网上找来的 mark之
No need for anything fancy. Any platform with a good C99 math library (like the iphone) already has everything you need -- specifically the erfc function, which is a slight variant:
#import <math.h>double cumulativeNormal(double x){return0.5* erfc(-x * M_SQRT1_2);}Note this is for the standard normal distribution (i.e. mean = 0, variance = 1), you'll need to use the appropriate scaling -- so for some mean and variance, it will be:
return0.5* erfc(((mean - x)/sqrt(variance))* M_SQRT1_2);If you don't need double precision, change the doubles to floats, use the erfcf function, and change the literals to single-precision literals.
Remember, Objective-C is an extremely light extension to C. Every C function is an Objective-C function, with no wrapping or other shenanigans required.
摘自http://stackoverflow.com/questions/2785944/cumulative-normal-distribution-function-in-objective-c
P.S. stackoverflow是我最近看的最多的网站……
