The reduce filter may needs a flat suffix

[kookma]

In response to a question in talk: https://talk.tiddlywiki.org/t/how-to-get-the-sum-of-field-values-by-filtering-based-on-prefix-of-field-names/6199/28

It was asked how to calculate the sum of field values based on filed name for given tiddlers. @ericshulman provided a solution as below

\define makesum() [<currentTiddler>fields[]prefix[ct-]] :map[<..currentTiddler>get<currentTiddler>] +[sum[]]

$text text={{{ [tag[tidsToSum]] :map[subfilter<makesum>] +[sum[]] }}} />

Which works fine. BUT, I think this question lends itself to reduce operator. So the

\define makesum() [<currentTiddler>fields[]prefix[ct-]] :reduce[<..currentTiddler>get<currentTiddler>add<accumulator>]

<$list filter="[tag[tidsToSum]] :reduce[subfilter<makesum>add<accumulator>]">

</$list>

Both solutions seem a little complex. BUT for reduce, if the input contains several values, it only takes the first value. For example, see below solution:

\define makesum() [<currentTiddler>fields[]prefix[ct-]] :map:flat[<..currentTiddler>get<currentTiddler>]

<$list filter="[tag[exmp195]] :reduce[subfilter<makesum>add<accumulator>]">

</$list>

The makesum here returns all values for all fields prefixed with ct-, BUT the :reduce only takes the first value and adds it to accumulator.

What do you think if we add a flat suffix to reduce?
I think @saqimtiaz was the developer behind the reduce operator.

[pmario]

hmm, But that would mean, that the :reduce run-prefix would behave like :map ...

:map and :reduce are already complex filter run prefixes. If they could be interchanged imo it would be even more complex.

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I would suggest, that we add the solution Eric provided to the docs. So there will be 1 way to solve that problem.

[kookma]

I think yes, we need such examples as the one by Eric to be added to doc. Understanding map and reduce needs more examples and I think both operators are quite powerful.

[ericshulman]

Here's another, more generic example of use the :map filter run prefix:

<!-- Get total size of all fields in a tiddler, including size of field NAMES as well as VALUES -->
\define size() [<currentTiddler>fields[]] :map[<..currentTiddler>get<currentTiddler>else[]length[]] [<currentTiddler>fields[]length[]] +[sum[]]

<!-- Get total size of all tiddlers -->
{{{ [all[tiddlers]] :map[subfilter<size>] +[sum[]] }}}