Loop performance disappearance

[Chris Ryland]

I always assumed (perhaps being too much of a newbie) that

for i in range(n):
   :

would automatically be recognized by the compiler and turned into a simple
loop control using i alone (like turning it into an xrange, I guess). Seems
simple enough and common enough that it should be automatic, no? (Forgive
any massive display of ignorance here.)

--
Cheers!
/ Chris Ryland, President / Em Software, Inc. / www.emsoftware.com
"Remco Gerlich" <scarblac-spamtrap at pino.selwerd.nl> wrote in message
news:slrn8cva9a.aj8.scarblac-spamtrap at flits104-37.flits.rug.nl...
> Mikael Johansson wrote in comp.lang.python:
> > I was just wondering what the reason for the huge performance decrease
> > in a loop execution when the number of steps exceeds some critical value
> > is. To give an example:
> >
> > for i in range(loops):
> >     pass
> >
> > If loops=500 000 (space added for clarity) the "program" executes in ~2
> > secs on my machine. But if loops is set to 5 000 000, the execution time
> > rises to substantially more than tenfold (I terminated it after one
> > minute). However the CPU-load is quite small, most of the time is spent
> > disk swapping like crazy!
>
> That's because range(loops) is a list. It actually has to construct a list
> that size, and then walks through it. 5 million doesn't fit in your memory
> and it starts swapping.
>
> > Any ideas of the reason for this, work-arounds?
>
> Use xrange instead of range. xrange doesn't return a list, but an object
> that generates values on demand, simulating a list. Slightly slower, but
> doesn't need the memory.
>
> --
> Remco Gerlich,  scarblac at pino.selwerd.nl
>   Murphy's Rules, "Which is why you get 'em so cheap":
>    In SPI's Universe, the sword is prohibited from use at any combat
>    range.