reduce: 티들리위키 — 줄 없는 개인 웹 공책

reduce Operator

2023년 3월 21일 오후 1:39

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 currentTiddler outside 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.

Tip

Literal filter parameters cannot contain square brackets but you can work around the issue by using a variable:

<$set name="sum-input" value="[add<accumulator>]">
{{{ =1 =2 =3 +[reduce<sum-input>] }}}
</$set>

Tip

Compare with the analogous named filter run prefix :reduce

\define num-items() [get[quantity]add<accumulator>]

[tag[shopping]reduce<num-items>]

is equivalent to:

[tag[shopping]] :reduce[get[quantity]add<accumulator>]

Tip

If the optional second parameter is not given, the initial accumulator value will be empty. Numerical operators treat empty input as if it was the number 0. See the multiply-input examples for how this can affect the result of reduce in some cases.

Examples