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# reverse bit order

 Is there any easy way to reverse the order of the bits in a byte in C++? (i.e. 00000001 becomes 10000000) Oct 9 '06 #1
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 mi******@gmail.com wrote: Is there any easy way to reverse the order of the bits in a byte in C++? (i.e. 00000001 becomes 10000000) http://www.google.com/search?hl=en&q=reverse+bits+byte V -- Please remove capital 'A's when replying by e-mail I do not respond to top-posted replies, please don't ask Oct 9 '06 #2
 mi******@gmail.com 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[i] = 0; for (int b = 0; b < 8; b++) { BitField[i] <<= 1; BitField[i] |= k & 1; k >>= 1; } } If you want safer code, use a vector instead of an array: vector
 Mike posted: Is there any easy way to reverse the order of the bits in a byte in C++? (i.e. 00000001 becomes 10000000) Here's some code I wrote last month. It's written in C, but you'll get the idea: http://groups.google.ie/group/comp.l...473fb7f?hl=en& -- Frederick Gotham Oct 10 '06 #4
 Martin Steen wrote: > int BitField[256]; for (int i = 0; i < 256; i++) { int k = i; BitField[i] = 0; for (int b = 0; b < 8; b++) { BitField[i] <<= 1; BitField[i] |= k & 1; k >>= 1; } } If you want safer code, use a vector instead of an array: vector
 Martin Steen wrote: mi******@gmail.com wrote: >Is there any easy way to reverse the order of the bits in a byte inC++?(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 -- Please remove capital 'A's when replying by e-mail I do not respond to top-posted replies, please don't ask Oct 10 '06 #6
 mi******@gmail.com 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 #include #include using std::numeric_limits; using std::cout; using std::hex; using std::showbase; using std::internal; using std::setw; template >= (n/2); return reverser_imp::reverse((val >(n/2)) & mask, mask) | (reverser_imp::reverse(val & mask, mask) << (n/2)); } }; template { static inline unsigned reverse(unsigned val, unsigned) { return val; } }; inline unsigned reverse(unsigned val) { return reverser_imp::digits>:: reverse(val, numeric_limits::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 Reference." For more information about this book, see www.petebecker.com/tr1book. Oct 10 '06 #7
 Pete Becker posted: inline unsigned reverse(unsigned val) { return reverser_imp::digits>:: reverse(val, numeric_limits::max()); } In my opinion, it's major overkill to use either of: numeric_limits::digits numeric_limits::max The former can be retrieved from: CHAR_BIT , while the latter can be retrieved from: (char unsigned)-1 or even: UCHAR_MAX -- Frederick Gotham Oct 10 '06 #8
 Frederick Gotham wrote: Pete Becker posted: >inline unsigned reverse(unsigned val) { return reverser_imp::digits>:: reverse(val, numeric_limits::max()); } In my opinion, it's major overkill to use either of: numeric_limits::digits numeric_limits::max The former can be retrieved from: CHAR_BIT , while the latter can be retrieved from: (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 Reference." For more information about this book, see www.petebecker.com/tr1book. Oct 10 '06 #9
 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 Oct 10 '06 #10
 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 Reference." For more information about this book, see www.petebecker.com/tr1book. Oct 10 '06 #11
 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(); } well... not in your header files in anyway. -- Frederick Gotham Oct 10 '06 #12
 Frederick Gotham wrote: Pete Becker posted: >It's far better to write long winded, unmaintainable, compiler-bustingtemplates 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(); } well... not in your header files in anyway. 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 Reference." For more information about this book, see www.petebecker.com/tr1book. Oct 10 '06 #13
 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 Oct 10 '06 #14
 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 is sometimes over my head. Best regards, Tom Oct 10 '06 #15
 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 Oct 10 '06 #16
 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 Reference." For more information about this book, see www.petebecker.com/tr1book. Oct 10 '06 #17
 "Pete Becker" Pete Becker posted: >>It's far better to write long winded, unmaintainable, compiler-bustingtemplates 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 littlemore basic than the CHAR_BIT family of macros. Even if you choose tonever us CHAR_BIT and the like, you'll never be able to write: namespace MyNamespace { int CHAR_BIT(); }well... not in your header files in anyway. 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? Oct 10 '06 #18
 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; what about: #undef CHAR_BIT int CHAR_BIT = 5; or is that Undefined Behavior? Nate Oct 10 '06 #19
 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; what about: #undef CHAR_BIT int CHAR_BIT = 5; or is that Undefined Behavior? Hahaha... it never stops, does it? :) Oct 10 '06 #20
 Jim Langston wrote: "Pete Becker" Frederick Gotham wrote: >>Pete Becker posted: It's far better to write long winded, unmaintainable, compiler-bustingtemplates 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 littlemore basic than the CHAR_BIT family of macros. Even if you choose tonever us CHAR_BIT and the like, you'll never be able to write: namespace MyNamespace { int CHAR_BIT(); }well... not in your header files in anyway. That's because macros are evil. Always use templates. Never use macros. Ifyou can't write it as a template you haven't tried hard enough. So tell me, do you use header guards? Never. Header guards are for wimps. If you can't get your #include directives right, you shouldn't be writing code. -- -- Pete Author of "The Standard C++ Library Extensions: a Tutorial and Reference." For more information about this book, see www.petebecker.com/tr1book. Oct 10 '06 #21

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