Implementing Bitwise Operators

In C use have available the not operator (~), then what is the use of implementing it. Or, are you talking about something else. Comments ???

~ is bitwise  NOT a.. while ! is logical NOT ..
the asker wants to implement logical NOT .. using the mentioned operators

you are trying to implement the logical NOT operator using the bitwise operators (<< >> + | & ~). Let me try to solve your problem. You try this simple module

bool NOT(bool x)
{
     x = !x;
     return(x);
}

Let me know if it works.

regards

Operators are implemented inside a compiler. It's part of the language. I don't think C provides facility to implement your own operators.
The only thing you can do is implement functions which mimics operators.

-Narendra

#define NOT(x) x > 0 ? 1 : 0

I totally agree with ssnkumar. Operators cannot be implemented. Thats' why I have provided you a function which mimics the logical NOT operator. Check it out...

I totally agree with ssnkumar. Operators cannot be implemented. Thats' why I have provided you a function which mimics the logical NOT operator. Check it out...

This is Computer Science 101.


Look in your textbook.

See, the thing is that I am trying to implement a function that will implement the logical NOT operator.  For instance, if I have anything other than zero, it produces zero, while a zero input produces 1.  The deal is that I can only use a certain number of operators, and I am stuck.  

~ace

Ok, in C a variable is considered true if even one of its bits is 1. It is false if all its bits are 0. So to create a NOT function we need to check every bit in the variable. Return 1 if all the bits are 0 and return 0 if even one bit is 1.

bool NOT(char x)
{
  char r = 0; // will hold the result
  char one = 1;
  int N = 8; // the size of the variable in bits
 
  // loop through every bit in the variable. test each
  // bit and then right shift it out so that the next
  // bit takes its place. OR it with r so that r
  // will equal 1 even if one bit is set to 1
  for(int i=0; i==N; i++) {
    r = r | (x & one);
    x = x>>1;
  }

  // now x is either 1 (if non zero) and 0 (if zero),
  // therefore we need to switch it, the best way to
  // do this is to xor it with 1. then 1^1 = 0 & 0^1 = 0.
  // because we cant use xor we have to make it:
  // A XOR B = (A & (~B)) | (B & (~A))
  r = (r & (~one)) | (one & (~r));

  return r; // this is now either 1 or 0 and will be
            // the same as the logical !
}

Well, I think that should work and I hope it's what you're after.

Prash

just realised that the function definition is wrong. It should be

char NOT(char x)

Prash

>> I totally agree with ssnkumar. Operators cannot be implemented. Thats' why I have provided you a function which mimics the logical NOT operator.

Ditto, this can be done in C++ by overloading an operator but this CANNOT be done in C by a method other than a function.

Exceter

He isn't trying to overload the operator, just trying to create a function...
How about this:

unsigned char not( unsigned char c ) {
    return (c & 0x01) ^ 0x01;
}

anding c with 1 should result in either 0x00 or 0x01 then if you xor 0x00 with 0x01 you'll get 0x01
conversely if you xor 0x01 with 0x01 you'll get 0x00

I think it should work

Looks like pn206 is you answer. Could have included a test shell to varify it works :)

>>>>>Ditto, this can be done in C++ by overloading an operator but this CANNOT be done in C by a method other than a function.


macro definitions {group of statements} is for that purpose :)

The negation function is very easy:-)
Just XOR the bit you want to negate with 1.
So, if you want to negate the complete 4 bytes of an integer, then XOR it with 0xffffffff. The result is NOT of that integer (the same you get when you use ~ operator with an integer).
Hope this answers your question.
Since this is very easy, I am not giving the code......I think you can do it very easily....:-)

Feedback is appreciated......

-Narendra

>> >>>>>Ditto, this can be done in C++ by overloading an operator but this CANNOT be done in C by a method other than a function.


>> macro definitions {group of statements} is for that purpose :)

Yeah, but a macro isn't an operator. :-)

Exceter

Thanks -

I just needed a hint on how to accomplish it.  Just out of curiosity - how did you know I couldn't use conditionals?  I take it you must have done this assignment.

It is ironic that many of you consider this basic computer science, believe it or not this is a 300 level course.  

Thanks all who helped.


~ace

Knowing the answer is a good hint toward knowing the question.

And having an understanding of the hardware goes a long way toward knowing the answer.  ;)


This was part of the first computer design class that I ever took (albeit 28 years ago).  The necessary math was a prereq and the heart of your question goes to the math.  It's kind of scary that this is now a 300 level class.

Kdo

I agree that this is scary that it is a 300 level class - Computer Architecture.  I don't agree with it - I think that basic programming should be taught before high level programming...it would make more logical sense to go in that order.  

It was just a bunch of puzzles...most likely prereqs to information that comes later...but still...

~ace