# reverse bit order

Discussion in 'C++' started by mike7411@gmail.com, Oct 9, 2006.

1. ### Guest

Is there any easy way to reverse the order of the bits in a byte in
C++?

(i.e. 00000001 becomes 10000000)

, Oct 9, 2006

2. ### Victor BazarovGuest

wrote:
> Is there any easy way to reverse the order of the bits in a byte in
> C++?
>
> (i.e. 00000001 becomes 10000000)

V
--

Victor Bazarov, Oct 9, 2006

3. ### Martin SteenGuest

wrote:
> Is there any easy way to reverse the order of the bits in a byte in
> C++?
>
> (i.e. 00000001 becomes 10000000)
>

No. There is only the hard way
You have to shift the bits one by one.
The best idea is to calculate an array
of 256 values that contain the reversed bits.
Then you can "look up" in that array. Here
is the code to make the array (named "BitField"):

int BitField[256];
for (int i = 0; i < 256; i++)
{
int k = i;
BitField = 0;
for (int b = 0; b < 8; b++)
{
BitField <<= 1;
BitField |= k & 1;
k >>= 1;
}
}

If you want safer code, use a vector instead of an array:
vector<int> BitField(256);

Best regards, Martin

P.S. normally, I don't do other people's homework - SCNR this time

Martin Steen, Oct 10, 2006
4. ### Frederick GothamGuest

Frederick Gotham, Oct 10, 2006
5. ### Pete BeckerGuest

Martin Steen wrote:
>
> int BitField[256];
> for (int i = 0; i < 256; i++)
> {
> int k = i;
> BitField = 0;
> for (int b = 0; b < 8; b++)
> {
> BitField <<= 1;
> BitField |= k & 1;
> k >>= 1;
> }
> }
>
> If you want safer code, use a vector instead of an array:
> vector<int> BitField(256);
>

How is that safer? Either way looks equally correct. If you're worried
about typos, it's far better to use a manifest constant instead of that
hardcoded value of 256.

--

-- Pete

Author of "The Standard C++ Library Extensions: a Tutorial and
www.petebecker.com/tr1book.

Pete Becker, Oct 10, 2006
6. ### Victor BazarovGuest

Martin Steen wrote:
> wrote:
>> Is there any easy way to reverse the order of the bits in a byte in
>> C++?
>>
>> (i.e. 00000001 becomes 10000000)
>>

>
> No. There is only the hard way

If there are only 8 bits in a byte (and nobody said it was the actual
requirement), then a table of 255 values would be the easiest way.

if (mybyte)
mybyte = mytable[mybyte];

Now, generation of the table can be done once, outside of your program.

> [..code not involving a table redacted..]

V
--

Victor Bazarov, Oct 10, 2006
7. ### Pete BeckerGuest

wrote:
> Is there any easy way to reverse the order of the bits in a byte in
> C++?
>
> (i.e. 00000001 becomes 10000000)
>

The one true way is to recognize that reversing any sequence of bits
involves only splitting it into two parts and returning a value whose
upper half is the reverse of the original lower half and whose lower
half is the reverse of the original upper half. Like this:

#include <limits>
#include <iomanip>
#include <iostream>
using std::numeric_limits;
using std::cout; using std::hex; using std::showbase;
using std::internal; using std::setw;

template <unsigned n> struct reverser_imp
{
static inline unsigned reverse(unsigned val, unsigned mask)
{
}
};

template <> struct reverser_imp<1>
{
static inline unsigned reverse(unsigned val, unsigned)
{
return val;
}
};

inline unsigned reverse(unsigned val)
{
return reverser_imp<numeric_limits<unsigned char>::digits>::
reverse(val, numeric_limits<unsigned char>::max());
}

void show_reversed(unsigned val)
{
cout.fill('0');
cout << hex << showbase << internal;
cout << setw(4) << val << ": "
<< setw(4) << reverse(val) << '\n';
}

int main()
{
show_reversed(0x0f);
show_reversed(0x80);
show_reversed(0x40);
show_reversed(0xC0);
return 0;
}

--

-- Pete

Author of "The Standard C++ Library Extensions: a Tutorial and
www.petebecker.com/tr1book.

Pete Becker, Oct 10, 2006
8. ### Frederick GothamGuest

Pete Becker posted:

> inline unsigned reverse(unsigned val)
> {
> return reverser_imp<numeric_limits<unsigned char>::digits>::
> reverse(val, numeric_limits<unsigned char>::max());
> }

In my opinion, it's major overkill to use either of:

numeric_limits<char unsigned>::digits
numeric_limits<char unsigned>::max

CHAR_BIT

(char unsigned)-1

or even:

UCHAR_MAX

--

Frederick Gotham

Frederick Gotham, Oct 10, 2006
9. ### Pete BeckerGuest

Frederick Gotham wrote:
> Pete Becker posted:
>
>> inline unsigned reverse(unsigned val)
>> {
>> return reverser_imp<numeric_limits<unsigned char>::digits>::
>> reverse(val, numeric_limits<unsigned char>::max());
>> }

>
>
> In my opinion, it's major overkill to use either of:
>
> numeric_limits<char unsigned>::digits
> numeric_limits<char unsigned>::max
>
>
> CHAR_BIT
>
>
> (char unsigned)-1
>
> or even:
>
> UCHAR_MAX
>

Haven't you gotten the word? Macros are evil. This is the 21st century.
Quaint C approaches should never be used. Templates, templates,
templates. Always.

--

-- Pete

Author of "The Standard C++ Library Extensions: a Tutorial and
www.petebecker.com/tr1book.

Pete Becker, Oct 10, 2006
10. ### Frederick GothamGuest

Pete Becker posted:

> Haven't you gotten the word? Macros are evil. This is the 21st century.
> Quaint C approaches should never be used. Templates, templates,
> templates. Always.

Exceptions, exceptions, exception -- and not the kind you throw!

C++ has many "warts" (if you wish to call them that) which perpetuate from
its origins in C. We have accepted these warts, documented them, and moved
forward.

When you want to give something a name in C++ (be it a function, an object,
a class), then you don't need to pick a very unique name, because all you
need do is enclose it in a namespace:

namespace MyNamespace { int cout; }

Macros muck this up completely. However, there is a finite list of macros
which the Standard defines, such as:

INT_MAX
UCHAR_MAX
CHAR_BIT

As always, the Standard can take liberties wherever it pleases, and it
chooses to define these macros. If you genuinely perfer the numeric_limits
method, then go ahead. However, I myself find it awfully tedious and
longwinded, and I prefer good ol' CHAR_BIT.

While one should hesitate to define macros (for the obvious reasons),
there's no need to hesitate to use the ones that are already there, and
which will _always_ be there. Never will we be able to write:

int CHAR_BIT = 5;

Also, you might notice that "max" and "min" don't evaluate to a compile-
time constant, which make INT_MAX and the like all the more attractive.

--

Frederick Gotham

Frederick Gotham, Oct 10, 2006
11. ### Pete BeckerGuest

Frederick Gotham wrote:
>
> Macros muck this up completely.

Yup. They're evil. Doesn't matter how you rationalize it.

[rationalizaton snipped]

It's far better to write long winded, unmaintainable, compiler-busting
templates than to resort to macros. That's the C++ way!

--

-- Pete

Author of "The Standard C++ Library Extensions: a Tutorial and
www.petebecker.com/tr1book.

Pete Becker, Oct 10, 2006
12. ### Frederick GothamGuest

Pete Becker posted:

> Frederick Gotham wrote:
>>
>> Macros muck this up completely.

>
> Yup. They're evil. Doesn't matter how you rationalize it.
>
> [rationalizaton snipped]

So _your_ rationalisation is this:

There's a undamaged wheel in a car-wreck, but we can't make use of it
because it's evil for a car to crash.

> It's far better to write long winded, unmaintainable, compiler-busting
> templates than to resort to macros. That's the C++ way!

Indeed it is, but not when it comes to the most basic of things, and there
is little more basic than the CHAR_BIT family of macros. Even if you choose
to never us CHAR_BIT and the like, you'll never be able to write:

namespace MyNamespace { int CHAR_BIT(); }

--

Frederick Gotham

Frederick Gotham, Oct 10, 2006
13. ### Pete BeckerGuest

Frederick Gotham wrote:
> Pete Becker posted:
>
>
>> It's far better to write long winded, unmaintainable, compiler-busting
>> templates than to resort to macros. That's the C++ way!

>
>
> Indeed it is,

Now you've seen the light.

> but not when it comes to the most basic of things, and there
> is little more basic than the CHAR_BIT family of macros. Even if you choose
> to never us CHAR_BIT and the like, you'll never be able to write:
>
> namespace MyNamespace { int CHAR_BIT(); }
>
>

That's because macros are evil. Always use templates. Never use macros.
If you can't write it as a template you haven't tried hard enough.

--

-- Pete

Author of "The Standard C++ Library Extensions: a Tutorial and
www.petebecker.com/tr1book.

Pete Becker, Oct 10, 2006
14. ### Frederick GothamGuest

Pete Becker posted:

> That's because macros are evil. Always use templates. Never use macros.
> If you can't write it as a template you haven't tried hard enough.

This has nothing to do with "Writing Templates" Vs "Writing Macros", and
everything to do with the Standard Library, which provides certain macros.

There's plenty of things you can do with macros which you _can't_ do with
templates.

--

Frederick Gotham

Frederick Gotham, Oct 10, 2006
15. ### Thomas TutoneGuest

Frederick Gotham wrote:

> Pete Becker posted:
>
> > That's because macros are evil. Always use templates. Never use macros.
> > If you can't write it as a template you haven't tried hard enough.

>
>
> This has nothing to do with "Writing Templates" Vs "Writing Macros", and
> everything to do with the Standard Library, which provides certain macros.
>
> There's plenty of things you can do with macros which you _can't_ do with
> templates.

Hey Fred, so that the rest of us can sleep easy, could you please post
something acknowledging that you knew that each of Pete's postings in
this subthread regarding templates vs. macros has been tongue-in-cheek.
Your responses have appeared to take Pete seriously, and perhaps you
have done so just to be funny, but if so I must confess that your humor

Best regards,

Tom

Thomas Tutone, Oct 10, 2006
16. ### Frederick GothamGuest

Thomas Tutone posted:

> Hey Fred, so that the rest of us can sleep easy, could you please post
> something acknowledging that you knew that each of Pete's postings in
> this subthread regarding templates vs. macros has been tongue-in-cheek.
> Your responses have appeared to take Pete seriously, and perhaps you
> have done so just to be funny, but if so I must confess that your humor
> is sometimes over my head.

I had no reason to think Pete was being sarcastic.

Maybe I'm a bit socially retarded today. . . or maybe I'm just used to the
wide variety of people (and therefore attitudes) I encounter on Usenet.

--

Frederick Gotham

Frederick Gotham, Oct 10, 2006
17. ### Pete BeckerGuest

Frederick Gotham wrote:
> Pete Becker posted:
>
>> That's because macros are evil. Always use templates. Never use macros.
>> If you can't write it as a template you haven't tried hard enough.

>
>
> This has nothing to do with "Writing Templates" Vs "Writing Macros", and
> everything to do with the Standard Library, which provides certain macros.

It's about writing code to reverse the bits in a byte. Don't you remember?

>
> There's plenty of things you can do with macros which you _can't_ do with
> templates.
>

If you can't do it with templates you shouldn't be doing it.

--

-- Pete

Author of "The Standard C++ Library Extensions: a Tutorial and
www.petebecker.com/tr1book.

Pete Becker, Oct 10, 2006
18. ### Jim LangstonGuest

"Pete Becker" <> wrote in message
news:...
> Frederick Gotham wrote:
>> Pete Becker posted:
>>
>>
>>> It's far better to write long winded, unmaintainable, compiler-busting
>>> templates than to resort to macros. That's the C++ way!

>>
>>
>> Indeed it is,

>
> Now you've seen the light.
>
>> but not when it comes to the most basic of things, and there is little
>> more basic than the CHAR_BIT family of macros. Even if you choose to
>> never us CHAR_BIT and the like, you'll never be able to write:
>>
>> namespace MyNamespace { int CHAR_BIT(); }
>>
>>

>
> That's because macros are evil. Always use templates. Never use macros. If
> you can't write it as a template you haven't tried hard enough.

So tell me, do you use header guards?

Jim Langston, Oct 10, 2006
19. ### Nate BarneyGuest

Frederick Gotham wrote:
> While one should hesitate to define macros (for the obvious reasons),
> there's no need to hesitate to use the ones that are already there, and
> which will _always_ be there. Never will we be able to write:
>
> int CHAR_BIT = 5;

#undef CHAR_BIT
int CHAR_BIT = 5;

or is that Undefined Behavior?

Nate

Nate Barney, Oct 10, 2006
20. ### Guest

Nate Barney wrote:
> Frederick Gotham wrote:
> > While one should hesitate to define macros (for the obvious reasons),
> > there's no need to hesitate to use the ones that are already there, and
> > which will _always_ be there. Never will we be able to write:
> >
> > int CHAR_BIT = 5;

>
>
> #undef CHAR_BIT
> int CHAR_BIT = 5;
>
> or is that Undefined Behavior?
>

Hahaha... it never stops, does it?

, Oct 10, 2006