On 17/02/2015 13:25, Dave Cragg wrote:
Alex,
Thanks for that.
Disbelieving soul that I am (sorry), I puzzled for a while over the results of
these two versions.
...
I had to use a pencil and paper to track what was in t and what the engine was referring to after
the "x" and "y" inserts. Then it became clear.
I feel a better person.
pencil and paper is good for your soul :-)
Though it would be kinda cool to do a quick LC simulation showing
visible "animation" of the variable and index as it goes through the loop.
That's top of my list for what to do the next time I have to go on a
long train journey ....
-- Alex.
Cheers
Dave
On 16 Feb 2015, at 23:08, Alex Tweedly <a...@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. (Oh, and btw, it does remember
how many chars were in the initial value, and uses that).
The fact that it does incremental scan rather than one initial scan can be seen
from the following (totally useless :-) bit of code :
on mouseup
put "abc" & CR & "def" & CR & "ghi" & CR into t
repeat for each line L in t
put the number of chars in L && L &CR after msg
if L = "abc" then
put "x" into line 2 of t
put "qwert" after line 3 of t
end if
end repeat
end mouseup
An initial scan would have produced :
3 abc
3 x
g
3 ijq
But we actually get:
3 abc
1 x
6 ghiqwe
It is significant that it doesn't do a complete scan, because that would be a
cost penalty at the start of the loop, and so could make it unnecessarily
expensive to do some operations. Again that can be seen from the following
(take out of context) snippet of not quite real code ....
put "millions and millions of lines of data" into tVar
repeat for each line L in tvar
if some condition then exit repeat
end repeat
If there were an initial scan, then you would do that whole costly scan even
though you could actually exit the loop very early; because it's an incremental
scan, you don't need to worry about that (so long as you are sure you will exit
soon).
-- Alex.
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