Alex,
Thanks for that.
Disbelieving soul that I am (sorry), I puzzled for a while over the results of
these two versions.
on mouseup
put empty into msg
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 "y" into line 3 of t
end if
end repeat
end mouseup
[no final CR on original t]
on mouseup
put empty into msg
put "abc" & CR & "def" & CR & "ghi" 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 "y" into line 3 of t
end if
end repeat
end mouseup
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.
Cheers
Dave
> On 16 Feb 2015, at 23:08, Alex Tweedly <[email protected]> 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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