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Conditional Operators

 2nd August 2019 at 1:27pm

New in: 5.1.20The 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]] evaluates to I am 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]] evaluates to default-value