Problem: scanf used for double

[Frank Chow]

Please see the following code:

/* main.c */

#include <stdio.h>

int main()
{
double d;
scanf("%f", &d);
printf("%g\n", d);
}

The problem is, whatever I input to stdin (even an illegal data), the
program just print a number that seems to be a data from "raw memory",
ie uninitialized memory (for example: "5.35162e-315").

And if I replace the statement "double d" with "double d = 3.14", then
I'll always get the output "3.14" for whatever I input (even an
illegal data).

Then I may get the conclusion that the "scanf" statement never does
its job. And I have tried this under 3 compilers and none of them give
the right result.

Please give me some explanation about this.

Thanks.

[Christopher Benson-Manica]

Frank Chow <fa*******@yaho o.com> spoke thus:

#include <stdio.h> int main()
{
double d;
scanf("%f", &d);
printf("%g\n", d);
}

If you had your compiler warnings turned on, you probably would have gotten
something like

test.c:6: warning: float format, double arg (arg 2)

The correct conversion specifier for doubles is %lf when using scanf. Also,
should return a value from main, a la

return( EXIT_SUCCESS );

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Eric Sosman]

Frank Chow wrote:

Please see the following code:

/* main.c */

#include <stdio.h>

int main()
{
double d;
scanf("%f", &d);

scanf("%lf", &d)

See Question 12.13 in the comp.lang.c Frequently Asked
Questions (FAQ) list

http://www.eskimo.com/~scs/C-faq/top.html
--
Er*********@sun .com

[Irrwahn Grausewitz]

Christopher Benson-Manica <at***@nospam.c yberspace.org> wrote:

<snip>

Also,
should return a value from main, a la

return( EXIT_SUCCESS );

after inclusion of stdlib.h, of course; alternatively

return 0;

is fine, too.
--
Irrwahn
(ir*******@free net.de)

[Dan Pop]

In <be************ **************@ posting.google. com> fa*******@yahoo .com (Frank Chow) writes:

Please see the following code:

/* main.c */

#include <stdio.h>

int main()
{
double d;
scanf("%f", &d);
printf("%g\n", d);
}

The problem is, whatever I input to stdin (even an illegal data), the
program just print a number that seems to be a data from "raw memory",
ie uninitialized memory (for example: "5.35162e-315").

Where did you get the idea that %f is the right conversion specifier for
a double? If you needed a float instead, what would you use?

For obvious reasons, scanf is not a perfect mirror of printf.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Christopher Benson-Manica]

Irrwahn Grausewitz <ir*******@free net.de> spoke thus:

return 0;
is fine, too.

I thought it was non-portable :(

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Irrwahn Grausewitz]

Christopher Benson-Manica <at***@nospam.c yberspace.org> wrote:

Irrwahn Grausewitz <ir*******@free net.de> spoke thus:

return 0;
is fine, too.

I thought it was non-portable :(

Nope, according to ISO/IEC 9899:1999 5.1.2.2.3 and 7.20.4.3#5
it is perfectly portable.

Regards
--
Irrwahn
(ir*******@free net.de)

[Glen Herrmannsfeldt]

"Frank Chow" <fa*******@yaho o.com> wrote in message
news:be******** *************** ***@posting.goo gle.com...

Please see the following code:

/* main.c */

#include <stdio.h>

int main()
{
double d;
scanf("%f", &d);
printf("%g\n", d);
}

The problem is, whatever I input to stdin (even an illegal data), the
program just print a number that seems to be a data from "raw memory",
ie uninitialized memory (for example: "5.35162e-315").

Others have said that %f is wrong for double, which is true.

Note that if scanf() doesn't find valid data to convert to a number it
doesn't store anything. Also, it returns the number of valid conversions,
which you can check.

-- glen

[Martin Ambuhl]

Frank Chow wrote:

Please see the following code:

/* main.c */

#include <stdio.h>

int main()
{
double d;
scanf("%f", &d);

You mean scanf("%lf", &d);


--
Martin Ambuhl

[Frank Chow]

Thank you all. And indeed I should first read the C-faq to avoid such
a naive mistake.