butlast filter operator

[History Buff]

All,

I have a general question about the butlast filter operator. I had a filter where I had the butlast operator as the last item with a variable for the parameter. All worked fine until the variable was 0.  I expected that it wouldn't remove any of the last items, but what I got was no results at all. I fixed it be making sure that the variable was never 0, but I don't understand why a 0 in the butlast operator would yield no results. Can anyone explain that to me? 

Thanks so much!

[Eric Shulman]

On Sunday, June 27, 2021 at 11:28:50 AM UTC-7 History Buff wrote:

I don't understand why a 0 in the butlast operator would yield no results

The butlast[] operator is defined in this shadow tiddler:

https://tiddlywiki.com/#%24%3A%2Fcore%2Fmodules%2Ffilters%2Flistops.js

Here's the javascript code from that tiddler:

exports.butlast = function(source,operator,options) {

var count = $tw.utils.getInt(operator.operand,1),

results = [];

source(function(tiddler,title) {

results.push(title);

});

return results.slice(0,-count);

};

The first line gets the operand value, defaulting to 1.

The next 4 lines copy the entire list of items into a results array

The last line uses the javascript slice() function to remove items from the results array and then return that array

Here's the documentation for the javascript slice() function:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice

Note the description of the "end" parameter:

Zero-based index before which to end extraction. slice extracts up to but not including end.

Thus, when you used butlast[0], the filter code invokes slice(0,-0),

i.e., "starting from the first item up to but not including the first item"

which results in no items being returned.

enjoy,

-e

[Damon Pritchett]

Thanks Eric,

I appreciate that. I looked at it, but I really don’t know JavaScript. Makes me want to learn though. 

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[TW Tones]

Another way to look at it ;  

Butlast purpose discard the last N input title(s)

So saying  "discard the last 0 input titles" is non sensical 

If you actually want to discard 3, then 2 then 1 perhaps you filter needs find another way once it reaches 0

Tones

[Damon Pritchett]

Hey Tony,

Yeah that’s what I eventually realized so I changed what happens when I reach zero. I was just wanting to understand the inner workings better. 

To view this discussion on the web visit https://groups.google.com/d/msgid/tiddlywiki/faaa280d-a866-47d3-abef-5ec8e51e5d5dn%40googlegroups.com.

[Saq Imtiaz]

This is essentially an oversight and will be fixed. Thank you for bringing this up.

https://github.com/Jermolene/TiddlyWiki5/issues/5833

[History Buff]

Thanks!