c++ - sprintf %g specifier gives too few digits after point

Question

I'm trying to write floating point vars into my ini file and i encountered a problem with format specifiers. I have a float value, let it be 101.9716. Now i want to write it to my ini file, but the problem is i have another float values, which have less preceision (such as 15.85), and that values are writing to ini file in the same loop. so i do:

sprintf(valLineY, "%g", grade[i].yArr[j]);

All my other variables become nice chars like "20" (if it was 20.00000), "13.85" (if it was 13.850000) and so on. But 101.9716 becomes "101.972" for some reason. Can you please tell me why does this happen and how to make it "101.9716" without ruining my ideology (which is about removing trailing zeroes and unneeded perceisions). Thanks for any help.

Answer 1

Why this happens?

I tested:

double f = 101.9716;
printf("%f
", f);
printf("%e
", f);
printf("%g
", f);

And it output:

101.971600
1.019716e+02 // Notice the exponent +02
101.972

Here's what C standard (N1570 7.21.6.1) says about conversion specifier g:

A double argument representing a floating-point number is converted in style f or e (or in style F or E in the case of a G conversion specifier), depending on the value converted and the precision. Let P equal the precision if nonzero, 6 if the precision is omitted, or 1 if the precision is zero. Then, if a conversion with style E would have an exponent of X:

— if P > X ≥ ?4, the conversion is with style f (or F) and precision P ? (X + 1).

— otherwise, the conversion is with style e (or E) and precision P ? 1.

So given above, P will equal 6, because precision is not specified, and X will equal 2, because it's the exponent on style e.

Formula 6 > 2 >= -4 is thus true, and style f is selected. And precision will then be 6 - (2 + 1) = 3.

How to fix?

Unlike f, style g will strip unnecessary zeroes, even when precision is set.

Finally, unless the # flag is used, any trailing zeros are removed from the fractional portion of the result and the decimal-point character is removed if there is no fractional portion remaining.

So set high enough precision:

printf("%.8g
", f);

prints:

101.9716