Reverse a list

Alex Tweedly alex at tweedly.net
Tue Feb 17 17:29:53 EST 2015


On 17/02/2015 15:43, Bob Sneidar wrote:
> Then past posts are incorrect on this matter. It was explicitly stated that the actual memory holding the variable data was “indexed” and that altering the variable data could relocate the variable in memory resulting in invalid data. I have seen this myself. The data returned is garbled nonsense. I don’t think setting the value of the variable back to what it was will correct this.
That is still consistent with what I described. What I am claiming (and 
*all* I am claiming) is that the scan for the next line is incremental 
rather than done once for all lines at the start of the loop. Whether 
the engine does this by maintaining a "character offset" or a direct 
memory pointer is way beyond the scope of my description/claim.

If (as seems likely) it does keep a direct memory pointer then it is 
indeed very possible for that to explain the problem you describe - some 
modification to the subsequent lines might require more memory than is 
available in place, so a new larger memory block is created and the 
string copied/expanded, and hence the pointer would become stale and the 
result would be total garbage.

I am *absolutely* not recommending that anyone should modify the 
variable in question within the loop - even if that seems to work in 
some cases, it is known to be dangerous, and so should just NOT BE DONE 
in "real" code.  But doing it in limited cases does allow one to deduce 
some things about how the engine is doing its tasks.

Of course, now that we have access to the open source, it's equally 
possible to just go look. And if I ever have time to go through a github 
tutorial, and get myself a copy of the source, I will try to do that and 
be in a position to actually know what the engine does, rather than 
deduce it indirectly. But I'm not holding my breath for when I'll have 
that much free time  :-)

-- Alex.
> But I’ve been mistaken before.
>
> Bob S
>
>
> On Feb 16, 2015, at 18:05 , Dr. Hawkins <dochawk at gmail.com<mailto:dochawk at gmail.com>> wrote:
>
> On Mon, Feb 16, 2015 at 3:08 PM, Alex Tweedly <alex at tweedly.net<mailto:alex at tweedly.net>> wrote:
>
> That's not quite correct. It doesn't do a single initial complete scan of
> the whole variable and keep all the pointers. What it does is (more like)
> keep track of how far it has currently processed, and then when it needs
> the next line, it scans from that remembered position forward until the
> next CR - and then remembers that as the current position.
>
>
> And if you repeat for each item, and change the itemDel in the loop, you
> had *better* remember to change it back before end repeat, or very strange
> things happen . . .
>
>
> --
> Dr. Richard E. Hawkins, Esq.
> (702) 508-8462
>
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