[tw5] Re: Difference between filter and subfilter?

[The Islander]

Hi Tones and Eric,

I greatly appreciate your detailed replies! Reading through them clears it 
up for me thank you.

It may be useful to add this sort of clarification to official 
documentation - the motivations behind some of the more complex filter 
operators. If anyone feels it's appropriate for me to request this as an 
open issue on github, please let me know.

The "filter" filter operator in particular is difficult to search for, due 
to its identical naming to the parent concept of a filter, and other 
related naming concepts like the "filter" filter run prefix.

Thanks!


On Sunday, August 1, 2021 at 3:05:31 AM UTC-7 Eric Shulman wrote:

> Here's a demonstration of the difference:
>
> 1) Create three tiddlers named A, B, and C, with text content "this is A", 
> "this is B" and "this is C", respectively.
>
> 2) Create a Test tiddler containing:
> <$vars F="[get[text]match[this is B]]">
>
> subfilter result 1 is: <$text text={{{ [enlist[A B C]subfilter] }}} 
> />
>
> <$list filter="[enlist[A B C]get[text]match[this is B]]" variable="result">
>subfilter result 2 is: <>
> 
>
> filter result 1 is: <$text text={{{ [enlist[A B C]filter] }}} />
>
> <$list filter="[enlist[A B C]]">
><$list filter="[get[text]match[this is 
> B]then]" variable="result">
>   filter result 2: is <>
>
> 
>
> 3) The output will be:
> *subfilter result 1 is: this is B*
>
> *subfilter result 2 is: this is B*
>
> *filter result 1 is: B*
>
> *filter result 2: is B*
>
> The difference:
>
>- *subfilter* applies filter F to each input title and returns the 
>TEXT that matches "this is B", and is *equivalent to inserting filter 
>F into a larger filter expression*
>- *filter *applies filter F to each input title and returns the TITLE 
>whose text matches "this is B" and is *equivalent to writing nested 
>filters*
>
> hope this helps...
>
> -e
>
>

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[tw5] Re: Difference between filter and subfilter?

[Eric Shulman]

Here's a demonstration of the difference:

1) Create three tiddlers named A, B, and C, with text content "this is A", 
"this is B" and "this is C", respectively.

2) Create a Test tiddler containing:
<$vars F="[get[text]match[this is B]]">

subfilter result 1 is: <$text text={{{ [enlist[A B C]subfilter] }}} 
/>

<$list filter="[enlist[A B C]get[text]match[this is B]]" variable="result">
   subfilter result 2 is: <>


filter result 1 is: <$text text={{{ [enlist[A B C]filter] }}} />

<$list filter="[enlist[A B C]]">
   <$list filter="[get[text]match[this is 
B]then]" variable="result">
  filter result 2: is <>
   


3) The output will be:
*subfilter result 1 is: this is B*

*subfilter result 2 is: this is B*

*filter result 1 is: B*

*filter result 2: is B*

The difference:

   - *subfilter* applies filter F to each input title and returns the TEXT 
   that matches "this is B", and is *equivalent to inserting filter F into 
   a larger filter expression*
   - *filter *applies filter F to each input title and returns the TITLE 
   whose text matches "this is B" and is *equivalent to writing nested 
   filters*

hope this helps...

-e

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[tw5] Re: Difference between filter and subfilter?

[TW Tones]

The Islander

I agree this should be better documented. To stimulate the discussion;

As the person who submitted that issue on github subfilter and subsequently 
closed it, the filter operator, and the subsequent :filter run prefix have 
changed this quite a bit. My concern then was "not having a filter to be 
sub of when used:, however as the first example show I have effectively 
gotten what I asked for 

The subfilter was first on the scene, and made imbedding a full filter 
inside another much easier, otherwise you would have to reconstruct the 
filter from filter fragments. 

>From the doco note it still uses the word subfilter but they look the same.
*Filter operator*
*Apply a subfilter to each input title and return the titles that return a 
non-empty result from the subfilter*  
*The filter operator runs a subfilter for each input title, and returns 
those input titles for which the subfilter returns a non-empty result (in 
other words the result is not an empty list). The results of the subfilter 
are thrown away.  *

*subfilter operator*
 select titles from the operand interpreted as a filter expression 

* Compare with the similar filter 
 operator which runs 
a subfilter against each title, returning those titles that return a 
non-empty list (and discards the results of the subfilter)*  

In the following example there is no apparent difference, remember there is 
an implied [all[].. at the beginning of reach run.

\define active-filter() [!tag[done]!tag[reference]]
{{{ [tag[todo]count[]] }}}

{{{ [tag[todo]subfiltercount[]] }}}

{{{ [tag[todo]!subfiltercount[]] }}}

{{{ [tag[todo]filtercount[]] }}}

{{{ [tag[todo]!filtercount[]] }}}

Regards
Tones
On Sunday, 1 August 2021 at 14:19:00 UTC+10 The Islander wrote:

> Hello, after reading the official documentation on subfilter and filter, 
> it's not clear to me what the difference is between these two operators.
>
>- Why would someone choose to use one over the other? A couple of 
>simple examples where one differentiates from the other would be very 
>useful.
>- Is subfilter suitable for use in the middle of a filter expression? 
>The subfilter documentation page itself shows an example of a subfilter in 
>the middle of a filter expression, but none of the examples do this
>- I'm further confused by the discussion in this issue on Github 
> which is 
>requesting filter as a simple alias of subfilter for readability
>
> On a side note, I thought there would be some consistency to availability 
> of currentTiddler within the operand of filter, but there isn't 
> . Are saqimtiaz's 
> improvements something slated to be included in 5.2.0?
>
> Thanks!
>

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