C bitwise operations relevant bits

Question

Sorry if this is a duplicate, I couldn't find exactly what I want.

I have the current value of a byte:

00110001 A

And I have to write another value:

10001001 B

but in the 2nd byte only some bits are relevant. The relevant bits are the ones with a one in this bit

11000000 C

The final byte will be B on the bits where C==1 and A on the bits where C==0. How can I do that with no if statements?

The answer is D 10110001

if is not a loop, it's kind of a branching statement. Anyway, are you looking for the bitwise AND operator? — user529758
– user529758, Commented Jul 16, 2013 at 14:27
There are no such thing as "if loops". You probably mean "if statements". — Jonathon Reinhart
– Jonathon Reinhart, Commented Jul 16, 2013 at 14:27

Carl Norum · Accepted Answer · 2013-07-16 14:54:28Z

Mask and combine:

finalByte = (B & C) | (A & ~C);

To break down how it works - the result of B & C is a byte containing all of the bits of B where bits of C are set (a normal masking operation). A & ~C yields a byte with all of the bits of A where bits of C are cleared - hence the ~ complement operation. The | combines the two into the final byte you're looking for.

Mike Seymour · Accepted Answer · 2013-07-16 14:28:43Z

(B & C) | (A & ~C)

The first expression keeps only the bits of B where C is set; the second keeps only the bits of A where C is not set; and the logical or combines those two bit-sets to give the result you want.

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