valueDiff for arrays?

[Monte Goulding]

Just to throw it out there I still want to add `each` to filter one day. So in this case I think it would be:

filter keys of tArray1 with expression \
      each is among the keys of tArray2 and \
      tArray1[each] is not tArray2[each]

> On 5 Aug 2018, at 11:23 am, Brian Milby via use-livecode <use-livecode at lists.runrev.com> wrote:
> 
> Here is code that only uses LCS to accomplish the goal (only returning the
> keys where they exist in both arrays but the values are different).  This
> is made to be similar to the way the existing functions work (with the
> option to mutate).
> 
> command valueDiff @pDestinationA, @pLeftA, pRightA
>   local tMutate, tLeft, tRight, tNewRight
>   if the paramCount is 3 then
>      put false into tMutate
>      put pLeftA into tLeft
>      put pRightA into tRight
>   else
>      put true into tMutate
>      put pDestinationA into tLeft
>      put pLeftA into tRight
>   end if
>   repeat for each key tKey in tLeft
>      if tKey is among the keys of tRight and \
>            tLeft[tKey] is not tRight[tKey] then
>         put tRight[tKey] into tNewRight[tKey]
>         next repeat
>      end if
>      delete variable tLeft[tKey]
>   end repeat
>   if tMutate then
>      put tLeft into pDestinationA
>      put tNewRight into pLeftA
>   else
>      put tLeft into pDestinationA["1"]
>      put tNewRight into pDestinationA["2"]
>   end if
> end valueDiff
> 
> My question from an engine perspective is whether it would be faster to
> just generate both arrays instead of removing keys from tLeft.  I decided
> to build the tNewRight to avoid iterating the keys of tRight.
> 
> I'd be pretty confident that Mark's solution will be faster, even with 3
> iterations over the keys.
> 
> On Sat, Aug 4, 2018 at 6:52 PM, Mark Waddingham via use-livecode <
> use-livecode at lists.runrev.com> wrote:
> 
>> On 2018-08-04 21:00, Richard Gaskin via use-livecode wrote:
>> 
>>> Mark Waddingham wrote:
>>> 
>>> Yes - so come up with LCS handlers which do that, share them, let's
>>>> work together to make them as efficient in LCS as possible and then
>>>> see what we can do in the engine to improve their performance (which
>>>> I'm guessing is as much the purpose for the proposition as the
>>>> syntax).
>>>> 
>>> 
>>> It is, primarily.  The minor convenience is nice too, but you know how
>>> much I love to see performance enhancements in the engine.
>>> 
>>> As for implementation, Mark Wieder's looks good:
>>> http://lists.runrev.com/pipermail/use-livecode/2018-August/248862.html
>>> 
>> 
>> One important point here (which I think is something which is often
>> overlooked in LC as it is just something we deal with implicitly case by
>> case)...
>> 
>> In your use-case for 'valueDiff' - do you need to tell the difference
>> between a key value being the empty string and a key value not being
>> present at all?
>> 
>> [ i.e. using an array key absence to emulate 'nothing': meaning you are
>> actually storing nothing or a value (even the empty string). ]
>> 
>> It might seem like a minor detail, but does change the operation somewhat
>> (and potential implementations).
>> 
>> Warmest Regards,
>> 
>> Mark.
>> 
>> --
>> Mark Waddingham ~ mark at livecode.com ~ http://www.livecode.com/
>> LiveCode: Everyone can create apps
>> 
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