Loss of precision assigning floating point values?

[BigMan]

How can I check if assignment of a float to a double (or vice versa)
will result in loss of precision?

[ulrich]

On Wed, 13 Apr 2005 10:28:11 +0200, Karl Heinz Buchegger
<kb******@gasca d.at> wrote:

BigMan wrote:

1. Does the C++ standard say that assigning a float to a double never
results in loss of precision? If so, where does it say so?

No. How could it.
Assume: sizeof( double ) == 8, sizeof( float ) == 4
How can one pack 8 bytes into 4 without loosing anything?

imho, BigMan meant it the other way round: assign a float to a double...

[Karl Heinz Buchegger]

ulrich wrote:

On Wed, 13 Apr 2005 10:28:11 +0200, Karl Heinz Buchegger
<kb******@gasca d.at> wrote:

BigMan wrote:

1. Does the C++ standard say that assigning a float to a double never
results in loss of precision? If so, where does it say so?

No. How could it.
Assume: sizeof( double ) == 8, sizeof( float ) == 4
How can one pack 8 bytes into 4 without loosing anything?

imho, BigMan meant it the other way round: assign a float to a double...

Sorry. Obviously I didn't pay close attention.

--
Karl Heinz Buchegger
kb******@gascad .at

[Victor Bazarov]

BigMan wrote:

1. Does the C++ standard say that assigning a float to a double never
results in loss of precision? If so, where does it say so?
It does not. Actually, it says the opposite: "An rvalue of type float
can be converted to an rvalue of type double. The value is unchanged."
(4.6/1)
2. Where could I find more info about reducing the error of operations
on FP numbers?

Any book in line with "C++ for Scientists and Engineers". "The Art of
Computer Programming". What Karl recommended. Other good books, many
on CAD or geometry modeling.

V

[Clark S. Cox III]

On 2005-04-13 10:02:23 -0400, Victor Bazarov <v.********@com Acast.net> said:

BigMan wrote:

1. Does the C++ standard say that assigning a float to a double never
results in loss of precision? If so, where does it say so?
It does not.

Really?
Actually, it says the opposite: "An rvalue of type float
can be converted to an rvalue of type double. The value is unchanged."
(4.6/1)

Isn't that exactly what BigMan said? If the value is "unchanged" how
can it be less precise? Wouldn't a loss of precision count as changing
the value?
--
Clark S. Cox, III
cl*******@gmail .com

[Victor Bazarov]

Clark S. Cox III wrote:

On 2005-04-13 10:02:23 -0400, Victor Bazarov <v.********@com Acast.net>
said:

BigMan wrote:

1. Does the C++ standard say that assigning a float to a double never
results in loss of precision? If so, where does it say so?

It does not.

Really?

Actually, it says the opposite: "An rvalue of type float
can be converted to an rvalue of type double. The value is unchanged."
(4.6/1)

Isn't that exactly what BigMan said? If the value is "unchanged" how can
it be less precise? Wouldn't a loss of precision count as changing the
value?

What do you want from me? So, I didn't read the question very carefully.
He got his answer. The Standard does say the assignment never results in
loss of precision, and I quoted it and gave him the paragraph number.

[Clark S. Cox III]

On 2005-04-13 11:24:19 -0400, Victor Bazarov <v.********@com Acast.net> said:

Clark S. Cox III wrote:

On 2005-04-13 10:02:23 -0400, Victor Bazarov <v.********@com Acast.net> said:

BigMan wrote:

1. Does the C++ standard say that assigning a float to a double never
results in loss of precision? If so, where does it say so?
It does not.

Really?

Actually, it says the opposite: "An rvalue of type float
can be converted to an rvalue of type double. The value is unchanged."
(4.6/1)

Isn't that exactly what BigMan said? If the value is "unchanged" how
can it be less precise? Wouldn't a loss of precision count as changing
the value?

What do you want from me? So, I didn't read the question very carefully.
He got his answer. The Standard does say the assignment never results in
loss of precision, and I quoted it and gave him the paragraph number.

No worries, I thought that I had missed something.

--
Clark S. Cox, III
cl*******@gmail .com

[E. Robert Tisdale]

BigMan wrote:

How can I check if assignment of a float to a double (or vice versa)
will result in loss of precision?

You probably mean accuracy.

In the typical implementation,
double precision is more precise that single [float] precision
so conversion from double to float *always* reduces precision
and conversion from float to double *always* increases precision.
Accuracy depends upon the number of significant digits [bits].
If all of the significant digits can be represented accurately
by type float, then accuracy will be maintained in the conversion
from double to float. If conversion from double to float
causes significant digits to be discarded,
the resulting inaccuracy may cause trouble.

You may be concerned about whether the conversion is *exact* or not.
IEEE floating-point specifies signals for inexact arithmetic
and you may be able trap an inexact conversion.

Consult you man pages for signal

SIGNAL(2) Linux Programmer’s Manual SIGNAL(2)
NAME
signal - ANSI C signal handling
SYNOPSIS
#include <signal.h>
typedef void (*sighandler_t) (int);
sighandler_t signal(int signum, sighandler_t handler);

and look for

SIGFPE
Some C++ compilers have options
for trapping inexact floating-point operations
but they probably won't help you much
because most floating-point operations are inexact.