created: 20140410101941871
modified: 20141009180223576
tags: Learning
title: Introduction to Filters
type: text/vnd.tiddlywiki

A step by step introduction to how [[Filters]] are used.

! Using Filters

Filters are a special language within WikiText for expressing lists of tiddlers.

Filters are used in the ListMacro, TabsMacro, ListWidget, CountWidget, and many other areas of TiddlyWiki.

For example, this is how the ListMacro would be used to display  the first example below:

```
<<list-links "HelloThere Introduction [[Title with Spaces]]">>
```

The easiest way to experiment with tiddler filters is by typing them into the "Filter" tab of the [[advanced search panel|$:/AdvancedSearch]].

! Simple Filters

The simplest example of a filter is a list of tiddler titles (if necessary quoted with double square brackets):

```
HelloThere Introduction [[Title with Spaces]]
```

The titles must be separated by one or more spaces and/or linebreaks.

! Filter Operators

Filter operators are used to select tiddlers based on some criteria. For example, this filter consists of a single operation that selects all tiddlers tagged "introduction":

```
[tag[introduction]]
```

The word "tag" is the ''operator'' and "introduction" is the ''operand''.

See [[Filters]] for a complete list of the available operators.

!! Default Filter Operator

The operator defaults to `title` if omitted, so `[[HelloThere]]` is equivalent to `[title[HelloThere]]`. If there are no spaces in the title, then the double square brackets can also be omitted: `HelloThere`.

! Negating Filter Operators

Filter operations can be negated by preceding the operator with an exclamation mark (!). This example selects all tiddlers that are not tagged "introduction":

```
[!tag[introduction]]
```

! Operator Suffixes

Some filter operators can take an optional suffix that provides further information for the operation. For example, the "field" operator takes a suffix indicating the field to be compared. The following filter returns all tiddlers that have "JeremyRuston" in the "modifier" field:

```
[field:modifier[JeremyRuston]]
```

! Field Operator Shortcut

If an unknown operator is used then it is instead interpreted as the suffix of the "field" operator. Thus, these two filters both return all the tiddlers that contain the string "create" in their "caption" field:

```
[caption[create]]
[field:caption[create]]
```

! Indirect Operands

If a filter operator is written with curly brackets around the operand then it is taken to be a TextReference to the actual value. For example, this filter selects all tiddlers containing the string contained in the tiddler titled "$:/temp/search"

```
[search{$:/temp/search}]
```

! Variable Operands

If a filter operator is written with angle brackets around the operand then it is taken to be the name of a variable containing the actual value. For example, this filter selects all tiddlers containing the title of the current tiddler:

```
[search<currentTiddler>]
```

(Note that the `currentTiddler` variable is used to track the current tiddler).

! ORing Multiple Filter Operators

You can use multiple filter operations at once. This example selects all tiddlers that are either tagged "introduction" or "demo":

```
[tag[introduction]] [tag[demo]]
```

Each separate operator is processed in turn, accumulating the tiddlers that they select.

Here's an example that returns tiddlers tagged ''alpha'' or ''beta'' that are also tagged ''task'' and not tagged ''done'':

```
[tag[alpha]] [tag[beta]] +[tag[task]!tag[done]]
```

! ANDing Multiple Filter Operators

A sequence of operators can be logically ANDed together by bashing them together and merging the outer square brackets. This is called a "run" of operations. For example, here we select tiddlers that are tagged "introduction" and also tagged "demo":

```
[tag[introduction]tag[demo]]
```

Here's another example that selects all tiddlers tagged "introduction" that are not tagged "demo":

```
[tag[introduction]!tag[demo]]
```

! Negating Runs of Filter Operators

Ordinarily, each run of filter operations adds to the accumulated results. Prefixing a run with `-` causes the list of tiddlers to instead be removed from the results. For example, this example returns all the tiddlers tagged "introduction" apart from `HelloThere` and `Title with Spaces`:

```
[tag[introduction]] -HelloThere -[[Title with Spaces]]
```

This example returns all tiddlers tagged "introduction" that are not also tagged "demo":

```
[tag[introduction]] -[tag[demo]]
```

! Working with Filter Results

Usually, each run of filter operations takes as its source the entire store of available tiddlers. Prefixing a run with `+` causes the accumulated results to be used as the source instead.

For example, this filter selects tiddlers tagged "introduction" or "demo" and then sorts the resulting list by the "title" field:

```
[tag[introduction]] [tag[demo]] +[sort[title]]
```
