Bit Manipulation: Set a bit to 0 in unknown number

Question

Let's say you have a binary number, 8 bits long. You don't know what it is.

xxxx xxxx

I want to set bit 4 to 0. How do I do this?

If I knew the values of x, I could go

xxxx xxxx AND xxxx 0xxx

But I don't know the values of x. How can I do this without knowing the values?

Thank You.

Answer 1

I must be overtired. You can AND the operation anyway.

1 and 1 = 1,

1 and 0 = 0

x and 0 = 0

x and 1 = x

Cool.

Answer 2

xxxx xxxx AND 1111 0111

The bitwise AND operator here will not "turn on" bits that are "off" on the left; it can only "turn off" bits that are otherwise "on". This will guarantee that bit 4 is "off", regardless of the input.

To elaborate:

1 AND 1 => 1
0 AND 1 => 0
1 AND 0 => 0
0 AND 0 => 0

Thus by setting every bit "on" in the number on the right, you're guaranteeing that those will either "stay on" or "stay off" -- i.e. they won't be changed. But the one that you set to "off" on the right, that one will always be "off", no matter what comes in on the left.

Answer 3

AND 1111 0111

The other bits, the ones you want to leave unchanged will not be altered.