caption: reduce
created: 20201004154131193
modified: 20210522162536854
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]]. Optional second parameter for initial value for accumulator
op-parameter-name: S
op-purpose: apply a subfilter to each input title in turn, accumulating a single value
tags: [[Filter Operators]]
title: reduce Operator
type: text/vnd.tiddlywiki

<<.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
* ''..currentTiddler'' - the value of the variable `currentTiddler` outside the subfilter. <<.from-version "5.2.0">>
* ''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 <<.op reduce>> operator receives no input, its output will be empty. The [[else Operator]] can be useful in such cases.

<<.tip "Literal filter operands 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 analagous named [[filter run prefix|Filter Expression]] `:reduce`">>

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

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

is equivalent to:

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

<$macrocall $name=".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 <<.op reduce>> in some cases."""/>

<<.operator-examples "reduce">>