Code is wrong or? in C

Question

I wrote this code:

float suma;
int centy;
int cele;
printf("Zadaj sumu a ja ti ju napisem a zaroven aj vysklonujem:\n");
scanf("%f",&suma);
cele=(int)suma;
centy= (suma-cele)*100;

switch ((int)suma) {
    case 1:
        printf("%d euro",(int)suma);
        break;
    case 2 ... 4:
        printf("%d eura",(int)suma);
        break;

    default:
        printf("%d eur",(int)suma);
        break;

}
switch (centy) {
    case 1:
        printf(" a %d cent\n",centy);
        break;
    case 2 ... 4:
        printf(" a %d centy\n",centy);
        break;

    default:
        printf(" a %d centov\n",centy);
        break;
}

But when I type 5.56 it will say that I have typed 5.55 in console. What do you think. What should I change? I am newbie so...

You expect `suma-cele` to be 0.56, but it might be 0.5599999999999999. — Bo Persson, Nov 04 '15 at 20:53
The actual value is 0.559999942779541015625. So multiply by 100 and round, e.g. `centy = round( (suma-cele) * 100.0 );` — user3386109, Nov 04 '15 at 20:58
Most important fact when dealing with currencies: **Do not use floating point**. — too honest for this site, Nov 04 '15 at 21:14
@Olaf That's a fine point of view, and I don't disagree with you. I suggest that you post an answer that shows how to input a fixed point number, because that's the part that's difficult for a beginner. — user3386109, Nov 04 '15 at 21:30
@user3386109: Noo time/too lazy. Feel free. But there are already dozens of Q/A here about this subject. They pop up regularily like new "security" laws. — too honest for this site, Nov 04 '15 at 21:33
@Olaf My point was that you shouldn't be downvoting bruceg's perfectly valid answer, unless you're willing to take the time to provide a better answer. — user3386109, Nov 04 '15 at 21:36
@user3386109: Nice idea to require providing a better answer if you DV. However, afaik there is no such rule, yet. And the answer does not cover the actual problem, but provides a temporal workaround (without mentioning it is a workaround) which will eventually create larger problems (if not for this project, but for a potential latter - give a fish vs. teach how to fish). Finally: It would have been better to comment at the answer then. — too honest for this site, Nov 04 '15 at 21:42
Ah, [Case Ranges - Using the GNU Compiler Collection (GCC)](https://gcc.gnu.org/onlinedocs/gcc/Case-Ranges.html#Case-Ranges). — Mr Lister, Nov 05 '15 at 06:46

bruceg · Accepted Answer · 2015-11-04T21:46:28.320

1

You have to correct for rounding errors when using floating point numbers. An easy way that often works is to add 0.5 before converting to an int. So, something like this should work better:

centy= (int) (0.5 + suma * 100.0) - cele * 100;

It's probably better to read the input as a string and parse it into euros and cents using the '.' as a delimiter. But this will get the code working easily. Adding 0.5 often works when you don't need exact precision, but close enough will do.

edited Nov 04 '15 at 21:46

answered Nov 04 '15 at 21:02

bruceg

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Thanks man. It works – RA3SK Nov 04 '15 at 21:10
No. When dealing with currencies or other precise values, never use floating point. Use fixed-point integers instead (e.g. calculate in cents or a fraction thereof). – too honest for this site Nov 04 '15 at 21:17
I agree. It's probably better to read the input as a string and parse it into euros and cents using the '.' as a delimiter, but I wanted to get his code working easily. Adding 0.5 often works when you don't need exact precision, but close enough will do. – bruceg Nov 04 '15 at 21:19
Please add that comment to the answer. I'll remove the DV then. – too honest for this site Nov 04 '15 at 21:45
1

There, I think that's an answer we can all live with, thanks @Olaf. – user3386109 Nov 04 '15 at 21:48
1

Still not happy with it, but removed the DV. Hope my comment above will help. And the question might be closed soon anyway as a dup. – too honest for this site Nov 04 '15 at 21:48

Code is wrong or? in C

1 Answers1