Conditional Statement using Bitwise operators

Question

So I see that this question has already been asked, however the answers were a little vague and unhelpful. Okay, I need to implement a c expression using only "& ^ ~ ! + | >> <<"

The expression needs to resemble: a ? b : c

So, from what I've been able to tell, the expression needs to look something like:

return (a & b) | (~a & c)

This works when a = 0, because anding it with b will give zero, and then the or expression will return the right side, (~a & c) which works because ~0 gives all ones, and anding c with all ones returns c.

However, this doesn't work when a > 0. Can someone try to explain why this is, or how to fix it?

Jerry Coffin · Accepted Answer · 2012-04-05 19:15:24Z

21

I would convert a to a boolean using !!a, to get 0 or 1. x = !!a.

Then I'd negate that in two's complement. Since you don't have unary minus available, you use the definition of 2's complement negation: invert the bits, then add one: y = ~x + 1. That will give either all bits clear, or all bits set.

Then I'd and that directly with one variable y & b, its inverse with the other: ~y & c. That will give a 0 for one of the expressions, and the original variable for the other. When we or those together, the zero will have no effect, so we'll get the original variable, unchanged.

answered Apr 5, 2012 at 19:15

Jerry Coffin

494k83 gold badges656 silver badges1.2k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

atb Over a year ago

This is actually perfect. So why does adding one make all bit set or clear? I understand why that needs to happen, but I don't understand how that happens.

Jerry Coffin Over a year ago

If we started with 0, then flipping the bits gives all ones. When we add one, all those turn back to zeros (and the carry gets set, but we ignore it). If we started with 1, flipping the bits gives 111...10. Adding 1 turns that last 0 to a 1, so all the bits are now 1's.

Anton Sergeyev Over a year ago

the y = ~x + 1 part got me confused; I finally figured it works due to integer overflow in case when x = 0, but it's not obvious. The more clear solution for me was to first shift lsb to msb with left shift (00000001 => 10000000) and then copy msb with right shift: y = (x << 31) >> 31

George Skoptsov · Accepted Answer · 2012-04-05 19:09:40Z

3

In other words, you need a to have all bits set to 0, if a is false (i.e. 0), and have all bits set to 1, if a is true (i.e. a > 0).

For the former case, the work is already done for you; for the latter -- try to work out result of the expression ~!1.

answered Apr 5, 2012 at 19:09

George Skoptsov

3,9811 gold badge29 silver badges46 bronze badges

1 Comment

atb Over a year ago

well that's what I thought at first. ~!1 would give all ones which is perfect. However, if I were to do ~!a, and a = 0, then ~!0 would give me 1110, so I'm not sure what to do from here /:

Collectives™ on Stack Overflow

Conditional Statement using Bitwise operators

2 Answers 2

3 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related