What kind of logarithm functions / methods are available in objective-c / cocoa-touch?

Question

I've tried searching for logarithm + objective-c, but all I get is math test pages from teachers, or explanations what a logarithm is ;)

I've got some measurements that are like 83912.41234 and others are 32.94232. I need to press down this huge spectrum into something between 0 and 100, and that 32.94232 would habe to be at least something bigger than 2, where the 83912.41234 would be something near 100. So I think a logarithm function will be my friend here.

UPDATE: I've came across the math.h file through "Open Quickly" (very nice command in Xcode: SHIFT + CMD + D), and there, big surprise:

extern double log ( double );
extern float logf ( float );

extern double log10 ( double );
extern float log10f ( float );

extern double log2 ( double );
extern float log2f ( float );

extern double log1p ( double );
extern float log1pf ( float );

extern double logb ( double );
extern float logbf ( float );

But: No text, no comments. I'm not such a math-freak. So some description would be good, i.e. what kind of logarithm for what case, how the curve looks like, etc... so any great links are highly appreciated!

Answer 1

Also, getting a logarithm with an arbitrary base:

float logx(float value, float base) 
{
   return log10f(value) / log10f(base);
}

Answer 2

math.h is a standard include. the wikipedia page has some documentation.

Another way to 'squish' the values is to fit your values to a straight line. For you example:

Straight line equation

y = mx + c
y = 'squished' value.
x = The value you want to squish
m = Gradient of the line
c = intercept on the y axis

A quick calculation on your values gives something like:

y = 1.17e-3 x + 1.96

Hope this helps.

Answer 3

How about grabbing a book on C standard library functions?

Otherwise, you can try man pages: man 3 logf, for example