[LinuxBIOS] Another dumb C question

[Joseph Smith]

Hello,
I have another newbie C question.
Is this the same as 2^9 ??

value = 9

(2 << (value - 1))

This will give me 512, but will it work no matter what "value" is?
Or does linuxbios already have a "to the power of" global function?

Thanks - Joe

[Peter Stuge]

On Thu, Jul 12, 2007 at 05:00:23AM -0400, Joseph Smith wrote:
> Hello,
> I have another newbie C question.
> Is this the same as 2^9 ??

Note that ^ means XOR in C. Exclusive or. Output 1 when only one
input bit is 1, but not both.

  00100011
^ 10101010
----------
  10001001
> value = 9
>
> (2 << (value - 1))
>
> This will give me 512, but will it work no matter what "value" is?

Yep.
> Or does linuxbios already have a "to the power of" global function?

No pow() in LB and the pow() in libm uses doubles so it's bad for the
task.


Bit shift is fine methinks but I would suggest:

(1 << value)

instead of

(2 << (value - 1))


//Peter

[Joseph Smith]

Quoting Peter Stuge <[e-mail]>:
> > value = 9
> >
> > (2 << (value - 1))

> Bit shift is fine methinks but I would suggest:
>
> (1 << value)
>
> instead of
>
> (2 << (value - 1))
>
> //Peter

Not sure I understand. I am trying to get 2 "to the power of" value.

if value = 9
(1 << value)
This equals 18???? Not 2^9 = 512
where
(2 << (value - 1))
(2 << (9 - 1))
2 << 8 = 512

Thanks - Joe

[Corey Osgood]

Joseph Smith wrote:
> Quoting Peter Stuge <[e-mail]>:
> > > value = 9
> > >
> > > (2 << (value - 1))
> > Bit shift is fine methinks but I would suggest:
> >
> > (1 << value)
> >
> > instead of
> >
> > (2 << (value - 1))
> >
> > //Peter
>
> Not sure I understand. I am trying to get 2 "to the power of" value.
>
> if value = 9
> (1 << value)
> This equals 18???? Not 2^9 = 512
> where
> (2 << (value - 1))
> (2 << (9 - 1))
> 2 << 8 = 512
>
> Thanks - Joe

remember that 2 = (1 << 1). so 2 << (value - 1) = 1 << value. I'm not
sure where you're getting an 18 from, that makes no sense at all (since
18 in either hex or dec has two non-zero bits, so it's impossible to get
that result with just that binary shift).

-Corey

[Joseph Smith]

Quoting Corey Osgood <[e-mail]>:
> Joseph Smith wrote:
> > Quoting Peter Stuge <[e-mail]>:
> > > > value = 9
> > > >
> > > > (2 << (value - 1))
> > > Bit shift is fine methinks but I would suggest:
> > >
> > > (1 << value)
> > >
> > > instead of
> > >
> > > (2 << (value - 1))
> > >
> > > //Peter
> >
> > Not sure I understand. I am trying to get 2 "to the power of" value.
> >
> > if value = 9
> > (1 << value)
> > This equals 18???? Not 2^9 = 512
> > where
> > (2 << (value - 1))
> > (2 << (9 - 1))
> > 2 << 8 = 512
> >
> > Thanks - Joe
>
> remember that 2 = (1 << 1). so 2 << (value - 1) = 1 << value. I'm not
> sure where you're getting an 18 from, that makes no sense at all (since
> 18 in either hex or dec has two non-zero bits, so it's impossible to get
> that result with just that binary shift).
>
> -Corey

Well lets see if value = 9, which is 1001 in binary.
If you bitshift 1001 one position to the left it becomes 10010, correct?
Which is 18 in decimal.

Thanks - Joe

[Joseph Smith]

Quoting Joseph Smith <[e-mail]>:
> Quoting Corey Osgood <[e-mail]>:
> > Joseph Smith wrote:
> > > Quoting Peter Stuge <[e-mail]>:
> > > > > value = 9
> > > > >
> > > > > (2 << (value - 1))
> > > > Bit shift is fine methinks but I would suggest:
> > > >
> > > > (1 << value)
> > > >
> > > > instead of
> > > >
> > > > (2 << (value - 1))
> > > >
> > > > //Peter
> > >
> > > Not sure I understand. I am trying to get 2 "to the power of" value.
> > >
> > > if value = 9
> > > (1 << value)
> > > This equals 18???? Not 2^9 = 512
> > > where
> > > (2 << (value - 1))
> > > (2 << (9 - 1))
> > > 2 << 8 = 512
> > >
> > > Thanks - Joe
> >
> > remember that 2 = (1 << 1). so 2 << (value - 1) = 1 << value. I'm not
> > sure where you're getting an 18 from, that makes no sense at all (since
> > 18 in either hex or dec has two non-zero bits, so it's impossible to get
> > that result with just that binary shift).
> >
> > -Corey
>
> Well lets see if value = 9, which is 1001 in binary.
> If you bitshift 1001 one position to the left it becomes 10010, correct?
> Which is 18 in decimal.
>
> Thanks - Joe

Nevermind I get it. I was looking at it bass ackwards :-)
Sorry, I'm a little tired.


Thanks - Joe