-1 in binary

jwagner

Hello all,

When I print out -1 in binary I get 11111111111111111111111111111111, as you can see in the following code.

$x = 0;
echo decbin($x);  //prints 0
echo decbin(~$x);  /prints 11111111111111111111111111111111

I am totally confused. Why are all bits set? As far as I expected only the first bit should have been set, and perhaps also the last one (to indicate the sign).

I guess I'm missing on a major piece of the puzzle.

Thanks to all of you out there.

laserlight

Search the Web for "two's complement".

jwagner

laserlight;10923019 wrote:
Search the Web for "two's complement".

Thanks so much.

So in other words, in negative parlance the more 0s you have, the more negative value you have, and as such a value of 111111111111111111110 is two, because the counting starts at -1 and not at -0, right?

laserlight

jwagner wrote:
So in other words, in negative parlance the more 0s you have, the more negative value you have

Eh, not really. That is like saying that for positive numbers, the more 1s you have the more positive the value, but 0100 is greater than 0011. It is still about place.

jwagner wrote:
because the counting starts at -1 and not at -0, right?

Yes, there is no negative zero in two's complement. However, search the Web for one's complement.

jwagner

laserlight;10923047 wrote:
Eh, not really. That is like saying that for positive numbers, the more 1s you have the more positive the value, but 0100 is greater than 0011. It is still about place.

Of course, yeah.

Anyway, thanks so much man. You are of great help. I really appreciate it.