when comparing the value to the options the switch statement returns one of the corresponding results. If a result cannot be found, a fail-over option is returned.

switch(
1,
0, 1, 2,
"Off","Running","Fault",
forceQuality("!BAD STATE!",0))

Checking Conditions

You can use expression bindings to return true or false based on different conditions.

Example 1

Consider the following expression that references the Tag with a path of 'Folder/Machine State':

{Folder/Machine State} = 0

The above expression simply tests the value of the Machine State Tag. If the value of the Machine State Tag is ever equal to 0 then the above expression would return true. In every case where Machine State is not equal to 0, then the expression would return false.  

Example 2

It is possible to check for multiple conditions in the same expression. If you have two boolean Tags and you only want the expression to return true if both Tags are true then the binding would look like:

{boolTag1}=True && {boolTag2}=True 

Example 3

Consider the following expression that references the Tag with a path of 'Folder/Machine State':

if({Folder/Machine State} = 0, 1, 0)

This also finds the opposite of the Machine State if it's a boolean. It returns whether or not the Machine State is 0.