Simple question about: ~

[00704a-Liber]

In article <1620006 at hpcilzb.HP.COM> tedj at hpcilzb.HP.COM (Ted Johnson) writes:
.
.Could someone please explain why the following statements both give the
.same answer?
.
.	short int x, y = 12;
.
.		x = -y -1;
.vs.
.
.	short int x, y = 12;
.
.		x = ~y;	
.
.
.Both ways end up assigning x the value of -13.  K&R say something about
.the  ~ operator taking the one's complement of a number, but I didn't
.follow their explanation...  
.
.P.S. This was on a machine where a short int is 16 bits.

This is because you are working on a 2's complement machine.

To take the negative of a 2's complement number, you
invert all the bits (ie, take the 1's complement of the number which is
using '~'), then you add 1 to the result.

Therefore, on a 2's complement machine, ~y +1 === -y or, as you stated,
~y === -y - 1.

This is a machine dependent feature and is NOT guaranteed to be consistent
under all implementations of C.
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