Ggtcf

I want to round a float to maximum 4 decimals places. It mean 0.333333333 will be 0.3333, but 0.33 is still 0.33

Use the std::round() function

The C++ standard library offers functions for performing rounding. For floats, it is:

float round ( float arg );

this will round arg to the nearest integral value. Now, you want a different decimal resolution. So don't round your value, round your value times 10000, so your singles digit is now the former 0.0001 digit. Or more generally:

float my_round(
 float x,
 int num_decimal_precision_digits) 

 float power_of_10 = std::pow(10, num_decimal_precision_digits);
 return std::round(x * power_of_10) / power_of_10;

Note that there may be accuracy issues, as floating-point computations and representations are only accurate to within a certain number of digits, and in my_round we have at least four sources of such inaccuracy: The power-of-10 calculation, the multiplication, the devision and the actual rounding.

Here is a solution, for example:

float ret = float(round(0.333333333 * 10000)) / 10000)

You can write it as a function. Maybe there would be a better way?

Cast it into a fixed-point type

If you want to have your results rounded, with a fixed number of decimal digits, you're hinting that you don't really need the "floating" aspect of floating point numbers. Well, in this case, cast your value to a type which represents such numbers. Essentially, that would be a (run-time-variable) integer numerator and a compile-time-fixed denominator (which in your case would be 10,000).

There's an old question here on the site about doing fixed-point math:

What's the best way to do fixed-point math?

but I would suggest you consider the CNL library as something recent/popular. Also, several proposals have been made to add fixed-point types to the standard library. I don't know which one is the farthest advance, but have a look at this one: Fixed-Point Real Numbers by John McFarlane.

Back to your specific case: Fixed-point types can typically be constructed from floating-point ones. Just do that.

Assuming you need print rounded number, this is one of a proper solutions:

cout << setprecision(4) << x << 'n';

std::setprecision documentation.

Live demo

Until more details are not provided it is impossible to provide a better answer.

Please note if you are planing to round number x then print it, it will end with big headache, since some corner cases can produce much longer results then expected.