Question

I’m trying to implement a data compression idea I’ve had, and since I’m imagining running it against a large corpus of test data, I had thought to code it in C (I mostly have experience in scripting languages like Ruby and Tcl.)

Looking through the O’Reilly ‘cow’ books on C, I realize that I can’t simply index the bits of a simple ‘char’ or ‘int’ type variable as I’d like to to do bitwise comparisons and operators.

Am I correct in this perception? Is it reasonable for me to use an enumerated type for representing a bit (and make an array of these, and writing functions to convert to and from char)? If so, is such a type and functions defined in a standard library already somewhere? Are there other (better?) approaches? Is there some example code somewhere that someone could point me to?

Thanks –

Answer 1

Following on from what Kyle has said, you can use a macro to do the hard work for you.

It is possible.

To set the nth bit, use OR:

x |= (1 << 5); // sets the 6th-from right

To clear a bit, use AND:

x &= ~(1 << 5); // clears 6th-from-right

To flip a bit, use XOR:

x ^= (1 << 5); // flips 6th-from-right

Or…

#define GetBit(var, bit) ((var & (1 << bit)) != 0) // Returns true / false if bit is set #define SetBit(var, bit) (var |= (1 << bit)) #define FlipBit(var, bit) (var ^= (1 << bit)) 

Then you can use it in code like:

int myVar = 0; SetBit(myVar, 5); if (GetBit(myVar, 5)) {   // Do something }