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C reversing bits - the simplest solution

post #1 of 11
Thread Starter 
I stumbled upon several resources on reversing the bits in a byte in C (for example), and I was shocked to see that among lookup tables and divide and conquer algorithms, noone ever suggested this:
Code:
char reversed = 0;
    for(i=0;i<8;i++)
    {
        reversed |= ((original>>i) & 0b1)<<(7-i);
    }
return reversed;

So here it is, IMO the simples way to reverse bits. Also works for byte, int, long, etc, with almost no modification.
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post #2 of 11
They said simple tongue.gif Lookup tables are often much easier to understand than bitwise, especially if you've not been programming for long, or haven't really used bitwise. But good job!
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post #3 of 11
Thread Starter 
Quote:
Originally Posted by tompsonn View Post

or haven't really used bitwise. But good job!

Well, I figure if you're doing something that requires inverting bits, you ought to learn bitwise anyway biggrin.gif Thanks thumb.gif
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post #4 of 11
Quote:
Originally Posted by ronnin426850 View Post

Well, I figure if you're doing something that requires inverting bits, you ought to learn bitwise anyway biggrin.gif Thanks thumb.gif

I do agree with that point indeed!
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post #5 of 11
Personally I think lookup tables are the best approach. But it has to have every possible number. All of them; including infinity.
post #6 of 11
Quote:
Originally Posted by Plan9 View Post

Personally I think lookup tables are the best approach. But it has to have every possible number. All of them; including infinity.

No it doesn't lol tongue.gif
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post #7 of 11
Quote:
Originally Posted by Plan9 View Post

Personally I think lookup tables are the best approach. But it has to have every possible number. All of them; including infinity.

? what do you mean.
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post #8 of 11
Quote:
Originally Posted by tompsonn View Post

No it doesn't lol tongue.gif

Yes it is. But you need every possible number in the universe. Including:
  • 1
  • 1001
  • 11001001
  • 100100011100010
  • 1010010010100010010101001
  • 1010100101100010001000100101010010010010100101001010010100101001010010101001010101001010101001010100100101010100101001001010010101
  • but not 1101 as that's a bad number.
post #9 of 11
The reason is probably performance. You have to perform a lot of operations for every bit in every byte, and you probably want to do this on a lot of data. Lookup tables are fast and the data set isn't that big.
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post #10 of 11
Thread Starter 
Quote:
Originally Posted by Coma View Post

The reason is probably performance. You have to perform a lot of operations for every bit in every byte, and you probably want to do this on a lot of data. Lookup tables are fast and the data set isn't that big.

If you work with a CPU that gets hogged from bitwise ops (2Mhz?), you probably work with very limited memory too. And CPU cycles can be organized and optimized, where taken memory stays taken biggrin.gif
Anyway, you're making a valid point and it's up to the programmer to decide which solution is best.

That's why solutions are generally labeled "simplest", "easiest" or "fasters", and rarely "best" smile.gif
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