# Standard deviation

Using an object to keep state:

``````class StdDevAccumulator(n=0, sum=0, sumofsquares=0) {
method <<(num) {
n += 1
sum += num
sumofsquares += num**2
self
}

method stddev {
sqrt(sumofsquares/n - pow(sum/n, 2))
}

method to_s {
self.stddev.to_s
}
}

var i = 0
var sd = StdDevAccumulator()
[2,4,4,4,5,5,7,9].each {|n|
say "adding #{n}: stddev of #{i+=1} samples is #{sd << n}"
}
``````

#### Output:

``````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
``````

Using static variables:

``````func stddev(x) {
static(num=0, sum=0, sum2=0)
num++
sqrt(
(sum2 += x**2) / num -
(((sum += x) / num)**2)
)
}

%n(2 4 4 4 5 5 7 9).each { say stddev(_) }
``````

#### Output:

``````0
1
0.9428090415820633658677924828064653857143
0.8660254037844386467637231707529361834714
0.9797958971132712392789136298823565567864
1
1.399708424447530341827019471260509366836
2
``````