C++: How to Convert From Float to String Without Rounding, Truncation or Padding? [duplicate]

Question

I am facing a problem and unable to resolve it. Need help from gurus. Here is sample code:-

float f=0.01f;
printf("%f",f);

if we check value in variable during debugging f contains '0.0099999998' value and output of printf is 0.010000.

a. Is there any way that we may force the compiler to assign same values to variable of float type?

b. I want to convert float to string/character array. How is it possible that only and only exactly same value be converted to string/character array. I want to make sure that no zeros are padded, no unwanted values are padded, no changes in digits as in above example.

Answer 1

It is impossible to accurately represent a base 10 decimal number using base 2 values, except for a very small number of values (such as 0.25). To get what you need, you have to switch from the float/double built-in types to some kind of decimal number package.

Answer 2

You could use boost::lexical_cast in this way:

float blah = 0.01;
string w = boost::lexical_cast<string>( blah );

The variable w will contain the text value 0.00999999978. But I can't see when you really need it.

It is preferred to use boost::format to accurately format a float as an string. The following code shows how to do it:

float blah = 0.01;
string w = str( boost::format("%d") % blah ); // w contains exactly "0.01" now