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.
<<.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)