Swap digits in a double in C

Question

What I want to do in C is swap two digits in a double.

For example, if the input is 54321.987 and I want to swap the 2nd with the 4th digit, the output should be 52341.987.

Example when too small: 12.34 would output 1002.34.

Answer 1

Using stringification approach:
There are more elegant ways, but you can see the steps (and improve on) this pseudo code to stringify, move values, and convert back to number.

char buf1[20];
char buf2[20];
char *dummy;
double val = 54321.987;

sprintf(buf1, "%9.3f", val );
//Now the number is in string form: "54321.987".  Just move the two elements  
buf2[0]=buf1[0];
buf2[1]=buf1[3];
buf2[2]=buf1[2];
buf2[3]=buf1[1]; //and so on
//now convert back:

val = strtod(buf2, &dummy);
printf("%9.3f\n", val);

Or, a function could be used to do essentially the same thing: (still stringification)

double swap_num_char(double num, int precision, int c1, int c2); //zero index for c1 and c2


int main(void)
{

    double val = 54321.987;

    printf("%9.3f\n", swap_num_char(val, 3, 1, 3));

    return 0;   
}

double swap_num_char(double num, int precision, int c1, int c2)
{
    char buf[25]; 
    char format[10];
    char *dummy;
    char c;

    sprintf(format, "%s0.%df", "%", precision);

    sprintf(buf, format, num);

    c = buf[c1];
    buf[c1] = buf[c2];
    buf[c2] = c;

    num = strtod(buf, &dummy);

    return num;
}