Printf only returns zeros

Question

I'm sitting right in front of my computer but I don't have a clue where my mistake is. So here is my code in C:

#include <stdio.h>
#include <math.h>

double f (double x);

int main ()
{
    double x;

    printf("Please type in a decimal number: ");
    scanf("&lf", &x);

    printf("%", f(x));

    return 0;
}

double f (double x)
{
    return (fabs(x) * sin(x) * 10e-2);
}

I read the code multiple times but I'm either too stupid or just not able to find the mistake. Please help me. I would be very glad if you'd help me.

Hopefully, you guys have an idea or a tip for me (even that would be helpful).

Check your compiler, you may need to enable more warnings. `printf("%f", ..` — Jongware, Nov 03 '18 at 15:08
Your format strings are both wrong; Take another look into your C book how they work. — tkausl, Nov 03 '18 at 15:08
Possible duplicate of [Correct format specifier for double in printf](https://stackoverflow.com/questions/4264127/correct-format-specifier-for-double-in-printf) — Jongware, Nov 03 '18 at 15:10
`I'm sitting right in front of my computer but I don't have a clue where my mistake is` Don't simply sit try to look at what compiler is telling. — kiran Biradar, Nov 03 '18 at 15:10
The bit about _"sitting right in front of your computer"_ is entirely redundant - all bugs are written by people sat in front of a computer, yet you sound surprised that sitting there does not resolve the problem. — Clifford, Nov 03 '18 at 15:21
Read [How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) and read *carefully* the documentation of every function you use, notably of [scanf](http://en.cppreference.com/w/c/io/fscanf), [printf](https://en.cppreference.com/w/c/io/fprintf), [sin](https://en.cppreference.com/w/c/numeric/math/sin), [fabs](https://en.cppreference.com/w/c/numeric/math/fabs). Compile with all warnings and debug info, so `gcc -Wall -Wextra -g` with [GCC](http://gcc.gnu.org/) — Basile Starynkevitch, Nov 03 '18 at 15:21

gsamaras · Accepted Answer · 2018-11-03T15:48:11.377

2

Change this:

scanf("&lf", &x);

to this:

scanf("%lf", &x);

and this:

printf("%", f(x));

to this:

printf("%lf", f(x));

Instead of seating in front of the computer, ask for the compiler's help, by compiling with -Wall flag to enable a significant amount of warnings. Then, your posted code should have given you:

prog.c: In function 'main':
prog.c:11:11: warning: too many arguments for format [-Wformat-extra-args]
   11 |     scanf("&lf", &x);
      |           ^~~~~
prog.c:13:13: warning: spurious trailing '%' in format [-Wformat=]
   13 |     printf("%", f(x));
      |             ^
prog.c:13:12: warning: too many arguments for format [-Wformat-extra-args]
   13 |     printf("%", f(x));
      |            ^~~

which would have helped you pinpoint the problem, and start searching the internet, until for example, Reading in double values with scanf in c, would have come around!

PS: I assumed you use GCC compiler. If not, make sure to enable the compilation warnings of your compiler.

edited Nov 03 '18 at 15:48

answered Nov 03 '18 at 15:09

gsamaras

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Thank youuuuuuuuuuuuuuuuuuuuuuuuuu. I didn't see that I wrote &lf instead of %lf and % instead of %lf. :D – Xandi200 Nov 03 '18 at 15:14
Good advice on compiler warnings, but it is not a given that the unspecified compiler in question is gcc (or other for which `-Wall` is a valid option). – Clifford Nov 03 '18 at 15:17
I agree @Clifford, updated! But I think, it's enough for a not so experienced person, for now, at least! – gsamaras Nov 03 '18 at 15:20
@gsamaras Possibly, but equally it would probably confuse a novice using Visual Studio who is not even aware that there is a command line to set options for. In fact until VS2015, VS did not even support format specifier checking. – Clifford Nov 03 '18 at 15:25
The gcc option `-Wall` does not enable "all warnings". It is a good idea to also use `-Wextra`. See http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html – Dipstick Nov 03 '18 at 15:35
1

@Dipstick correct, updated. But again, for a start `-Wall` is very good. – gsamaras Nov 03 '18 at 15:48

Printf only returns zeros

1 Answers1