log base 2?

Projects Programming
1

I am trying to do a log2(x) but cannot seem to find any library function that does this. Any hints (yes I tried searching with no luck)?

Thanks,
memotick

2

Hint: AVR Libc Home Page

3

A little less digging perhaps?

http://www.nongnu.org/avr-libc/user-manual/group__avr__math.html

4

First principles:

logxn = logy n / logy x

Where y could be, say 10 or e

5

GoForSmoke:
A little less digging perhaps?

Thing is, that's probably the single most valuable skill every programmer should have.

6

Thing is, that's probably the single most valuable skill every programmer should have.

And the second is, whilst digging, not being distracted by... oh look! It's a video of a kitten!

7

GoForSmoke:
A little less digging perhaps?

avr-libc: <math.h>: Mathematics

Thanks everyone, but this was especially helpful. I didn't realize math.h was even on my system until I tried it!

8

And of course you checked that site out to see what else you have and bookmarked it?

There's a link directly to the Libc library page on the Arduino Reference page. It's probably that way so you don't have to hunt for 2 links just to get to the useful info.

9

AWOL:
... not being distracted by... oh look! It's a video of a kitten!

Star Wars according to a 3 year old.

10

AWOL:
First principles:

logxn = logy n / logy x

Exactly, so:

void setup () 
{
  Serial.begin(115200);
  Serial.println (log (65536) / log (2));
  }
void loop () {}

Gives:

16.00

So there are 16 bits in 65536, so to speak.

11

Some interesting bithacks for log2 operations: - Bit Twiddling Hacks -

e.g. Find integer log base 2 of a 32-bit IEEE float : - Bit Twiddling Hacks -