
Boost Users : 
From: Kevin Carroll (kcarroll_at_[hidden])
Date: 20051114 12:04:02
> On Sun, 20051113 at 16:02 0800, Robert Ramey wrote:
>
> > Truth is, I don't even know how one goes about assigning a NaN or a
> +/inf to floating/double variable !!
Not an expert here like you guys, but how about:
#include <limits>
double x_NaN = std::numeric_limits<double>::quiet_NaN();
double y_PlusInfinity = std::numeric_limits<double>::infinity();
double z_MinusInfinity = std::numeric_limits<double>::infinity();
...
Also, here is some concise info on IEEE standard floating point numbers
available from the Internet:
The following notes are reproduced from
http://stevehollasch.com/cgindex/coding/ieeefloat.html
Infinity
The values +infinity and infinity are denoted with an exponent of all
1s and a fraction of all 0s. The sign bit distinguishes between negative
infinity and positive infinity. Being able to denote infinity as a
specific value is useful because it allows operations to continue past
overflow situations. Operations with infinite values are well defined in
IEEE floating point.
Not A Number
The value NaN (Not a Number) is used to represent a value that does not
represent a real number. NaN's are represented by a bit pattern with an
exponent of all 1s and a nonzero fraction. There are two categories of
NaN:
QNaN (Quiet NaN) and SNaN (Signalling NaN).
A QNaN is a NaN with the most significant fraction bit set. QNaN's
propagate freely through most arithmetic operations. These values pop
out of an operation when the result is not mathematically defined.
An SNaN is a NaN with the most significant fraction bit clear. It is
used to signal an exception when used in operations. SNaN's can be handy
to assign to uninitialized variables to trap premature usage.
Semantically, QNaN's denote indeterminate operations, while SNaN's
denote invalid operations.
The definition of NaNs, signed zero and infinity, and denormalized
numbers from "IEEE Standard for Binary FloatingPoint Arithmetic",
ANSI/IEEE Standard 7541985, Institute of Electrical and Electronics
Engineers, August 1985., is reproduced below.
In the following, 'S' stands for the sign bit, 'E' for the exponent, and
'F' for the fractional part. The symbol 'u' stands for an undefined bit
(0 or 1).
Type S (1 bit) E (11 bits) F (52 bits)
signaling NaN u 2047 (max) .0uuuuuu (with at
least one 1 bit)
quiet NaN u 2047 (max) .1uuuuuu
negative infinity 1 2047 (max) .0000000
positive infinity 0 2047 (max) .0000000
negative zero 1 0 .0000000
positive zero 0 0 .0000000
Kevin Carroll
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