Hi Christian,
On Saturday 18 September 2010 14:49:29 Christian Brabandt wrote:
> Hi John!
>
> On Fr, 17 Sep 2010, John Little wrote:
> > On Sep 18, 3:05 am, Bee <[email protected]> wrote:
> > function! Random()
> >
> > if !exists("s:seeded")
> >
> > call libcallnr("libc.so.6", "srand", localtime() )
> > let s:seeded = 1
> >
> > endif
> > return libcallnr("libc.so.6", "rand", 0 ) % 65536
> >
> > endfun
> >
> > You might have to adjust that "libc.so.6" for your OS.
>
> This reminds me, that I have wished a rand() function for very long.
>
> So here is a very simple patch. It doesn't add much to the footprint. I
> am not sure, how portable the code is, though:
According to man rand on Linux:
{{{
The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99,
POSIX.1-2001. The function rand_r() is from POSIX.1-2001.
POSIX.1-2008 marks rand_r() as obsolete.
}}}
And man time:
{{{
CONFORMING TO
SVr4, 4.3BSD, C89, C99, POSIX.1-2001. POSIX does not specify any error
conditions.
}}}
However, see below:
>
>
> --- a/runtime/doc/eval.txt
> +++ b/runtime/doc/eval.txt
> @@ -4522,6 +4522,9 @@
> This can be used to avoid some things that would remove the
> popup menu.
>
> +rand() *rand()*
> + Returns a pseudo-random positive integer Number.
> +
> *E726* *E727*
> range({expr} [, {max} [, {stride}]]) *range()*
> Returns a |List| with Numbers:
> diff --git a/src/eval.c b/src/eval.c
> --- a/src/eval.c
> +++ b/src/eval.c
> @@ -653,6 +653,7 @@
> static void f_prevnonblank __ARGS((typval_T *argvars, typval_T *rettv));
> static void f_printf __ARGS((typval_T *argvars, typval_T *rettv));
> static void f_pumvisible __ARGS((typval_T *argvars, typval_T *rettv));
> +static void f_rand __ARGS((typval_T *argvars, typval_T *rettv));
> static void f_range __ARGS((typval_T *argvars, typval_T *rettv));
> static void f_readfile __ARGS((typval_T *argvars, typval_T *rettv));
> static void f_reltime __ARGS((typval_T *argvars, typval_T *rettv));
> @@ -7829,6 +7830,7 @@
> {"prevnonblank", 1, 1, f_prevnonblank},
> {"printf", 2, 19, f_printf},
> {"pumvisible", 0, 0, f_pumvisible},
> + {"rand", 0, 0, f_rand},
> {"range", 1, 3, f_range},
> {"readfile", 1, 3, f_readfile},
> {"reltime", 0, 2, f_reltime},
> @@ -14224,6 +14226,18 @@
> rettv->vval.v_number = 1;
> #endif
> }
> +/*
> + * "rand()" function
> + */
> + static void
> +f_rand(argvars, rettv)
> + typval_T *argvars;
> + typval_T *rettv;
> +{
> + rettv->v_type = VAR_NUMBER;
> + srand((varnumber_T)time(NULL));
> + rettv->vval.v_number = rand();
> +}
This is wrong. You're calling srand with the current time on every invocation
which will:
1. Yield non-predictable and non-reproducible results.
2. If two calls are done one right after the other within the same wall-clock
second, it will yield the exact same value.
A better idea would be to provide a binding for srand as well, so the user can
set a seed and get predictable results. An even better idea may be to provide
a consistent pseudo-random-number generator which will yield the same results
on all platforms. See:
http://www.haifux.org/lectures/79/
Regards,
Shlomi Fish
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