在我的一个回答中，我感叹我一直想要某种方法以十进制形式打印浮点值中的所有有效数字，这与问题所要求的方式大致相同。好吧，我终于坐下来写了。它不是很完美，这是打印附加信息的演示代码，但它主要适用于我的测试。如果您（即任何人）想要驱动它进行测试的整个包装程序的副本，请告诉我。
static unsigned int
ilog10(uintmax_t v);
/*
* Note: As presented this demo code prints a whole line including information
* about how the form was arrived with, as well as in certain cases a couple of
* interesting details about the number, such as the number of decimal places,
* and possibley the magnitude of the value and the number of significant
* digits.
*/
void
print_decimal(double d)
{
size_t sigdig;
int dplaces;
double flintmax;
/*
* If we really want to see a plain decimal presentation with all of
* the possible significant digits of precision for a floating point
* number, then we must calculate the correct number of decimal places
* to show with "%.*f" as follows.
*
* This is in lieu of always using either full on scientific notation
* with "%e" (where the presentation is always in decimal format so we
* can directly print the maximum number of significant digits
* supported by the representation, taking into acount the one digit
* represented by by the leading digit)
*
* printf("%1.*e", DBL_DECIMAL_DIG - 1, d)
*
* or using the built-in human-friendly formatting with "%g" (where a
* '*' parameter is used as the number of significant digits to print
* and so we can just print exactly the maximum number supported by the
* representation)
*
* printf("%.*g", DBL_DECIMAL_DIG, d)
*
*
* N.B.: If we want the printed result to again survive a round-trip
* conversion to binary and back, and to be rounded to a human-friendly
* number, then we can only print DBL_DIG significant digits (instead
* of the larger DBL_DECIMAL_DIG digits).
*
* Note: "flintmax" here refers to the largest consecutive integer
* that can be safely stored in a floating point variable without
* losing precision.
*/
#ifdef PRINT_ROUND_TRIP_SAFE
# ifdef DBL_DIG
sigdig = DBL_DIG;
# else
sigdig = ilog10(uipow(FLT_RADIX, DBL_MANT_DIG - 1));
# endif
#else
# ifdef DBL_DECIMAL_DIG
sigdig = DBL_DECIMAL_DIG;
# else
sigdig = (size_t) lrint(ceil(DBL_MANT_DIG * log10((double) FLT_RADIX))) + 1;
# endif
#endif
flintmax = pow((double) FLT_RADIX, (double) DBL_MANT_DIG); /* xxx use uipow() */
if (d == 0.0) {
printf("z = %.*s\n", (int) sigdig + 1, "0.000000000000000000000"); /* 21 */
} else if (fabs(d) >= 0.1 &&
fabs(d) <= flintmax) {
dplaces = (int) (sigdig - (size_t) lrint(ceil(log10(ceil(fabs(d))))));
if (dplaces < 0) {
/* XXX this is likely never less than -1 */
/*
* XXX the last digit is not significant!!! XXX
*
* This should also be printed with sprintf() and edited...
*/
printf("R = %.0f [%d too many significant digits!!!, zero decimal places]\n", d, abs(dplaces));
} else if (dplaces == 0) {
/*
* The decimal fraction here is not significant and
* should always be zero (XXX I've never seen this)
*/
printf("R = %.0f [zero decimal places]\n", d);
} else {
if (fabs(d) == 1.0) {
/*
* This is a special case where the calculation
* is off by one because log10(1.0) is 0, but
* we still have the leading '1' whole digit to
* count as a significant digit.
*/
#if 0
printf("ceil(1.0) = %f, log10(ceil(1.0)) = %f, ceil(log10(ceil(1.0))) = %f\n",
ceil(fabs(d)), log10(ceil(fabs(d))), ceil(log10(ceil(fabs(d)))));
#endif
dplaces--;
}
/* this is really the "useful" range of %f */
printf("r = %.*f [%d decimal places]\n", dplaces, d, dplaces);
}
} else {
if (fabs(d) < 1.0) {
int lz;
lz = abs((int) lrint(floor(log10(fabs(d)))));
/* i.e. add # of leading zeros to the precision */
dplaces = (int) sigdig - 1 + lz;
printf("f = %.*f [%d decimal places]\n", dplaces, d, dplaces);
} else { /* d > flintmax */
size_t n;
size_t i;
char *df;
/*
* hmmmm... the easy way to suppress the "invalid",
* i.e. non-significant digits is to do a string
* replacement of all dgits after the first
* DBL_DECIMAL_DIG to convert them to zeros, and to
* round the least significant digit.
*/
df = malloc((size_t) 1);
n = (size_t) snprintf(df, (size_t) 1, "%.1f", d);
n++; /* for the NUL */
df = realloc(df, n);
(void) snprintf(df, n, "%.1f", d);
if ((n - 2) > sigdig) {
/*
* XXX rounding the integer part here is "hard"
* -- we would have to convert the digits up to
* this point back into a binary format and
* round that value appropriately in order to
* do it correctly.
*/
if (df[sigdig] >= '5' && df[sigdig] <= '9') {
if (df[sigdig - 1] == '9') {
/*
* xxx fixing this is left as
* an exercise to the reader!
*/
printf("F = *** failed to round integer part at the least significant digit!!! ***\n");
free(df);
return;
} else {
df[sigdig - 1]++;
}
}
for (i = sigdig; df[i] != '.'; i++) {
df[i] = '0';
}
} else {
i = n - 1; /* less the NUL */
if (isnan(d) || isinf(d)) {
sigdig = 0; /* "nan" or "inf" */
}
}
printf("F = %.*s. [0 decimal places, %lu digits, %lu digits significant]\n",
(int) i, df, (unsigned long int) i, (unsigned long int) sigdig);
free(df);
}
}
return;
}
static unsigned int
msb(uintmax_t v)
{
unsigned int mb = 0;
while (v >>= 1) { /* unroll for more speed... (see ilog2()) */
mb++;
}
return mb;
}
static unsigned int
ilog10(uintmax_t v)
{
unsigned int r;
static unsigned long long int const PowersOf10[] =
{ 1LLU, 10LLU, 100LLU, 1000LLU, 10000LLU, 100000LLU, 1000000LLU,
10000000LLU, 100000000LLU, 1000000000LLU, 10000000000LLU,
100000000000LLU, 1000000000000LLU, 10000000000000LLU,
100000000000000LLU, 1000000000000000LLU, 10000000000000000LLU,
100000000000000000LLU, 1000000000000000000LLU,
10000000000000000000LLU };
if (!v) {
return ~0U;
}
/*
* By the relationship "log10(v) = log2(v) / log2(10)", we need to
* multiply "log2(v)" by "1 / log2(10)", which is approximately
* 1233/4096, or (1233, followed by a right shift of 12).
*
* Finally, since the result is only an approximation that may be off
* by one, the exact value is found by subtracting "v < PowersOf10[r]"
* from the result.
*/
r = ((msb(v) * 1233) >> 12) + 1;
return r - (v < PowersOf10[r]);
}