caption: reduce
created: 20201004154131193
modified: 20201006174749170
op-input: a [[selection of titles|Title Selection]] passed as input to the filter
op-output: the final result of running the subfilter <<.place S>>
op-parameter: a [[filter expression|Filter Expression]]
op-parameter-name: S
op-purpose: apply a subfilter to each input title in turn, accumulating a single value
op-suffix: Initial value for accumulator
op-suffix-name: V
tags: [[Filter Operators]]
title: reduce Operator
type: text/vnd.tiddlywiki

\define reduce-tip()
The <<.op reduce>> operator will always produce output, even if its input was empty. If its input is empty, the output of <<.op reduce>> will be the initial value of the accumulator, i.e. the value of the suffix. One result of this fact is that the <<.op else>> operator will never be triggered if it follows a <<.op reduce>>. The "Empty input" examples show what happens when <<.op reduce>> receives no input.
\end

<<.from-version "5.1.23">> The <<.op 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 <<.op 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
* ''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

<$macrocall $name=".tip" _=<<reduce-tip>> />

<<.operator-examples "reduce">>