问题
While I'm using %g as format specifier in printf(), sometimes it rounds up upto 2 places after decimal point, sometimes upto 3 places , someimes upto 4 places...how it does so?
Actually where we should use %g instead of %f or %e for floating point numbers?
回答1:
The %g format specifier does its rounding just like %f would do, but if %f would result in 4.234000, then %g will omit the trailing zeros and print 4.234.
%g should be used when it makes the most sense in your output format that some numbers are printed as 12345.6, while a slightly bigger number would be printed as 1.235e04.
回答2:
%g automatically 'flips' between using %e and %f depending on the value, in an attempt to display as much information as possible, in the same way as hand-held calculators do.
Also, with %g trailing zeroes and decimal point are not included.
回答3:
For the %f conversion, the “precision” is the number of digits after the decimal point, while for %g it is the number of significant digits.
The default precision is 6 in both cases.
回答4:
From the printf manual:
"The double argument is converted in style f or e (or F or E for G conversions). The precision specifies the number of significant digits. If the precision is missing, 6 digits are given; if the precision is zero, it is treated as 1. Style e is used if the exponent from its conversion is less than -4 or greater than or equal to the precision. Trailing zeros are removed from the fractional part of the result; a decimal point appears only if it is followed by at least one digit."
I don't mean to "RTFM", but you'll probably find what you're looking for in the manual sections on controlling precision and length.
来源：https://stackoverflow.com/questions/3839408/how-g-rounds-up-floating-point-numbers