Unexpected setprecision behavior

I'm having some cases where setprecision in combination with iostreams
gives some unepected behavior. Consider the following program:

#include <iostream>
#include <iomanip>

int _tmain(int argc, _TCHAR* argv[])
{
float value = 0.063397526741027832;

std::cout << value << std::endl;
std::cout << std::setprecision( 7 ) << value << std::endl;
std::cout << fixed << value << std::endl;
std::cout << std::scientific << value << std::endl;
}

which gives the following output:

0.0633975
0.06339753
0.0633975
6.3397527e-002

The first line makes sense (the default precision is 6), the second
line adds a digit of precision in response to the setprecision(7)
call, the third line calls for a fixed output and as a result seems to
only give 6 digits of precision. The final lines seems to give 8
digits of precision. When using scientific is the "precision" of the
number refer to digits right of the decimal point. But why when using
fixed and a precision of 7, does it only give 6 digits of precision.

Anjo


Anjo

re: Unexpected setprecision behavior

Anjo Gasa wrote:The behaviour is expected. At the moment you setprecision()
the first time, you are neither in fixed, nor in scientific mode.
There, setprecision sets the number of _all_ digits, not those
after the decimal point, AFAIR. Try _first_ going into fixed,
then setprecision().

HTH,
- J.

re: Unexpected setprecision behavior

Anjo Gasa <anjogasa@gmail.comwrote:IIUC when you are using fixed, then the precision tells you the total
number of digits after the decimal point. So, since the precision is 7,
there are 7 digits after the decimal point (you must count the first 0
after the decimal point).

--
Marcus Kwok
Replace 'invalid' with 'net' to reply

re: Unexpected setprecision behavior

On Feb 1, 11:39 am, "Anjo Gasa" <anjog...@gmail.comwrote:
precision for the general format specifies max digits
while for float and scientific notation, it specifies
the max digits after decimal points. See below example

void print_floats()
{
float f1 = 54.325617;
float f2 = 183.3;
float f3 = -1545.545;
float f4 = 3434;
float f5 = 3434.123456789;
float f6 = 12345678.543543543;
float f7 = 0;

// general format : max 6 digits
// 54.3256 183.3 -1545.55 3434 3434.12 1.23457e
+07 0

std::cout << f1 << " " << f2 << " " << f3 << " " << f4
<< " " << f5 << " " << f6 << " " << f7 << "\n";

// setprecision with general format decides max num of digits
// 54.325619 183.3 -1545.545 3434 3434.1235
12345679 0

std::cout.precision(8);
std::cout << f1 << " " << f2 << " " << f3 << " " << f4
<< " " << f5 << " " << f6 << " " << f7 << "\n";

// setprecision with fixed format decides max num of digits after
decimal point
// 54.33 183.30 -1545.55 3434.00 3434.12
12345679.00 0.00

std::cout.setf(std::ios_base::fixed, std::ios_base::floatfield);
std::cout.precision(2);
std::cout << f1 << " " << f2 << " " << f3 << " " << f4
<< " " << f5 << " " << f6 << " " << f7 << "\n";
}