How to let a loop run for a while before checking for breakcondition?

[Hendrik van Rooyen]

 "Claudio Grondi" <claudio.grondi at freenet.de> wrote:


| Diez B. Roggisch wrote:
| > Claudio Grondi schrieb:
| >
| >>
| >> Sometimes it is known in advance, that the time spent in a loop will
| >> be in order of minutes or even hours, so it makes sense to optimize
| >> each element in the loop to make it run faster.
| >> One of instructions which can sure be optimized away is the check for
| >> the break condition, at least within the time where it is known that
| >> the loop will not reach it.
| >>
| >> Any idea how to write such a loop?
| >>
| >> e.g.
| >>
| >> counter = 2*64
| >>
| >> while counter(BUT DON'T CHECK IT THE FIRST ONE HOUR LONG):
| >
| >
| > now = time.time()
| > while time.time() - now < 3600.0 or some_other_condition:
| >    ...
| >
| >
| > The short circuiting of the or will prevent the execution of
| > some_other_condition.
| >
| >>   ... do something ... # and decrease the counter
| >>
| >> Thanks for any hint, but in particular if related to timers on the
| >> Windows 2000/XP system I am mainly working with.
| >>
| >> What do you think about this idea? Does it make sense?
| >
| > What idea?
| This one you haven't probably got from what I have written.
| I thought, that the introductory text gives enough context to be able to
| see what I mean, but I was apparently wrong.
|
| The idea is to speed up a loop by using a timer interrupt interfering
| with the loop, so that only after the timer interrupt would occur, the
| loop will start to check its break condition in each iteration.
| No checking of any kind in the loop should happen up to that time to
| minimize the number of operations in each iteration within the loop
| itself (i.e. the loop more or less won't know, that there is a timer on
| its way to change the loops behavior at a later time).
|
| I hope this above helps to understand what I would like to achieve.
|
| Claudio Grondi

I don't think this is usefully possible in python - the problem is that you will
simply replace one check - The expiry of the counter - with another - to see if
the interrupt has occurred already -

That said - the way I would do it would be to do something like this (in
horrible pseudo code):

loop_start:
    do_something()
    jump loop_start
    if counter > end_value:
        break
    jump loop_start
loop_end:


Interrupt_routine:
    replace the first jump to loop_start with a bunch of no - ops
    return

I don't think you can do this in python - it involves altering the running
loop - but hey maybe I can learn something here...

This example sort of exposes the break for what it is - a jump statement in
disguise - "look you cant recognise me - I am wearing dark glasses" - and
"continue" is exactly like that too - the only difference is that the one jumps
to the end, and the other to the beginning of the loop...

- Hendrik