From: Nobuyoshi Nakada <nobu@...>
Date: 2009-11-03T18:57:30+09:00
Subject: [ruby-core:26491] Re: Add Standard Deviation Function to Math Module

Hi,

At Tue, 3 Nov 2009 11:05:36 +0900,
Daniel Cohen wrote in [ruby-core:26490]:
> I was thinking that a Statistics module would be a good addition to Ruby. If
> you decide that it would be worth creating one, I would be happy to
> contribute.

statistics/extconf.rb and statistics/statistics.c.


require 'mkmf'
create_makefile('statistics')

#include <math.h>
#include <ruby.h>

struct stat_stdev_args {
    double sum, sum2;
    long num;
};

static VALUE
stdev_i(VALUE elem, VALUE arg, int argc, VALUE *argv)
{
    struct stat_stdev_args *a = (void *)arg;
    double x = NUM2DBL(elem);
    a->sum += x;
    a->sum2 += x*x;
    a->num++;
    return Qnil;
}

/*
 *  call-seq:
 *     Statistics.stdev(enum)   =>   double
 *
 *  Computes the standard deviation of the elements in an <i>enum</i>
 *  If they are all numbers.
 */
 
static VALUE
stat_stddev(VALUE obj, VALUE list)
{
    long i;
    struct stat_stdev_args args;
    double mean, stddev;

    args.sum = args.sum2 = 0.0;
    args.num = 0;
    rb_iterate(rb_each, list, stdev_i, (VALUE)&args);
    mean = args.sum / args.num;
    stddev = sqrt(args.sum2 / args.num - mean*mean);
    return DBL2NUM(stddev);
}

void
Init_statistics(void)
{
    VALUE mStatistics = rb_define_module("Statistics");
    rb_define_singleton_method(mStatistics, "stddev", stat_stddev, 1);
}


-- 
Nobu Nakada