round to the nearest power of 4

[naren2345]

Would this expression round an integer n to the nearest power of 4 ?

((n-1)|3) + 1

[=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=]

[e-mail] wrote:

Would this expression round an integer n to the nearest power of 4 ?

((n-1)|3) + 1

No. Just try it with some simple numbers (1 through 10), then you'll
get an idea of what it's supposed to do (but doesn't always get right).

[Bob Martin]

in 716941 20070129 062444 na*******@yahoo.com wrote:

>Would this expression round an integer n to the nearest power of 4 ?

((n-1)|3) + 1

add 3 then lose 2 low-order bits by shifting (note my assembler background?)

[Richard Bos]

na*******@yahoo.com wrote:

Would this expression round an integer n to the nearest power of 4 ?

((n-1)|3) + 1

No, it rounds to something-else of 4.

Also, for powers, define "nearest": arithmetically, geometrically, or
unusually?

Richard

[David T. Ashley]

<na*******@yahoo.comwrote in message
news:11**********************@l53g2000cwa.googlegr oups.com...

>
Would this expression round an integer n to the nearest power of 4 ?

((n-1)|3) + 1

You need to define what you mean by "power" and by "round" and by "nearest".

Did you actually mean "multiple" (0, 4, 8, 12, 16, etc.)?

Or did you actually mean "power" (1, 4, 16, 64, etc.)?

Looking at your code ...

0 --0
1 --4
2 --4
3 --4
4 --4
5 --8
6 --8

So in some sense it does round to a near muliple (not power!) of 4. But not
the nearest one.

There are an endless variety of code constructs, depending on how efficient
one needs to be. Certainly

switch (n & 3)

comes to mind as a starting point.

Dave.

--
David T. Ashley (dt*@e3ft.com)
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