log2

The Lady of Shallots

Hello everyone, I was wondering if there was a way to calculate the log2 of a number. I know there are functions for calculating the ln, log and log10, but don't see one for log2...maybe I'm missing it or there's a way to calc it using one of the other functions? Thanks! "It seems that we're getting to the bottom of the barrel of squirrels." - my boss

Mike Dimmick

Logarithms in different bases can be done using the formula logy x = logz x / logz y So you can use the log or log10 function to produce an approximate log2 function:

double log2(double val)
{
return log(val) / log(2.0);
}

Stability. What an interesting concept. -- Chris Maunder

Navin

Use the Magic Log Forumla: Logb x = Loga x/Loga b A funcion that can get an arbitrary log:

double MyLog(double base, double x)
{
  return log10(x) / log10(base);
}

...
double a = MyLog(2, 16);      // should be 4

Sometimes I feel like I'm a USB printer in a parallel universe.

Navin

Argh, beat me to it! You must have posted that while I was in the process of replying. :-D Sometimes I feel like I'm a USB printer in a parallel universe.

The Lady of Shallots

Perfect! Thanks so much! "It seems that we're getting to the bottom of the barrel of squirrels." - my boss

toxcct

just a curiosity and "personal culture" question... what should we need a Base-2 log for ?? very thank you

TOXCCT >>> GEII power

jmkhael

A whole world of it: Check on google log neperien In breif: Solving mathematical equations Mathematical modeling of the movement of protons Spatial Interpolation, Mechanical modeling Probability and statistics .... Papa while (TRUE) Papa.WillLove ( Bebe ) ;

toxcct

wow woww stop it man, did you read my post ??? i didn't ask for log neperien (which is a Base-e log), but the need for having a log2 one !!!!

TOXCCT >>> GEII power

jmkhael

Sorry for missreading your post log2 is used in music among others for as octave utility and for floating point arithmetics: "By knowing that the exponent and mantissa are both two's compliment values and also knowing that a floating point number is a very close approximation of the same number expressed in log2 form, a quick inversions and square roots can be implimented with simple functions. For example a quick inverse is calculated by negating the exponent and mantissa bits." Papa while (TRUE) Papa.WillLove ( Bebe ) ;

toxcct

powerful ! (paweurfoul :-D:):cool::doh: thnk u vry much

TOXCCT >>> GEII power

David Crow

toxctt wrote: what should we need a Base-2 log for ?? Since computers use base-2 numbers, binary logarithms come in very handy. For example, what is the next highest power of two for the number 79?

int x = 1 + (int) log2(79); // assuming log2() returns a double

If a binary tree has N nodes, what is the minimum number of levels in the tree? log2(13) + 1 yields the answer of 4. http://www.encyclopedia4u.com/b/binary-logarithm.html

"The pointy end goes in the other man." - Antonio Banderas (Zorro, 1998)

toxcct

oh, that's nice ! lol but since it's very useful, how to find such expression ? did you find such by yourself, or with docs ?

TOXCCT >>> GEII power

David Crow

toxcct wrote: oh, that's nice ! lol Are you laughing facetiously? toxcct wrote: how to find such expression ? What expression?

"The pointy end goes in the other man." - Antonio Banderas (Zorro, 1998)

toxcct

DavidCrow wrote: Are you laughing facetiously? no, i promise you i find all these very nice... DavidCrow wrote: What expression? such expression : int x = 1 + (int) log2(79);

TOXCCT >>> GEII power

David Crow

toxcct wrote: such expression : int x = 1 + (int) log2(79); I just made that statement up. A log2() function does not exist in the standard library, so if you needed one, it would have to be created.

"The pointy end goes in the other man." - Antonio Banderas (Zorro, 1998)

toxcct

DavidCrow wrote: log2() function does not exist in the standard library yes, that was implicit and understood. what i want to know is about the entiere expression (int x = 1 + (int) log2(79);) ; did you find it after been documented, or by a thinking of your own ?

TOXCCT >>> GEII power

David Crow

toxcct wrote: did you find it after been documented, or by a thinking of your own ? The latter. Since log2(79) is 6.3, I needed to take the integer part of that and add one, yielding 7. Thus 27 is 128, the next highest power of 2 for the number 79.

"The pointy end goes in the other man." - Antonio Banderas (Zorro, 1998)

Antti Keskinen

This expression is a solution to the problem DavidCrow presented: "-- What is the next highest power of two for the number 79?" The expression first evaluates the power to which 2 must be raised to reach 79, then adds 1 to this. It all bases on the way logarithms are defined: "Base N logarithm from number Y will result in the power to which N must be raised to result in Y". Most obviously, he just made up a quick problem, and solved it, to give an example. So, he thought it on his own, made it up. -Antti Keskinen ---------------------------------------------- The definition of impossible is strictly dependant on what we think is possible.

Robert A T Kaldy

Aaaarrrrgggghhh!! you really want to compute binary logarithm of integers with float logarithm function? I think that integer approach would be much much more quicker:

int log2(int x)
{
int lg;
// if (x <= 0) throw something...
for(lg = -1; x != 0; x /= 2, lg++);
return lg;
}

Robert-Antonio "Science is a differerntial equation. Religion is a boundary condition."

David Crow

Robert A. T. Káldy wrote: ...with float logarithm function The log2() function I demonstrated was theoretical. Were it to be implemented, ints would surely be used over doubles. Robert A. T. Káldy wrote: I think... But without actual metrics, we don't really know.

"The pointy end goes in the other man." - Antonio Banderas (Zorro, 1998)