CODEWITHSUNDEEP
c++bit-manipulation
Russell
Russell

Reputation: 4085

Is there a bitwise trick to increase only zero to one?

In C++, is it possible to achieve the following with a bitwise trick?

(i == 0) ? 1 : i

Assume that i is an unsigned 32-bit integer.

EDIT: This is out of curiosity. The goal is not to optimize and clearly not to improve readability.

Upvotes: 2

Views: 255

Answers (2)

Qaz
Qaz

Reputation: 61900

Until I see real results that say otherwise, I believe your code is much clearer and just as fast, but either of these accomplish the same thing without the conditional:

i + !i

Or

i | !i

If i is 0, !i is 1. Combining 0 with 1 through addition or bitwise OR always gives 1. If i is non-zero, !i is 0. Combining a non-zero value with 0 through addition or bitwise OR always gives that value. Thus, the result is 1 if i is 0 and i if not.

Again, I will be surprised if either of these actually turn out to be better for the compiler.

Upvotes: 8

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726479

On computers with two's complement representation of negative numbers, you can use this formula:

(!(n & -2)) | n

Demo.

Explanation: Bit pattern for -2 has ones in all bit positions except the least significant one. Therefore, the expression n & -2 will be zero only when n is equal to zero or one. The ! will make one from zero and zero from any other number. Hence, zero or one will be OR-ed with one, while any other number will remain intact.

Upvotes: 4

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