Help me with a logic equation in C

[leonake]

Message for otuzsubat

Hi otuzsubat!
I saw a message from you about tips and tricks in C programming and I consider you a great programmer.
Could you clarify a question for me?
y*=2.25 means y = (y << 1) + (y >> 2) or y = (y << 1) + (y >> 3)
Think a litlle bit. Maby I wrong.
Thanks a lot. See you next time.

 

[checkmate]

Re: Message for otuzsubat

Do you mean y=2.25y?
The RHS is just (2^1)y + (2^-2)y
If so, then your first statement is correct. It's a common trick to make base-2 multiplication of addition even more efficient. In digital circuits, such a trick is even more widely used since hardware multipliers are costly but shift-registers arent.

 

[checkmate]

Re: Message for otuzsubat

Hi checkmate!

In our case y is the same variable in memory. Shifting y means

y << 1 => y x 2 => y was modified in memory
after that
y >> 3 => y / 8 => y(initial) x 0.25

Good luck

Hi,

could you clarify what language you are using? Because I naturally assumed it to be C/C++.

Or maybe you are assuming instead
y = (y <<= 1) + (y >>= 3)
Then the new y value would be 0.375 of the original y value as the evaluation precedence is from R-to-L.