stringise DBL_DIG

[Malcolm]

/*
converts a double to decimal with no loss of accuracy.
*/
void accurate(char *out, double x)
{
char fmt[32];

sprintf(fmt, "%%.%dg", DBL_DIG);
sprintf(out, fmt, x);
}

Any way of getting rid of the first call to sprintf() ?

[Ben Pfaff]

Re: stringise DBL_DIG

"Malcolm" <malcolm@55bank .freeserve.co.u k> writes:
[color=blue]
> sprintf(fmt, "%%.%dg", DBL_DIG);
> sprintf(out, fmt, x);
>
> Any way of getting rid of the first call to sprintf() ?[/color]

sprintf(out, "%.*g", (int) DBL_DIG, x);
--
"I don't have C&V for that handy, but I've got Dan Pop."
--E. Gibbons

[Malcolm]

Re: stringise DBL_DIG


"Ben Pfaff" <blp@cs.stanfor d.edu> wrote in message[color=blue]
>
> sprintf(out, "%.*g", (int) DBL_DIG, x);
>[/color]
Thanks. I was barking up the wrong tree with the stringise (#) operator,
which of course gives me the string "DBL_DIG".

[Martin Ambuhl]

Re: stringise DBL_DIG

Malcolm wrote:
[color=blue]
> /*
> converts a double to decimal with no loss of accuracy.
> */
> void accurate(char *out, double x)
> {
> char fmt[32];
>
> sprintf(fmt, "%%.%dg", DBL_DIG);
> sprintf(out, fmt, x);
> }
>
> Any way of getting rid of the first call to sprintf() ?[/color]

#define ACCURATE(s,x) sprintf((s),"%. *g",DBL_DIG,(x) )


--
Martin Ambuhl

[Peter Nilsson]

Re: stringise DBL_DIG

"Malcolm" <malcolm@55bank .freeserve.co.u k> wrote in message
news:bvjpbq$b8c $1@newsg4.svr.p ol.co.uk...[color=blue]
>
> "Ben Pfaff" <blp@cs.stanfor d.edu> wrote in message[color=green]
> >
> > sprintf(out, "%.*g", (int) DBL_DIG, x);
> >[/color]
>
> Thanks. I was barking up the wrong tree with the stringise (#) operator,
> which of course gives me the string "DBL_DIG".[/color]

That's easily fixed (See FAQ 11.17), but you're still in trouble.

AFAIK, there's nothing barring an implementation from doing any of these in
a header source...

#define DBL_DIG __DBL_DIG
#define DBL_DIG (10)
#define DBL_DIG (FLT_DIG*2)

So code like...

#define STR(x) #x
#define STRSTR(x) STR(x)

sprintf(out, "%." STRSTR(DBL_DIG) "g", x);

....is not as maximally portable as it might seem.

--
Peter

[Old Wolf]

Re: stringise DBL_DIG

> > /*[color=blue][color=green]
> > converts a double to decimal with no loss of accuracy.
> > */
> > void accurate(char *out, double x)
> > {
> > char fmt[32];
> >
> > sprintf(fmt, "%%.%dg", DBL_DIG);
> > sprintf(out, fmt, x);
> > }[/color]
>
> #define ACCURATE(s,x) sprintf((s),"%. *g",DBL_DIG,(x) )[/color]

#define ACCURATE(s,n,x) snprintf((s),(n ),"%.*g",DBL_DI G,(x))

I don't mean to be a spelling nazi, but IMHO a buffer overflow
(eg. when the code is ported to a system that has a higher
value for DBL_DIG) is a more serious portability "error"
than casting malloc (which often does get pointed out on here).

Please tell me if I should refrain from future comments on this
issue :)

Also, you could go
#define ACCURATE_PF(x) "%.*g",DBL_DIG, (x)
and then
printf(ACCURATE _PF(x));
or
snprintf(s, n, ACCURATE_PF(x)) ;
etc.

[Peter Nilsson]

Re: stringise DBL_DIG

oldwolf@inspire .net.nz (Old Wolf) wrote in message news:<843a4f78. 0402021315.4efb 61a4@posting.go ogle.com>...[color=blue][color=green][color=darkred]
> > > /*
> > > converts a double to decimal with no loss of accuracy.
> > > */
> > > void accurate(char *out, double x)
> > > {
> > > char fmt[32];
> > >
> > > sprintf(fmt, "%%.%dg", DBL_DIG);
> > > sprintf(out, fmt, x);
> > > }[/color]
> >
> > #define ACCURATE(s,x) sprintf((s),"%. *g",DBL_DIG,(x) )[/color]
>
> #define ACCURATE(s,n,x) snprintf((s),(n ),"%.*g",DBL_DI G,(x))
>
> I don't mean to be a spelling nazi, but IMHO a buffer overflow
> (eg. when the code is ported to a system that has a higher
> value for DBL_DIG) is a more serious portability "error"
> than casting malloc (which often does get pointed out on here).[/color]

Only trouble is, snprintf is only available in C99. It's a common
extension for C90 implementations , but unfortunately, it's return
value is not consistent across such implementations .
[color=blue]
> Please tell me if I should refrain from future comments on this
> issue :)
>
> Also, you could go
> #define ACCURATE_PF(x) "%.*g",DBL_DIG, (x)
> and then
> printf(ACCURATE _PF(x));[/color]

This is generally not considered good form as it obfuscates the printf
call.
[color=blue]
> or
> snprintf(s, n, ACCURATE_PF(x)) ;
> etc.[/color]

--
Peter