created: 20140410101941871
modified: 20150228104558000
tags: Learning Filters
title: Introduction to filter notation
type: text/vnd.tiddlywiki

<<.preamble """This explains the basics of writing a [[filter|Filters]] to select a set of tiddlers. For a more technical presentation, see [[Filter Syntax]].""">>

<$macrocall $name=".tip" _="""Filters do nothing if you just type them into a tiddler on their own. They need a context. An easy way to experiment with filters is to type them into the <<.advancedsearch-tab Filter>> tab of [[Advanced Search|$:/AdvancedSearch]]."""/>

The simplest case is where you already know exactly which tiddlers you want. Type each title in double square brackets, with a space between each one and the next:

> `[[Recipe book]] [[ScrambledEggs]] [[Mom's apple pie]]`

You can omit the square brackets when a title doesn't contain any spaces:

> `[[Recipe book]] ScrambledEggs [[Mom's apple pie]]`

The double square brackets are actually a shorthand for this:

> `[title[ScrambledEggs]]`

... which gives us the <<.def "general model">> for any filter:

> `[operator[parameter]]`

For instance, here's how to select all the tiddlers that have been tagged <<.tag Recipe>>:

> `[tag[Recipe]]`

We can reverse the meaning by adding an exclamation mark `!` just before the operator. For example, we can select any tiddlers that do <<.em not>> have the <<.tag Recipe>> tag:

> `[!tag[Recipe]]`

Tiddlers can be filtered by other fields than just title and tags:

> `[field:serving[4]]`

That example will select any tiddlers that have <<.value 4>> in their <<.field serving>> field.

As the word "serving" isn't a standard filter operator (and isn't likely to become one), you can safely omit the `field:` prefix:

> `[serving[4]]`

!Combinations

The filters we've looked at so far have involved just one step each. But you can <<.def run>> several steps together like this:

> `[tag[Vegetarian]!tag[soup]serving[4]]`

Notice how the entire run is contained in a single pair of square brackets.

A tiddler has to match <<.em all>> of the steps in a run. So the example above retrieves vegetarian recipes (other than soups) for 4 people.

A sequence of separate runs will select the tiddlers that match <<.em any>> of the runs. We can use this to find recipes that serve either 3, 4 or 5 people:

> `[serving[3]] [serving[4]] [serving[5]]`

If we want to ignore vegetarian recipes that serve 4, we can say this:

> `[serving[3]] [serving[4]!tag[Vegetarian]] [serving[5]]`

By default, each run considers every tiddler in the wiki. But we can use a `+` sign to force a run to consider only the tiddlers that were selected by the preceding runs:

> `[serving[3]] [serving[4]] [serving[5]] +[tag[Vegetarian]] +[sort[title]]`

This selects recipes for 3, 4 or 5 people, then filters <<.em those>> to remove the vegetarian ones, and finally sorts any that are left into alphabetical order of title.

In a similar way, we can use a `-` sign to <<.em remove>> a run's tiddlers from the result so far. Here we select all vegetarian recipes apart from two:

> `[tag[Vegetarian]] -[title[ScrambledEggs]] -BeansOnToast`

!Special parameters

The parameter of each step we've seen so far has been in square brackets, meaning that ~TiddlyWiki treats it literally. But two other kinds of bracket are possible:

<<.def "Curly brackets">> `{}` mean that the parameter is a TextReference, and that its value is to be looked up in a specified tiddler. For example, if we have a tiddler called <<.tid Preference>> whose text happens to be the word <<.value Vegetarian>>, we can say

> `[tag{Preference}]`

as an alternative to `[tag[Vegetarian]]`. This allows the preference to change over time.

<<.def "Angle brackets">> `<>` mean that the parameter is the name of a [[variable|Variables]] whose value is to be used instead. Here we use the built-in <<.vlink currentTiddler>> variable in a filter that selects any tiddlers whose text contains the title of the current one:

> `[search<currentTiddler>]`
