Cumulative standard deviation: Difference between revisions

Using an object to keep state:

<lang ruby>class StdDevAccumulator(n=0, sum=0, sumofsquares=0) {

method <<(num) {

n += 1;

sum += num;

sumofsquares += num**2;

self;

}

method stddev {

Math.sqrt(sumofsquares/n - Math.pow(sum/n, 2));

}

method to_s {

self.stddev.to_s;

}

}

var i = 0;

var sd = StdDevAccumulator.new;

[2,4,4,4,5,5,7,9].each {|n|

say "adding #{n}: stddev of #{i+=1} samples is #{sd << n}"

}</lang>

{{out}}

<pre>

adding 2: stddev of 1 samples is 0

adding 4: stddev of 2 samples is 1

adding 4: stddev of 3 samples is 0.9428090415820633658677924828064653857143

adding 4: stddev of 4 samples is 0.8660254037844386467637231707529361834714

adding 5: stddev of 5 samples is 0.9797958971132712392789136298823565567864

adding 5: stddev of 6 samples is 1

adding 7: stddev of 7 samples is 1.399708424447530341827019471260509366836

adding 9: stddev of 8 samples is 2

</pre>

Using ''static'' variables:

static(num=0, sum=0, sum2=0);