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Conditional Operators

 11th July 2023 at 8:23am

Introduced in v5.1.20 The conditional filter operators allow if-then-else logic to be expressed within filters.

The foundation is the convention that an empty list can be used to represent the Boolean value false and a list with at one (or more) entries to represent true.

The conditional operators are:

  • then Operator replaces any input values with a constant string. For example:
    • [[HelloThere]is[missing]then[FOO]] evaluates to (empty)
    • [[Missing Tiddler]is[missing]then[FOO]] evaluates to FOO
  • else Operator if the title list is empty then returns a constant string, otherwise returns the original title list
    • [[HelloThere]is[tiddler]else[BAR]] evaluates to HelloThere
    • [[Missing Tiddler]is[tiddler]else[BAR]] evaluates to BAR

These operators can be combined. For example:

[[New Tiddler]is[missing]then[I am missing]else[No I am not missing]]

The else operator can be used to apply a defaults for missing values. In this example, we take advantage of the fact that the get operator returns an empty list if the field or tiddler does not exist:

[[HelloThere]get[custom-field]else[default-value]]

Filter Run Prefixes

The :then and :else named filter run prefixes serve a similar purpose as the conditional operators.

Also see: Named Filter Run Prefix