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- Subject: Re: for vs while
- From: "mlkesl" <kaishaku13@...>
- Date: Sat, 16 Feb 2002 14:23:08 -0000
Thanks for the explanation.
Now I see the difference between for and while
is not that great, about 7s vs 6s with that loop.
I still don't quite see exactly why it would be so
'wrong' to allow the for loop to work with a local
variable for the index instead of its own, but oh well.
I think many users could be confused by this, as they
may also have been 'contaminated' by C. :p
I guess the solution is to not use/support for loops.
I haven't got too far into lua, but I hope there are
not other loops/blocks in lua that act this way, creating
their own little scope to run in... *wince*
--- In lua-l@y..., John Mckenna <jmckenna@r...> wrote:
> mlkesl <kaishaku13@h...> writes:
> >I can understand it autoincrementing the index,
> >but that doesn't exactly mean changing the index
> >in the for loop is erroneous...just odd... right?
> "The behavious is undefined if you assign to the index inside the
> Have you had previous contamination from C? It sounds like you want
> for loops to behave as they do in there - as thinly disguised
> A for loop simply says "do this bit of code this many times". As a
> convenience, it lets you read to the index. The whole point of a
> loop is that the number of iterations is known before the loop is
> entered. This is a very useful property. It makes it much easier
> check that your program is correct (or at least, that bit won't do
> anything obviously stupid).
> Now, according to the rules, the behaviour of your example
> function F(input)
> return input + 1
> for i=1,10000000 do
> i = F(i)
> is undefined. But I was curious to see what it would do. It turns
> out that i is incremented twice each loop, and so the for loop only
> runs half as many iterations as your while loop. I think that goes
> a long way towards explaining the difference in speed you observed.
> It's still wrong. Don't assign to the index. It won't necessarily
> do the same thing on different versions of Lua.
> -Virus scanned and cleared ok