C ++: what is the printf () format specification for "float"?

Question

C ++: what is the printf() format specification for float ? (Visual C ++)

I used to use %g for float and %lg for double .

It looks like the spec has changed, and float is undefined and double is %g .

I have a bit in memory that I print, so casting is not an option.

Is there a way to print float values ​​using printf() ?

Update:

This code was written for unit testing of common C ++ libraries used in the embedded system. Here is what I need to do to get the float to work. The code is in the template function:

 template <typename T,typename TTYP,typename Ttyp,int bits,bool IsSigned> Error testMatrixT() { ... 

Here is the code snippet:

 if (typeid(Ttyp) == typeid(float)) { float64 c = *(float32*)&Tp(row,col); float64 a1 = *(float32*)&Arg1(row,col); float64 a2 = *(float32*)&Arg2(row,col); float64 e = *(float32*)&Exp(row,col); m_b = (c == e); _snprintf(m_acDiag, sizeof(m_acDiag)-1 , "add(Arg1,Arg2): arg1=%g, arg2=%g, Expected=%g, Actual=%g, Result: %s" , a1, a2, e, c, BOOL_PF(m_b)); } else { ... 

Pretty ugly, isn't it? Using float as args gives a bad result. Perhaps due to the use of _snprintf() ? A few years ago I would use %lg and everything would be fine. No more.

Answer 1

double and float use the same format specifiers with printf ( %a , %e , %f and %g ). This is because printf is a variational function. Any float arguments are implicitly advanced before doubling before the call; you cannot actually pass float to printf .

Answer 2

To the question in the title: None / "% f"

To the question in the message body: Yes and no.

printf is a variational function. All float arguments are automatically raised to double s. You print the float by passing it to printf, but the float does not actually reach the printf function.

Answer 3

 printf("float: %f", floatValue); 

or if you want a sample record:

 printf("float: %e", floatValue); 

Answer 4

From: http://www.cplusplus.com/reference/clibrary/cstdio/printf/

% f Decimal floating point 392.65

Answer 5

Others have indicated a format for printing float using printf , but since you are using C ++, my suggestion is to avoid this completely.

Use C ++ streams instead, then you don't have to worry about format specifiers. It is also type safe, but printf is not.

 #include <iostream> int main() { float f = 1.234; std::cout << f << std::endl; } 

If you have an array of bytes containing float, memcpy is in the variable float and use the code above.

Answer 6

%g has always been the correct format for float or double (since float arguments are raised to double for variational functions like printf ).

%lg was added as a synonym for %g (in C99 I don't know when C ++ was or was adopted), probably for better symmetry with the *scanf family.

$Lg for a long double.

You can replace g with f or e above, depending on which output format you want.