created: 20201004154413968
modified: 20201006181831622
tags: [[Operator Examples]] [[reduce Operator]]
title: reduce Operator (Examples)
type: text/vnd.tiddlywiki

\define add-price() [get[price]multiply{!!quantity}add<accumulator>]
\define num-items() [get[quantity]add<accumulator>]
\define join-with-commas() [<index>compare:number:gt[0]then<accumulator>addsuffix[, ]addsuffix<currentTiddler>else<currentTiddler>]
\define display-variable(name)
''<$text text=<<__name__>>/>'': <code><$text text={{{ [<__name__>getvariable[]] }}}/></code>
\end
\define reduce-tip()
Remember that <<.op reduce>> always produces output, so <<.op else>> will never trigger after <<.op reduce>>.
\end

These examples use the following predefined variables:

* <<display-variable add-price>>
* <<display-variable num-items>>
* <<display-variable join-with-commas>>

They also use the following data tiddlers:

<ul>
<$list filter="[tag[shopping]!has[draft.of]]">
<li>
''<$link><$text text=<<currentTiddler>>/></$link>'' quantity: <$text text={{!!quantity}}/>,  price: <$text text={{!!price}}/>
</li>
</$list>
</ul>

Number of items: 

<<.operator-example 1 "[tag[shopping]reduce<num-items>]">>

Total price:

<<.operator-example 2 "[tag[shopping]reduce<add-price>]">>

Using `<index>` to act differently on the first item than the rest:

<<.operator-example 3 "[tag[shopping]reduce<join-with-commas>]">>

Empty input, no suffix:

<<.operator-example 4 "[tag[non-existent]reduce<add-price>]">>

Note how the output contains a single item with no text. This is not "empty output" for the purposes of the <<.op else>> operator.

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

Empty input, no suffix, followed by <<.op else>>:

<<.operator-example 5 "[tag[non-existent]reduce<add-price>else[0]]">>

Note how the output still contains a single item with no text: <<.op else>> did not trigger. If you want the value to be 0 when <<.op reduce>> has no items to process, you need to specify 0 as the initial value by passing it as a suffix to <<.op reduce>>.

Empty input, suffix provided:

<<.operator-example 6 "[tag[non-existent]reduce:0<add-price>]">>