created: 20190802113703788
modified: 20230711082337975
tags: Filters
title: Conditional Operators
type: text/vnd.tiddlywiki

<<.from-version "5.1.20">> 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 <<.value false>> and a list with at one (or more) entries to represent <<.value true>>.

The conditional operators are:

* [[then Operator]] replaces any input values with a constant string. For example:
** <<.inline-operator-example "[[HelloThere]is[missing]then[FOO]]">>
** <<.inline-operator-example "[[Missing Tiddler]is[missing]then[FOO]]">>
* [[else Operator]] if the title list is empty then returns a constant string, otherwise returns the original title list
** <<.inline-operator-example "[[HelloThere]is[tiddler]else[BAR]]">>
** <<.inline-operator-example "[[Missing Tiddler]is[tiddler]else[BAR]]">>

These operators can be combined. For example:

<<.operator-example 1 "[[New Tiddler]is[missing]then[I am missing]else[No I am not missing]]">>

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

<<.operator-example 2 "[[HelloThere]get[custom-field]else[default-value]]">>

''Filter Run Prefixes''

The [[:then|Then Filter Run Prefix]] and [[:else|Else Filter Run Prefix]] named filter run prefixes serve a similar purpose as the conditional operators. 

Also see: [[Named Filter Run Prefix]]
