bitwise indexing in C?

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 -

c coding-style bit-manipulation

Edit Report

Theory

There is no C syntax for accessing or setting the n-th bit of a built-in datatype (e.g. a 'char'). However, you can access bits using a logical AND operation, and set bits using a logical OR operation.

As an example, say that you have a variable that holds 1101 and you want to check the 2nd bit from the left. Simply perform a logical AND with 0100:

1101
0100
---- AND
0100

If the result is non-zero, then the 2nd bit must have been set; otherwise is was not set.

If you want to set the 3rd bit from the left, then perform a logical OR with 0010:

1101
0010
---- OR
1111

You can use the C operators && (for AND) and || (for OR) to perform these tasks. You will need to construct the bit access patterns (the 0100 and 0010 in the above examples) yourself. The trick is to remember that the least significant bit (LSB) counts 1s, the next LSB counts 2s, then 4s etc. So, the bit access pattern for the n-th LSB (starting at 0) is simply the value of 2^n. The easiest way to compute this in C is to shift the binary value 0001 (in this four bit example) to the left by the required number of places. As this value is always equal to 1 in unsigned integer-like quantities, this is just '1 << n'

Example

unsigned char myVal = 0x65; /* in hex; this is 01100101 in binary. */

/* Q: is the 3-rd least significant bit set (again, the LSB is the 0th bit)? */
unsigned char pattern = 1;
pattern <<= 3; /* Shift pattern left by three places.*/

if(myVal && (char)(1<<3)) {printf("Yes!\n");} /* Perform the test. */

/* Set the most significant bit. */
myVal |= (char)(1<<7);

This example hasn't been tested, but should serve to illustrate the general idea.

answered 2008-09-15T13:48:54.373

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