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Write a CEL expression once as a User-Defined Function and call it by name wherever you need it, keeping logic consistent and reducing duplication across your workspace.

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Before you begin

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How reusable expression logic works

A User-Defined Function is a reusable CEL expression that can be called by name inside the expressions of Calculated Channels, Rules, and other User-Defined Functions. It does not produce a signal on its own. It acts as a building block that encapsulates a piece of logic, such as a threshold calculation or a normalization formula, that you want to apply consistently across multiple places in your workspace. Each input in a User-Defined Function is assigned a shorthand variable ($1, $2, and so on) based on the order you define them. You use these variables to write the expression, just as you would in a Calculated Channel.

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Example

For example, say you want to convert temperature readings from Celsius to Fahrenheit across multiple assets. Instead of writing ($1 * 9/5) + 32 in every Calculated Channel and Rule separately, you define it once as a User-Defined Function called celsius_to_fahrenheit with the expression ($1 * 9/5) + 32. You can then reference it in Calculated Channels, Rules, and other User-Defined Functions:
  • In a Calculated Channel: celsius_to_fahrenheit($1)
  • In a Rule: celsius_to_fahrenheit($1) > 200
If the conversion logic changes, you update the function once and the change applies everywhere it is referenced automatically.

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Create a User-Defined Function

  1. In Sift’s homepage, click the User-Defined Functions tab.
  2. Click New Function.
  3. In the User-defined function name box, enter a name for the function. For naming requirements, see User-Defined Functions settings.
  4. Optional: In the Description box, enter a description of what the function does.
  5. In the Input Channels box, select a data type for the first input ($1).
    • To add more inputs, click Add input Channel and select a data type for each additional input.
  6. In the Expression box, write the CEL expression using the defined input variables. For supported syntax, see Expression syntax.
  7. Optional: In the Preview section, click Select Run to select a Run, then review the plot to confirm the function produces the expected values against the input Channels.
  8. Click Save.

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Verify the function

You can confirm the User-Defined Function is working correctly in two ways:
  • During creation: In the Preview section, select a Run, and review the output to confirm the expression produces the expected values before saving.
  • After saving: The function appears in the User-Defined Functions section of the Expression Syntax panel. Call it in a Rule, Calculated Channel, or other User-Defined Function to confirm it behaves as expected.

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Reference