CODEWITHSUNDEEP
javascriptbit-manipulationbitwise-operatorsbit-shift
Dane
Dane

Reputation: 287

Bitwise operation positive number to negative

Performance and optimisation debates aside.

I'm looking for a way in Javascript to take a number let it be 5 and using bitwise operations change it to -5.

The problem with flipping the bits is that 0 becomes 1 ergo ~5 becomes -6.

Is there a way to do this purely through bitwise ops? I feel that adding the 1 back on would negate the minor improvement that I may or may not gain.

Upvotes: 0

Views: 4370

Answers (3)

Jay
Jay

Reputation: 1309

In case anyone is wondering how you would go back and forth between a negative & positive number (n) without using the + or - signs ...

n = ~n
add = 1
while add: 
     diff = add^n
     carry = (add&n)<<1
     add = carry
     n = diff
print(n)

Upvotes: 0

Guffa
Guffa

Reputation: 700382

The negative number as an integer is the two's complement. To flip the sign using two's complement you do like this:

"To get the two's complement of a binary number, the bits are inverted, or "flipped", by using the bitwise NOT operation; the value of 1 is then added to the resulting value, ignoring the overflow which occurs when taking the two's complement of 0."

In JavaScript that would be:

n = ~n + 1;

If you are looking for performance, that is not likely to be the fastest way. That would most likely be to use the operator specifically designed for that:

n = -n;

Upvotes: 4

exyi
exyi

Reputation: 510

It's not soo much faster. It does not make sense. When you need to negate the number noone can do it better than your CPU. http://jsperf.com/minus-vs-not

Upvotes: 0

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