modf function 无法正常工作

Question

I'm trying to store all digits of a decimal number.I have decided to use modf for this purpose.My code segment is;

struct high_precision scan_high(int *j)
{
    int i,a;
    struct high_precision mynum;
    double num1, fracpart, intpart;
    printf("Enter the values> ");
    scanf("%lf", &num1);
    if( num1 < 0 )
        mynum.sign = -1;
    else
        mynum.sign = 1;
    num1 = fabs(num1);
    fracpart = modf(num1, &intpart);
    if ( intpart > 0 && intpart < 10 )
        a = 1;
    while( intpart == 0 ) {
        fracpart *= 10;
        fracpart = modf(fracpart, &intpart);
        a -= 1;
    }
    for(i=0;fracpart > 0 && intpart != 0;i++){
        if( intpart > 0 ){
            mynum.digits[i] = intpart;
        }
        fracpart *= 10;
        fracpart = modf(fracpart, &intpart);
    }
    *j = i;
    mynum.decpt = a;

    return(mynum);
}

But somehow it doesn't work as I want.For instance;

Enter the values> 0.009876
    0.876000 9.000000
    0.760000 8.000000
    0.600000 7.000000

It must stop at this line.But, it is continuing to count;

1.000000 5.000000
1.000000 9.000000
1.000000 9.000000
1.000000 9.000000
1.000000 9.000000
1.000000 9.000000
0.999998 9.000000
0.999977 9.000000
0.999767 9.000000
0.997669 9.000000
0.976694 9.000000
0.766942 9.000000
0.669420 7.000000
0.694198 6.000000
0.941983 6.000000
0.419827 9.000000
0.198267 4.000000
0.982671 1.000000
0.826707 9.000000
0.267069 8.000000
0.000000 2.000000
0.000000 0.000000

Answer 1

That's because floating point numbers aren't stored exactly (eg aren't exactly representable). Here's an example you can use to illustrate this:

#include <stdio.h>

int main() {
  double x = 0.009876;
  printf("%.20lf\n",x);
  return 0;
}

---------- Capture Output ----------
> "c:\windows\system32\cmd.exe" /c c:\temp\temp.exe
0.00987599999999999940

> Terminated with exit code 0.

Answer 2

I think you are not doing things properly, let me tell why: scanf("%lf", &num1); .

Here you are already converting the input to double... so you won't get this "high precision scan" you are trying because it is getting "lost" in that first conversion.

If you want the actual decimal numbers, you should find the . in the ascii string and convert it to and int there.

EDIT I coded a little program, is this what you had in mind?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int split_decimal(const char * in, int * intpart, unsigned int * decimals) {

    char * dot = strchr(in, '.');

    if (!dot) {
        *intpart = atoi(in);
        *decimals = 0;
        return 0;
    }

    *decimals = atoi(dot+1);
    *intpart = atoi(in);

    return 0;
}

int main(int argc, char ** argv) {

    int intpart;
    unsigned int decimals;

    split_decimal("-1.337", &intpart, &decimals);
    printf("%d.%d\n", intpart, decimals);
    split_decimal("50", &intpart, &decimals);
    printf("%d.%d\n", intpart, decimals);

    return 0;
}

Output:

-1.337
50.0