#include <stdio.h>
#include <stdlib.h>
#include <math.h>
double roundToSignificantDigits(double value, int nSignificantDigits)
{
int normalizationDegree = (int)floor(log10(fabs(value))); // or frexp * log2
int nDigitsAfterDotNormalized = nSignificantDigits - 1;
int nDigitsAfterDot = nDigitsAfterDotNormalized - normalizationDegree;
double multiplier = pow(10, nDigitsAfterDot);
value = floor(value * multiplier + 0.49999999999999994) / multiplier;
return value;
}
int main()
{
double d1 = atof("5.50624235984287224709987640380859375e-17");
double d2 = atof("1980704062856.605712890625");
printf("%.20e\n", d1);
printf("%.20e\n", roundToSignificantDigits(d1, 7));
printf("%.20e\n", (float)d1);
printf("%.20e\n", (float)roundToSignificantDigits(d1, 7));
printf("%.20e\n", d2);
printf("%.20e\n", roundToSignificantDigits(d2, 7));
printf("%.20e\n", (float)d2);
printf("%.20e\n", (float)roundToSignificantDigits(d2, 15));
return 0;
}
