--- David le Blanc
<[EMAIL PROTECTED]> wrote:
>
> Just for the sake of it, and I apologise profusely
> for the top posting..
>
stuart>Hi David,
> Here is a summary of the posted prime calculators
> for testing your
> results against.
>
> >(1) Ohad
stuart>I saw his regexp soln. I was attempting to
stuart>solve it with arrays, loops and maybe hashes.
stuart>I figure that the exercise wouldn't be in the
stuart>book before the regexp chapter if I couldn't do
stuart>it easily (or without TOO much difficulty)
stuart>without them. That's all.
> >(2) Joseph
stuart>I thought this one was a joke. Partly because
stuart>of the context it was sent in, partly because
stuart>of what I saw it do. I ran it, and it printed
stuart>out a slew of numbers. It didn't stop after a
stuart>few minutes, so I ended the program.
> >(3) me (Hey, it's a Mee Too!)
>
#-------------------------------------------------------
> $max=shift||17393;
> %notprime=();
> @prime=();
>
> for $n (2..$max){
> unless($notprime{$n}){
> push @prime,$n;
> $notprime{$_*$n}=1 for 2..$max/$n;
> }
> }
>
> print "Found $#prime prime numbers..".$/;
> print "@prime\n";
>
#-------------------------------------------------------
>
> Brief explanation..
> (1)
sounds neat.
> (2) Joseph posted a calculator which uses the
> feature of prime numbers
> that the
> largest number you need to divide it by is the
> square root of itself
> ($_**0.5)
> and that you only need to test if the number you
> are examining is
> divisible
> (ie mod==0) by another prime you have already
> found. Hence, very
> quick and
> very cpu/memory cheap. Why not test against
> every number? If you
> test against
> a non-prime number, you are simply testing
> against a multiple of a
> prime of
> course.
stuart>Ahh, this makes sense, sortof. I'm going to
stuart>need to look at the code again and your
stuart>explanation. This was printing many, many,
stuart>many numbers, some of which sure didn't look
stuart>like they were primes. The sheer number plus
stuart>the proximity of them to each other in
stuart>cardinal? order (2341 2347 2443 etc) (these
stuart>numbers are used to illustrate my probably
stuart>improper use of cardinal, I'm not implying that
stuart>those numbers were outputted by the program)
stuart>made me think that some weren't primes.
>
> (3) A very simple implementation of an erogenous
> sieve (iud? did I say
> that wrong?)
> which keeps all primes in '@primes' and all
> 'eliminated' entries in
> '%notprime'.
> quick, but likely more memory intensive than
> (2).
This one is a bit confusing.
$max=shift||17393;
stuart>I thought shift was to remove an element from
stuart>the beginning of an array, but it's being used
stuart>in scalar context here. Why? And what is it
stuart>doing?
%notprime=();
> @prime=();
>
> for $n (2..$max){
stuart>this makes me think that you were somehow
stuart>assigning a number to $max above. But where
stuart>does 17393 come in? Why not 17394, or 18000?
> unless($notprime{$n}){
> push @prime,$n;
> $notprime{$_*$n}=1 for 2..$max/$n;
stuart>I'm not sure I understand this line directly
stuart>above. What I think it says is that it is
stuart>accessing the value within the %notprime hash
stuart>where the key equals the last inputted number
stuart>which I suppose is $n and $_ if $n was pushed
stuart>onto @prime, but just $_ if $n was not pushed
stuart>onto @prime. I don't get the "=1" part
stuart>though. What is that doing? I'm assuming that
stuart>is a for loop there that is working like a
stuart>foreach loop? but I'm not quite sure what it's
stuart>doing.
> Cheers.
> David
>
>
__________________________________
Do you Yahoo!?
Yahoo! Search - Find what you’re looking for faster
http://search.yahoo.com
--
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
<http://learn.perl.org/> <http://learn.perl.org/first-response>
