Random Number Generator

Random Number Generator

[Emil Block]

Does anyone know why a call to RAND with G77 returns a different value
than a call to RAND with F77 using the same seed?  The computer is a SUN
Sparc with OS 2.7. The same code is compiled in one case with G77 and in 
the other case with F77.

G77  rand (iseed)     returns 0.563463211
F77  rand (iseed)     returns 0.843506

where iseed = 1375.


Emil Block

Re: Random Number Generator

[Toon Moene]

Emil Block wrote:

> Does anyone know why a call to RAND with G77 returns a different value
> than a call to RAND with F77 using the same seed?  The computer is a SUN
> Sparc with OS 2.7. The same code is compiled in one case with G77 and in
> the other case with F77.
> 
> G77  rand (iseed)     returns 0.563463211
> F77  rand (iseed)     returns 0.843506
> 
> where iseed = 1375.

It would help if you could figure out how the Sun Fortran run time library
does this.

G77's actually does:

srand(iseed);
return rand();

-- 
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
Maintainer, GNU Fortran 77: http://gcc.gnu.org/onlinedocs/g77_news.html
Join GNU Fortran 95: http://g95.sourceforge.net/ (under construction)

Re: Random Number Generator

[Emil Block]

Toon Moene wrote:
> 
> Emil Block wrote:
> 
> > Does anyone know why a call to RAND with G77 returns a different value
> > than a call to RAND with F77 using the same seed?  The computer is a SUN
> > Sparc with OS 2.7. The same code is compiled in one case with G77 and in
> > the other case with F77.
> >
> > G77  rand (iseed)     returns 0.563463211
> > F77  rand (iseed)     returns 0.843506
> >
> > where iseed = 1375.
> 
> It would help if you could figure out how the Sun Fortran run time library
> does this.
> 
> G77's actually does:
> 
> srand(iseed);
> return rand();
> 
> --
> Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
> Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
> Maintainer, GNU Fortran 77: http://gcc.gnu.org/onlinedocs/g77_news.html
> Join GNU Fortran 95: http://g95.sourceforge.net/ (under construction)


Toon

Here's what the documentation contains:

Sun WorkShop Compilers FORTRAN 77 5.0

rand, drand, irand: Return Random Values

rand returns real values in the range 0.0 through 1.0.
drand returns double precision values in the range 0.0 through 1.0. 
irand returns positive integers in the range 0 through 2147483647. 

These functions use random(3) to generate sequences of random numbers.
The three functions share the same 256 byte state array. The only advantage
of these functions is that they are widely available on UNIX systems. For
better random number generators, compare lcrans, addrans, and shufrans. See
also random(3), and the Numerical Computation Guide 

  i = irand( k ) 
  r = rand( k ) 
  d = drand( k ) 
 
  k         INTEGER*4                     Input
                   k=0: Get next random number in the sequence 
                   k=1: Restart sequence, return first number 
                   k>0: Use as a seed for new sequence, return first number 

  rand      REAL*4                        Output
  drand     REAL*8                        Output
  irand     INTEGER*4                     Output

No mention of srand but the funtion exists as well as srandom.

Is G77 (rand_.c and srand_.c) doing an additional draw?

Emil Block

Re: Random Number Generator

[Toon Moene]

Emil Block wrote:

> Toon Moene wrote:

>>G77's actually does:
>>
>>srand(iseed);
>>return rand();

>   i = irand( k ) 
>   r = rand( k ) 
>   d = drand( k ) 
>  
>   k         INTEGER*4                     Input
>                    k=0: Get next random number in the sequence 
>                    k=1: Restart sequence, return first number 
>                    k>0: Use as a seed for new sequence, return first number 
> 
>   rand      REAL*4                        Output
>   drand     REAL*8                        Output
>   irand     INTEGER*4                     Output
> 
> No mention of srand but the funtion exists as well as srandom.
> 
> Is G77 (rand_.c and srand_.c) doing an additional draw?

Sorry for answering this so late ...

 From your description of whan Sun does, I'd say they do the same.  They 
must use *some* function to restart the sequence, and one would hope 
that it were srand.

However, given the current evidence, it's hard to say.  Perhaps you 
could ask Sun ?

-- 
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
Maintainer, GNU Fortran 77: http://gcc.gnu.org/onlinedocs/g77_news.html
Join GNU Fortran 95: http://g95.sourceforge.net/ (under construction)