Re: Seeding a Random Number Generator
On Sat, Dec 06, 2003 at 04:38:02PM -0800 John W. Krahn wrote:
Pd Schloss wrote:
I'm reviewing a perl script that someone wrote to do a statistical
analysis. I know it's bad form, but I was wondering if anyone knows
what the default seed is for the random number generator in Perl. They
haven't seeded it with srand - what does this do? It still seems to
pick random numbers, any ideas?
Download the source code for Perl if you don't already have it and have
a look at the pp.c file.
The actual seed is calculated in Perl_seed() in util.c. The calculation
of the seed itself is highly dependent on the platform used. It involves
various things, such as gettimeofday(2), the PID, a value read from a
random device such as /dev/urandom, the position of PL_stack_sp in
memory etc. For most cases it should be quite ok.
Tassilo
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Re: Seeding a Random Number Generator
Tassilo Von Parseval wrote: On Sat, Dec 06, 2003 at 04:38:02PM -0800 John W. Krahn wrote: Download the source code for Perl if you don't already have it and have a look at the pp.c file. The actual seed is calculated in Perl_seed() in util.c. The calculation of the seed itself is highly dependent on the platform used. It involves various things, such as gettimeofday(2), the PID, a value read from a random device such as /dev/urandom, the position of PL_stack_sp in memory etc. For most cases it should be quite ok. I did a grep -i -r 'seed' *.c and the only hit I got was in pp.c. John -- use Perl; program fulfillment -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] http://learn.perl.org/ http://learn.perl.org/first-response
Re: Seeding a Random Number Generator
On Sun, 07 Dec 2003 03:29:02 -0800
John W. Krahn [EMAIL PROTECTED] wrote:
I did a grep -i -r 'seed' *.c and the only hit I got was in pp.c.
FWIW
[EMAIL PROTECTED] perl-5.8.2]# grep -r seed
snip
util.c:Perl_seed(pTHX)
util.c:* is enough real entropy to fill the seed. */
util.c:Perl_get_hash_seed(pTHX)
util.c: UV myseed = 0;
util.c: myseed = (UV)Atoul(s);
util.c: /* Compute a random seed */
util.c: (void)seedDrand01((Rand_seed_t)seed());
util.c: myseed = (UV)(Drand01() * (NV)UV_MAX);
util.c: myseed +=
util.c: if (myseed == 0) { /* Superparanoia. */
util.c: myseed = (UV)(Drand01() * (NV)UV_MAX); /* One more chance. */
util.c: if (myseed == 0)
util.c: PL_rehash_seed_set = TRUE;
util.c: return myseed;
snip
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Owen
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Re: Seeding a Random Number Generator
Owen wrote:
On Sun, 07 Dec 2003 03:29:02 -0800
John W. Krahn [EMAIL PROTECTED] wrote:
I did a grep -i -r 'seed' *.c and the only hit I got was in pp.c.
FWIW
[EMAIL PROTECTED] perl-5.8.2]# grep -r seed
snip
util.c:Perl_seed(pTHX)
util.c:* is enough real entropy to fill the seed. */
util.c:Perl_get_hash_seed(pTHX)
util.c: UV myseed = 0;
util.c: myseed = (UV)Atoul(s);
util.c: /* Compute a random seed */
util.c: (void)seedDrand01((Rand_seed_t)seed());
util.c: myseed = (UV)(Drand01() * (NV)UV_MAX);
util.c: myseed +=
util.c: if (myseed == 0) { /* Superparanoia. */
util.c: myseed = (UV)(Drand01() * (NV)UV_MAX); /* One more chance. */
util.c: if (myseed == 0)
util.c: PL_rehash_seed_set = TRUE;
util.c: return myseed;
It must have been moved there for version 5.8 because it is not there in
5.6.
John
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Seeding a Random Number Generator
Hi, I'm reviewing a perl script that someone wrote to do a statistical analysis. I know it's bad form, but I was wondering if anyone knows what the default seed is for the random number generator in Perl. They haven't seeded it with srand - what does this do? It still seems to pick random numbers, any ideas? Thanks, Pat -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] http://learn.perl.org/ http://learn.perl.org/first-response
Re: Seeding a Random Number Generator
Pd Schloss wrote: I'm reviewing a perl script that someone wrote to do a statistical analysis. I know it's bad form, but I was wondering if anyone knows what the default seed is for the random number generator in Perl. They haven't seeded it with srand - what does this do? It still seems to pick random numbers, any ideas? Hi. I wrote this back in March. I assume I was right :) Rob Dixon wrote: Interesting. Digging into the code for Perl v5.6.1, 'srand' will try to read four bytes from the /dev/urandom device. If that fails, then it will read the current time of day and mix it up with the PID, a Perl stack pointer and a few arbitrary constants. So, as it should be, there is no 'default' seed. This probably doesn't help, but at least should end your search! Cheers, Rob -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] http://learn.perl.org/ http://learn.perl.org/first-response
Re: Seeding a Random Number Generator
Pd Schloss wrote: Hi, Hello, I'm reviewing a perl script that someone wrote to do a statistical analysis. I know it's bad form, but I was wondering if anyone knows what the default seed is for the random number generator in Perl. They haven't seeded it with srand - what does this do? It still seems to pick random numbers, any ideas? Download the source code for Perl if you don't already have it and have a look at the pp.c file. John -- use Perl; program fulfillment -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] http://learn.perl.org/ http://learn.perl.org/first-response
