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For loop and while loop. which one is faster?

For loop and while loop. which one is faster?
I see many articles fighting over it and different people come up with
different results.

0
mgcclx (1)
3/26/2007 10:45:59 PM
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mgcclx@gmail.com wrote:

> For loop and while loop. which one is faster?
> I see many articles fighting over it and different people come up with
> different results.

Unless you're designing a compiler, don't worry about that. Worry about the
algorithms instead.

That said, the only possible difference between a for loop and a while loop
is the jump prediction in your architecture's code instruction set. e.g.
the processor can suppose that the condition for a while loop is going to
be true, so it feeds the instructions after the conditional jump (that is,
it's supposing that the jump condition will be successful) and starts
running them in the first stages of the instruction pipeline, saving a
couple of pipeline stages.

Do you have to worry about processor pipelines and jump prediction? Not at
all, unless you're into assembler, or heavy low-level programming. If you
haven't understood a word of the last paragraph, don't worry about loop
speed.


In other words: trust your compiler (or JIT compiler, or interpreter). It
knows about optimizations of conditional jumps better than you (the same
applies to memory caching). Focus on the algorithmics if you want to
shorten processor time.

-- 
----------------------------------
Iv�n S�nchez Ortega -ivansanchez-algarroba-escomposlinux-punto-org-

Give a man a fire and he'll be warm for a day; set him on fire and he'll be
warm the rest of his life.
0
ISO
3/26/2007 11:48:14 PM
In article <eu9m2t$af5$1@hercules.cohp1>,
 Iv�n S�nchez Ortega 
 <ivansanchez-alg@rroba-escomposlinux.-.punto.-.org> wrote:

> mgcclx@gmail.com wrote:
> 
> > For loop and while loop. which one is faster?
> > I see many articles fighting over it and different people come up with
> > different results.
> 
> Unless you're designing a compiler, don't worry about that. Worry about the
> algorithms instead.
> 
> That said, the only possible difference between a for loop and a while loop
> is the jump prediction in your architecture's code instruction set. e.g.
> the processor can suppose that the condition for a while loop is going to
> be true, so it feeds the instructions after the conditional jump (that is,
> it's supposing that the jump condition will be successful) and starts
> running them in the first stages of the instruction pipeline, saving a
> couple of pipeline stages.
> 
> Do you have to worry about processor pipelines and jump prediction? Not at
> all, unless you're into assembler, or heavy low-level programming. If you
> haven't understood a word of the last paragraph, don't worry about loop
> speed.
> 
> In other words: trust your compiler (or JIT compiler, or interpreter). It
> knows about optimizations of conditional jumps better than you (the same
> applies to memory caching). Focus on the algorithmics if you want to
> shorten processor time.

This is quite right. I never worry about such things - that's the 
computer's job.

I use whichever is appropriate - try, in other words, to give a hint to 
yourself or whoever is going to look at that code in a year's time. If I 
have n items and I want to do something for each of them, I use a for 
loop. If I need to loop around until some condition is satisfied (maybe 
one of several conditions in different parts of the loop), then I use a 
while.

-- tim
0
tim.streater (535)
3/27/2007 10:01:04 AM
mgcclx@gmail.com wrote:

> For loop and while loop. which one is faster?
> I see many articles fighting over it and different people come up with
> different results.

Why don't you test them if its that important?

C.
0
3/28/2007 8:59:48 PM
Also sprach mgcclx@gmail.com:

> For loop and while loop. which one is faster?

I don't think it makes much of a difference internally.

for ( $i = 0; $i < 10; $i++ )
{
    // do stuff
}

is probably treated just like the - less elegant -

$i = 0;
while ( $i < 10 )
{
    // do stuff
    $i++;
}

with no performance gain or loss. On the other hand, it makes a huge 
difference in performance if you write

for ( $i = 0; $i < count( $a ); $i++ ) { /* do stuff */ }

or

for ( $i = count( $a ); $i--; ) { /* do stuff */ }

the latter being much faster (function calls - count() in this example - 
cost time and the second version is one statement shorter). Of course, it 
iterates backwards over $a, but there are many cases where this does not 
matter. So, if you want to optimize your code, reduce the number of function 
calls, reduce the number of statements and reduce the number of variables 
(especially for intermediate results).

Greetings,
Thomas 


0
thomas4089 (261)
3/29/2007 4:30:43 PM
mgcclx@gmail.com wrote:
: For loop and while loop. which one is faster?
: I see many articles fighting over it and different people come up with
: different results.

What ever you find, it will likely be wrong next year, And different again
the year after that.

Use the construct that makes the most sense.

0
yf110 (571)
3/30/2007 5:21:34 AM
On Mar 27, 1:45 am, mgc...@gmail.com wrote:
> For loop and while loop. which one is faster?
> I see many articles fighting over it and different people come up with
> different results.

a bit late, but...
AFAIK it is faster to count down than up. Dont know about while or
for, but they are pretty much the same.

Personally I use:
$i=88883467568457642645;
while(0<$i--)
  blabla;


0
sonnich (155)
3/30/2007 3:19:50 PM
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