float? double?

[Erick->]

hi all...

I've readed some lines about the difference between float and double
data types... but, in the real world, which is the best? when should we
use float or double??

thanks
Erick

[Lew Pitcher]

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Erick-wrote:

hi all...

I've readed some lines about the difference between float and double
data types... but, in the real world, which is the best? when should we
use float or double??

That's a fairly open-ended question.

However, considering that C floatingpoint math is conducted in
double-precision (with conversions to and from single precision only to
store or retrieve values from variables), and that (for certain types
of function) single-precision floatingpoint values ("float") are
promoted to double-precision values ("double") automagically, I'd have
to say that the only benefit float has over double is that float may
take less storage space than double.

For my money, unless space is an issue, it's probably better to stick
with double as the default floatingpoint format for your code.

Just my 2cents worth
- --
Lew Pitcher
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[Walter Roberson]

In article <11************ *********@m79g2 000cwm.googlegr oups.com>,
Erick-<er*********@ya hoo.eswrote:

>I've readed some lines about the difference between float and double
data types... but, in the real world, which is the best? when should we
use float or double??

Which is best, a pickup truck, or a half-tonne truck?
float and double are defined in terms of minimum precision allowed
for each. On any given platform, it is not required that there is
*any* difference between the two: if the float data type meets the
minimum requirements that C imposes on the double data type,
then the two could be exactly the same.

Traditionally, float was faster than double but offered less
precision. double never offers -less- precision than float, but
these days, it is not uncommon to find computers on which double
is as fast (or faster than) float. Also, float never occupies -more-
permanent storage than does double, and sometimes memory size
is the biggest factor (but these days you usually just go out and
buy more memory if you need it.)

There are also still computers which do not implement either float or
double in hardware, so from a speed perspective, sometimes both
are significantly worse than using integer arithmetic instead. But
there are also numerous computers these days on which integer arithmetic
is slower than double -- computers being sold into markets where
(say) 95% of the operations requiring speed are likely to be
floating point operations, so the development resources are spent
primarily on accelarating floating point. Thus, from a speed
perspective, you cannot trust that floating point with be either
slower or faster than integer arithmetic. But there -are- times
when using integer arithmetic can be absolutely crucial for
preserving required accuracy.

All of which is to say, "it depends" ;-)
Size, speed, precision: for any given task, any of them might be
the key factor. Speed is particularily variable: a CPU upgrade
without changing anything else might completely alter the speed factors.
--
If you lie to the compiler, it will get its revenge. -- Henry Spencer

[Walter Roberson]

In article <11************ **********@m79g 2000cwm.googleg roups.com>,
Lew Pitcher <lp******@sympa tico.cawrote:

>However, considering that C floatingpoint math is conducted in
double-precision (with conversions to and from single precision only to
store or retrieve values from variables),

That's incorrect, at least for C89 (I haven't checked C99.)

The "usual arithmetic conversions" choses long double if necessary,
then double if necessary, then:

Otherwise, if either operand has type float, the other operand
is converted to float.

The descriptions of operations such as binary + refer to the
usual arithmetic conversions, but definitely do NOT say that
float is promoted to double for the purposes of the calculation.
--
If you lie to the compiler, it will get its revenge. -- Henry Spencer

[Tim Prince]

Erick-wrote:

hi all...

I've readed some lines about the difference between float and double
data types... but, in the real world, which is the best? when should we
use float or double??

float would be used when better performance is required (faster, or less
storage used). double, on most current platforms, gives more range and
precision, and generally requires less care about numerical issues.

[jmcgill]

Tim Prince wrote:

Erick-wrote:

>hi all...

I've readed some lines about the difference between float and double
data types... but, in the real world, which is the best? when should we
use float or double??

float would be used when better performance is required (faster, or less
storage used).

Be certain that this is true for your platform before making such a
decision. It's entirely possible that your FPU datapath is more
efficient with doubles than with a shorter format. It's also possible
that float and double are the same datatype.

I would suggest the possibility that for a given application, increased
precision or range could conceivably be a liability. And in projects
I've worked on, I've seen routines that were dependent on assumptions
about the internal representation of the data (bad, not my fault, not in
my power to fix).

Anyway, check before assuming that float is more efficient than double.
It's entirely possible that the assumptions are backwards from the
results.

double, on most current platforms, gives more range and
precision, and generally requires less care about numerical issues.

I don't know about "most current platforms" because every time I make an
assumption about that, I'm reminded that what I think is the predominate
platform may be far from correct... I would seriously consider checking
into whether the current ARM chips have double fpu registers, and how
the compilers for that platform define float and double...

Now granted, in my tiny corner of the world, floats are 4 bytes and
doubles are 8 and long doubles 12.
Shoot me if I ever write anything that depends on that. (Wait, no, I
*have to* do that still, don't shoot.)

[jacob navia]

Erick-wrote:

hi all...

I've readed some lines about the difference between float and double
data types... but, in the real world, which is the best? when should we
use float or double??

thanks
Erick

double means DOUBLE PRECISION. Note the word precision here.

When you want more precision, use double precision, and if that
doesn't cut it use long double.

Precision means the number of significant digits you get
for the calculations. float gives you 6 digits precision,
(assuming IEEE 754) double gives you 15, and long double
more than that, using Intel/Amd implementation gives you 18.

If precision is not important (you are only interested
in a rough approximation) float is great.

[jmcgill]

jacob navia wrote:

double means DOUBLE PRECISION. Note the word precision here.

When you want more precision, use double precision, and if that
doesn't cut it use long double.

Precision means the number of significant digits you get
for the calculations. float gives you 6 digits precision,
(assuming IEEE 754) double gives you 15, and long double
more than that, using Intel/Amd implementation gives you 18.

I'm not up to date on the ISO specs, but I don't remember any
requirements like the ones you mention; only that long doubles be at
least as long as doubles, which themselves must be at least as long as
floats, and that there is a minimum range of -10^37 through 10^37.

It may be reasonable to assume IEEE754 on some (very common) platforms,
but is it strictly compliant to do so?

[jacob navia]

jmcgill wrote:

jacob navia wrote:

>double means DOUBLE PRECISION. Note the word precision here.

When you want more precision, use double precision, and if that
doesn't cut it use long double.

Precision means the number of significant digits you get
for the calculations. float gives you 6 digits precision,
(assuming IEEE 754) double gives you 15, and long double
more than that, using Intel/Amd implementation gives you 18.

I'm not up to date on the ISO specs, but I don't remember any
requirements like the ones you mention; only that long doubles be at
least as long as doubles, which themselves must be at least as long as
floats, and that there is a minimum range of -10^37 through 10^37.

It may be reasonable to assume IEEE754 on some (very common) platforms,
but is it strictly compliant to do so?

Do not confuse C99 standard (ISO) and IEEE 754 (floating point)

[jmcgill]

jacob navia wrote:

Do not confuse C99 standard (ISO) and IEEE 754 (floating point)

I wouldn't, or at least, I would be very explicit about my intentions if
doing so. But I've seen comments on this thread today that kind of
scare me. (By "kind of", I mean, I don't really give a damn, since none
of the posters work for me or are my students ;-)