[Tutor] toy program to find standard deviation of 2 columns of a sqlite3 database

[Manprit Singh]

Dear Sir,

Actually it started with- how to write an aggregate function in python for
sqlite3 . That i have got a fair idea now,So a task can be done in many
ways, each way has its own merits and demerits. I have got a fair idea of
this too with this example .

Series of mails have taught me a lot of things .

I am thankful to wonderful people like Dennis Lee Beiber,  Peter otten  and
avi.e.gross at gmail.com .

Regards
Manprit Singh

On Tue, Jul 5, 2022 at 1:10 PM Peter Otten <__peter__ at web.de> wrote:

> On 04/07/2022 19:14, Manprit Singh wrote:
> > Dear Sir,
> >
> > Finally I came up with a solution which seems more good to me, rather
> than
> > using the previous approach. In this solution I have used shortcut method
> > for calculating the standard deviation.
> >
> > import sqlite3
> >
> > class StdDev:
> >
> >      def __init__(self):
> >          self.cnt = 0
> >          self.sumx = 0
> >          self.sumsqrx = 0
> >
> >      def step(self, x):
> >          self.cnt += 1
> >          self.sumx += x
> >          self.sumsqrx += x**2
> >
> >      def finalize(self):
> >          return ((self.sumsqrx - self.sumx**2/self.cnt)/self.cnt)**0.5
> >
> > conn = sqlite3.connect(":memory:")
> > cur = conn.cursor()
> > cur.execute("create table table1(X1 int, X2 int)")
> > ls = [(2, 5),
> >        (3, 7),
> >        (4, 2),
> >        (5, 1),
> >        (8, 6)]
> > cur.executemany("insert into table1 values(?, ?)", ls)
> > conn.commit()
> >
> > conn.create_aggregate("stdev", 1, StdDev)
> > std_dev, = cur.execute("select stdev(X1), stdev(X2) from table1")
> > print(std_dev)
> > cur.close()
> > conn.close()
> >
> >
> > gives  output
> >
> > (2.0591260281974, 2.315167380558045)
> >
> > That's all.  This is what I was looking for .So what will be the best
> > solution to this problem ? This one or the previous one posted by me ?
>
> As always -- it depends. I believe the numerical error for the above
> algorithm tends to be much higher than for the one used in the
> statistics module. I'd have to google for the details, though, and I am
> lazy enough to leave that up to you.
>
> > The whole credit goes to Dennis lee bieber & avi.e.gross at gmail.com
>
> I think I mentioned it first ;)
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