# More fun with floating points

• By coincidence this is on a similar topic to today's CodeSOD. I present to you this rounding algorithm copy-pasted 100s of times through our codebase.

`    roundIt = 1;    sprintf_s(rndx, "%50.12f", givenDoubleValue);    if (rndx[strlen(rndx) - 12] == '0')      if (rndx[strlen(rndx) - 11] == '0')        roundIt = 0;    if (roundIt)    {      rndx[strlen(rndx) - 12] = '0';      rndx[strlen(rndx) - 11] = '0';    }    memset (x, '\0', sizeof x);    strncpy_s(x, rndx, (strlen(rndx) - 10));    givenDoubleValue = atof(x);    if (roundIt)      givenDoubleValue += 1.00;`

• By about the 3rd line of this code, I'd be thinking -- there must be a better way of doing this. Converting a numeric value into a string for further processing is almost always a code smell.

• @Mithious said:

... copy-pasted 100s of times through our codebase.

• So 1.01 rounds to 2?  It is similar to truncating to two decimal places and then running through ceil().

Fun things happen to negative numbers.  -0.99 rounds to 1!

And hopefully you never have large numbers (like 1e50) or it will abort your program.

• @jnz said:

And hopefully you never have large numbers (like 1e50) or it will abort your program.
%f would (should) never return exponential formatted numbers - you need %g for that:

```[pjh@pjh tmp]\$ cat ./format.c
#include <stdio.h>
int main(void){
double d =1e100;
printf("%.12f\n", d);
printf("%.12g\n", d);
return 0;
}
[pjh@pjh tmp]\$ make format
cc     format.c   -o format
[pjh@pjh tmp]\$ ./format
10000000000000000159028911097599180468360808563945281389781327557747838772170381060813469985856815104.000000000000
1e+100
[pjh@pjh tmp]\$```

• @jnz said:

Fun things happen to negative numbers.  -0.99 rounds to 1!

I was wondering if anyone would notice that one.

• I thought you were supposed to always round down and then deposit the extra into your own bank account - after beating the crap out of your copier, of course.

Or am I 'jumping to conclusions'?

• @DrPepper said:

By about the 3rd line of this code, I'd be thinking -- there must be a better way of doing this. Converting a numeric value into a string for further processing is almost always a code smell stench.
FTFY

• @jnz said:

So 1.01 rounds to 2?  It is similar to truncating to two decimal places and then running through ceil().

Actually, it's just like running through ceil(). And then adding 1 if the original number was negative, just for giggles.
@jnz said:
And hopefully you never have large numbers (like 1e50) or it will abort your program.

You can't tell that for sure; it depends on the buffer sizes and we were never told them. (Most likely true though.)

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