21st March 2023 at 1:39pm
| purpose | apply a subfilter to each input title in turn, accumulating a single value |
|---|---|
| input | a selection of titles passed as input to the filter |
| parameter | S=a filter expression. Optional second parameter for initial value for accumulator |
| output | the final result of running the subfilter S |
Learn more about how to use Filters
Introduced in v5.1.23 The reduce operator runs a subfilter for each input title, passing the result of the previous subfilter run as a variable. The initial value of the accumulator can optionally be specified. It returns the result of the final subfilter run.
The reduce operator is used to flatten a list of items down to a single item by repeatedly applying a formula. A typical use is to add up the values in a given field of a list of tiddlers.
The following variables are available within the subfilter:
- accumulator - the result of the previous subfilter run
- currentTiddler - the input title
- ..currentTiddler - the value of the variable
currentTiddleroutside the subfilter. Introduced in v5.2.0 - index - the numeric index of the current list item (with zero being the first item in the list)
- revIndex - the reverse numeric index of the current list item (with zero being the last item in the list)
- length - the total length of the input list
If the reduce operator receives no input, its output will be empty. The else Operator can be useful in such cases.
<$set name="sum-input" value="[add<accumulator>]">
{{{ =1 =2 =3 +[reduce<sum-input>] }}}
</$set>\define num-items() [get[quantity]add<accumulator>]
[tag[shopping]reduce<num-items>]is equivalent to:
[tag[shopping]] :reduce[get[quantity]add<accumulator>]