[Coco] RE: Learning MW C (REALLY!)

[James Jones]

Ries, Rich [S&FS] wrote:
> My other problem was remembering if | was logical or bitwise OR. The
> same for && and ^. I finally wrote a header file that I can #include in
> my files. (Drives everyone else crazy, but...) Here it is:
> 
> [code]
> /* Logical operators */
>     #define L_AND &&
>     #define L_OR  ||
>     #define L_NOT !
> 
> /* Bitwise operators */
>     #define b_and &
>     #define b_or |
>     #define b_xor ^
>     #define b_not ~

To people thinking of learning C: I urge you not to use such macros. 
Eventually, you'll find yourself working on someone else's code who 
doesn't use them, so you'll have to learn the operators eventually, and 
conversely, someone else will have to work on your code, and the macros 
that allegedly make the code clearer for you will be immediately ripped 
out, and the other person will be much harder to work with afterwards.

The "Boolean" operators are &&, ||, and !. && and || are also 
short-circuit; it's guaranteed that

     x && y    behaves exactly like (x != 0) ? (y != 0) : 0
     x || y    behaves exactly like (x != 0) ? 1 : (y != 0)

i.e. if the value can be determined just from x, y isn't evaluated.

^ is exclusive or, and because you always have to evaluate both 
operands, there's no "short-circuit" version--there can't be. &, |, and 
~ are bitwise and, or, and not respectively.

> /*
>  * Bit operations -
>  *   Note there is no limit to how many bits are in a byte.
>  *
>  *           tobit(20000) will work....
>  *
>  */
>     #define tobit(x) (1 << ((x)))

That's not correct. 1 has type int, so that

long
setbit(ell, bitno)
long ell;
int bitno;
{
     return ell | tobit(bitno);
}

will not work if bitno > 8 * sizeof(int). Strictly speaking, for such 
values, the result is undefined; a standard-conforming compiler could 
insert code to phone out for anchovy pizza or start the self-destruct 
sequence.

	James