caption: lookup
created: 20170907103639431
modified: 20210116081305739
op-input: a [[selection of titles|Title Selection]]
op-output: the lookup values corresponding to each lookup tiddler
op-parameter: prefix applied to input titles to yield title of lookup tiddler from which value is retrieved. Now accepts 1 or 2 parameters, see below for details
op-parameter-name: P, T
op-purpose: applies a prefix to each input title to yield the title of a tiddler from which the final value is retrieved. With a single parameter, the default field is "text" and the default index is "0". If a second parameter is provided, that becomes the target field or index.
op-suffix: the default value to be used for missing lookups. This operator can now accept a second suffix of `:index`, see below for details
op-suffix-name: D, I
tags: [[Filter Operators]]
title: lookup Operator
type: text/vnd.tiddlywiki

<<.from-version "5.1.15">>

The action of this operator is as follows with 1 parameter:

* Apply the specified prefix to each input tiddler title, yielding a new list of tiddler titles
* Transclude the value of the `text` field each of those tiddlers
** Substitute the default value for missing or empty values
* Return the list of values

<<.from-version "5.2.2">>

The use of the `:index` second suffix changes the default lookup location from field: `text` to index: `0`. This is used if no 2nd parameter is provided.

The action of this operator is as follows with 2 parameters:

If there are two parameters provided, use the second parameter as the target field or index.

<<.note """If there is only one parameter given, the filter checks for a second suffix equal to "index". If this suffix is found, the default target index is "0".
In all other cases, the default target field is "text".""">>

Then:

* Apply the specified prefix to each input tiddler title, yielding a new list of tiddler titles
* Transclude the value of the target field or index
** Substitute the default value for missing or empty values
* Return the list of values

<<.operator-examples "lookup">>