====== Standard deviation ======
{{tag>math}}
===== Description =====
Example how to count standard deviation of list elements

**Source**:  PROLOG str. 190 Wydawnictwo PLJ Warszawa 1991 ISBN 83-85190-63-5
===== Download =====
Program source code: {{standard_deviation.pl}}
===== Listing =====
<code prolog>
sred(L, Sr) :- suma(L, S, 0, I, 0), Sr is S/I.

suma([], Sum, Sum, Ilosc, Ilosc).
suma([H|T], Sum, Sum1, Ilosc, Ilosc1) :- Nilosc is Ilosc + 1,
Nsuma is Sum1 + H, suma(T, Sum, Nsuma, Ilosc, Nilosc).

wari(L, W) :- sred(L, Sr), rozn2(L, H, Sr), suma(H, Suma, 0, Ilosc, 0),
W is Suma/Ilosc.
rozn2([], [], _).
rozn2([H|T], [R|T1], Sr) :- R is (H - Sr)*(H - Sr), rozn2(T, T1, Sr).

odch(L, O) :- wari(L, War), O is sqrt(War).

test :- odch([1, 2, 3, 4, 5], O).</code>
===== Comments =====