# Converting Float to unsigned short and Back

Discussion in 'C Programming' started by avsrk@mailcity.com, May 16, 2006.

1. ### Guest

Hello Folks ,

General C data types question , more geared up towards embedded folks .

I have a positive float quantity with a fractional part (one decimal
point) which occupies
4 bytes . Now i want to stuff it in to a two byte quantity like
unsigned short and then
reconvert it back to float .
We can use any method like type casting or directly doing low level
operations on the bytes ,
or any other way you can think of ..
The question is , is it possible to get the floating quantity back
including its fractional part .

Ex

float fval = 52.3 ;

unsigned short sval = (unsigned short ) fval ;

float fvalnew = (float) sval ;

will the fvalnew be 52.3 either by the above way( i know it is not we
lose the fractional part) or
any other way .

Thanks
subra

, May 16, 2006

2. ### John DevereuxGuest

writes:

> Hello Folks ,
>
> General C data types question , more geared up towards embedded folks .
>
> I have a positive float quantity with a fractional part (one decimal
> point) which occupies
> 4 bytes . Now i want to stuff it in to a two byte quantity like
> unsigned short and then
> reconvert it back to float .
> We can use any method like type casting or directly doing low level
> operations on the bytes ,
> or any other way you can think of ..
> The question is , is it possible to get the floating quantity back
> including its fractional part .
>
> Ex
>
> float fval = 52.3 ;
>
> unsigned short sval = (unsigned short ) fval ;
>
> float fvalnew = (float) sval ;
>
>
> will the fvalnew be 52.3 either by the above way( i know it is not we
> lose the fractional part) or
> any other way .

Well you could "pre-multiply" by 10, so that you keep the first digit
after the decimal point.

float fval = 52.3 ;

unsigned short sval = (unsigned short) (10*fval) ;

/* sval == 523 */

float fvalnew = (float) sval / 10 ;

--

John Devereux

John Devereux, May 16, 2006

opined:

> Hello Folks ,
>
> General C data types question , more geared up towards embedded folks
> .
>
> I have a positive float quantity with a fractional part (one decimal
> point) which occupies
> 4 bytes . Now i want to stuff it in to a two byte quantity like
> unsigned short and then
> reconvert it back to float .
> We can use any method like type casting or directly doing low level
> operations on the bytes ,
> or any other way you can think of ..
> The question is , is it possible to get the floating quantity back
> including its fractional part .
>
> Ex
>
> float fval = 52.3 ;
>
> unsigned short sval = (unsigned short ) fval ;
>
> float fvalnew = (float) sval ;
>
>
> will the fvalnew be 52.3 either by the above way( i know it is not we
> lose the fractional part) or
> any other way .

It seems what you're really after is fixed point arithmetic. If you use
it, you can get rid of floating point variables altogether. Just
choose a suitable multiplication factor, and use integer arithmetics
throughout. Since you say you only have one decimal place to care
about, 10 sounds OK, but if your range allows, a 100 may be better
especially if you do a lot of calculations.

numbers. GMP/bignum may be one, but you may want something leaner for
embedded use. Depending on what you need it may be better to roll your
own.

--
How do I type "for i in *.dvi do xdvi i done" in a GUI?
(Discussion in comp.os.linux.misc on the intuitiveness of interfaces.)

<http://clc-wiki.net/wiki/Introduction_to_comp.lang.c>

4. ### Flash GordonGuest

wrote:
> Hello Folks ,
>
> General C data types question , more geared up towards embedded folks .
>
> I have a positive float quantity with a fractional part (one decimal
> point) which occupies
> 4 bytes . Now i want to stuff it in to a two byte quantity like
> unsigned short and then
> reconvert it back to float .

When you can fit a quart in to a pint pot you can guarantee to do it.
Until then it depends on the possible range. If 10*float will fit in to
an unsigned short (which also means the float must be positive) you can
do it, but in that case why are you using a float in the first place?

> We can use any method like type casting or directly doing low level
> operations on the bytes ,
> or any other way you can think of ..
> The question is , is it possible to get the floating quantity back
> including its fractional part .
>
> Ex
>
> float fval = 52.3 ;
>
> unsigned short sval = (unsigned short ) fval ;
>
> float fvalnew = (float) sval ;
>
>
> will the fvalnew be 52.3 either by the above way( i know it is not we
> lose the fractional part) or
> any other way .

It would be better if you told us the problem you are actually trying to
solve.
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

Flash Gordon, May 16, 2006
5. ### CBFalconerGuest

wrote:
>

.... snip ...
>
> I have a positive float quantity with a fractional part (one
> decimal point) which occupies 4 bytes . Now i want to stuff it
> in to a two byte quantity like unsigned short and then reconvert
> it back to float . We can use any method like type casting or
> directly doing low level operations on the bytes , or any other
> way you can think of .. The question is , is it possible to get
> the floating quantity back including its fractional part .

If you have a gallon of fuel (4 liters) can you put it in a one
quart (one liter) can, transport it somewhere, and then fill
another gallon can from it? To quote Hercule Poirot, apply the
little gray brain cells.

--
"If you want to post a followup via groups.google.com, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the

CBFalconer, May 16, 2006
6. ### Joe SmithGuest

"Vladimir Oka" <> wrote in message
news:...
> opined:
>
>> Hello Folks ,
>>
>> General C data types question , more geared up towards embedded folks
>> .
>>
>> I have a positive float quantity with a fractional part (one decimal
>> point) which occupies
>> 4 bytes . Now i want to stuff it in to a two byte quantity like
>> unsigned short and then
>> reconvert it back to float .
>> We can use any method like type casting or directly doing low level
>> operations on the bytes ,
>> or any other way you can think of ..
>> The question is , is it possible to get the floating quantity back
>> including its fractional part .
>>
>> Ex
>>
>> float fval = 52.3 ;
>>
>> unsigned short sval = (unsigned short ) fval ;
>>
>> float fvalnew = (float) sval ;
>>
>>
>> will the fvalnew be 52.3 either by the above way( i know it is not we
>> lose the fractional part) or
>> any other way .

>
> It seems what you're really after is fixed point arithmetic. If you use
> it, you can get rid of floating point variables altogether. Just
> choose a suitable multiplication factor, and use integer arithmetics
> throughout. Since you say you only have one decimal place to care
> about, 10 sounds OK, but if your range allows, a 100 may be better
> especially if you do a lot of calculations.
>
> You may even find ready-made libraries to deal with fixed point
> numbers. GMP/bignum may be one, but you may want something leaner for
> embedded use. Depending on what you need it may be better to roll your
> own.

Won't machine epsilon become an issue if he wants to reach too far down on
those floats? joe

Joe Smith, May 17, 2006

Joe Smith wrote:
> "Vladimir Oka" <> wrote in message
> news:...
> > opined:
> >
> >> Hello Folks ,
> >>
> >> General C data types question , more geared up towards embedded folks
> >>
> >> I have a positive float quantity with a fractional part (one decimal
> >> point) which occupies 4 bytes . Now i want to stuff it in to a two byte quantity like
> >> unsigned short and then reconvert it back to float . We can use any method like
> >> type casting or directly doing low level operations on the bytes , or any other way
> >> you can think of ..
> >> The question is , is it possible to get the floating quantity back
> >> including its fractional part .

<snip>

> > It seems what you're really after is fixed point arithmetic. If you use
> > it, you can get rid of floating point variables altogether. Just
> > choose a suitable multiplication factor, and use integer arithmetics
> > throughout. Since you say you only have one decimal place to care
> > about, 10 sounds OK, but if your range allows, a 100 may be better
> > especially if you do a lot of calculations.
> >
> > You may even find ready-made libraries to deal with fixed point
> > numbers. GMP/bignum may be one, but you may want something leaner for
> > embedded use. Depending on what you need it may be better to roll your
> > own.

>
> Won't machine epsilon become an issue if he wants to reach too far down on
> those floats? joe

Well, you win some, you lose some.

With fixed point you have to be even more careful than with floating
point when it comes to precision. If OP really has to squeeze that last
bit of size/performance from an embedded system, it may be worth the
extra (algorithmic) effort.

8. ### Joe SmithGuest

> Joe Smith wrote:
>> "Vladimir Oka" <> wrote in message
>> news:...
>> > opined:
>> >
>> >> Hello Folks ,
>> >>
>> >> General C data types question , more geared up towards embedded folks
>> >>
>> >> I have a positive float quantity with a fractional part (one decimal
>> >> point) which occupies 4 bytes . Now i want to stuff it in to a two
>> >> byte quantity like
>> >> unsigned short and then reconvert it back to float . We can use any
>> >> method like
>> >> type casting or directly doing low level operations on the bytes , or
>> >> any other way
>> >> you can think of ..
>> >> The question is , is it possible to get the floating quantity back
>> >> including its fractional part .

>
> <snip>
>
>> > It seems what you're really after is fixed point arithmetic. If you use
>> > it, you can get rid of floating point variables altogether. Just
>> > choose a suitable multiplication factor, and use integer arithmetics
>> > throughout. Since you say you only have one decimal place to care
>> > about, 10 sounds OK, but if your range allows, a 100 may be better
>> > especially if you do a lot of calculations.
>> >
>> > You may even find ready-made libraries to deal with fixed point
>> > numbers. GMP/bignum may be one, but you may want something leaner for
>> > embedded use. Depending on what you need it may be better to roll your
>> > own.

>>
>> Won't machine epsilon become an issue if he wants to reach too far down
>> on
>> those floats? joe

>
> Well, you win some, you lose some.
>
> With fixed point you have to be even more careful than with floating
> point when it comes to precision. If OP really has to squeeze that last
> bit of size/performance from an embedded system, it may be worth the
> extra (algorithmic) effort.

I hope OP does clarify what he's after in such a manner that survives the
"buckets of water" objection. I rather doubt somebody is paying him to do
something so Sysypheun (sp?) joe

Joe Smith, May 17, 2006

Joe Smith wrote:
> > Joe Smith wrote:
> >> "Vladimir Oka" <> wrote in message
> >> news:...
> >> > opined:
> >> >
> >> >> Hello Folks ,
> >> >>
> >> >> General C data types question , more geared up towards embedded folks
> >> >>
> >> >> I have a positive float quantity with a fractional part (one decimal
> >> >> point) which occupies 4 bytes . Now i want to stuff it in to a two
> >> >> byte quantity like
> >> >> unsigned short and then reconvert it back to float . We can use any
> >> >> method like
> >> >> type casting or directly doing low level operations on the bytes , or
> >> >> any other way
> >> >> you can think of ..
> >> >> The question is , is it possible to get the floating quantity back
> >> >> including its fractional part .

> >
> > <snip>
> >
> >> > It seems what you're really after is fixed point arithmetic. If you use
> >> > it, you can get rid of floating point variables altogether. Just
> >> > choose a suitable multiplication factor, and use integer arithmetics
> >> > throughout. Since you say you only have one decimal place to care
> >> > about, 10 sounds OK, but if your range allows, a 100 may be better
> >> > especially if you do a lot of calculations.
> >> >
> >> > You may even find ready-made libraries to deal with fixed point
> >> > numbers. GMP/bignum may be one, but you may want something leaner for
> >> > embedded use. Depending on what you need it may be better to roll your
> >> > own.
> >>
> >> Won't machine epsilon become an issue if he wants to reach too far down
> >> on
> >> those floats? joe

> >
> > Well, you win some, you lose some.
> >
> > With fixed point you have to be even more careful than with floating
> > point when it comes to precision. If OP really has to squeeze that last
> > bit of size/performance from an embedded system, it may be worth the
> > extra (algorithmic) effort.

> I hope OP does clarify what he's after in such a manner that survives the
> "buckets of water" objection. I rather doubt somebody is paying him to do
> something so Sysypheun (sp?) joe

If OP's sure his values can fit in a 16 bit unsigned value (multiplied
by 10), I'd still advise him to stick to fixed point. It's always
possible to temporarily expand the width during calculations if he's
worried about lack of precision (e.g. use 32 bits with scaling factor
of 1000 or more for all calculations). This strategy may fail if he
needs to do some complicated maths on it (like sine or suchlike).