[Edu-sig] adventures in using Python as a calculator
kirby urner
kirby.urner at gmail.com
Sun Oct 7 17:52:17 EDT 2018
A stereotype some people have, of computers, is they're like giant
calculators on steroids.
Then they find as much "text processing" goes on as "number crunching" -- ala
"regular expressions" -- and forget about the calculator model.
Calculators don't do strings, let alone much in the way of data files. Of
course that line got blurred in the golden age of programmable calculators
(HP65 etc.).
However, the Python docs start with Using Python as a Calculator in Guido's
tutorial.
Perhaps we could do more to fulfill the "on steroids" expectations (that
the computer is a "beefed up" calculator device) by exploring our number
types in more depth, and in particular by exploiting their superhuman
abilities in the extended precision department.
Yes, the int type is awesome, now that it integrates long int (for those
new to Python, these used to be separate), but lets not forget Decimal and
also... gmpy, or actually gmpy2.
I've probably lost even some seasoned Python teachers in jumping to that
relatively obscure third party tool, providing extended precision numeracy
beyond what Decimal (in Standard Library) will do. gmpy has a long
history, predating Python. I'm not the expert in the room.
In particular, it does trig.
Our Python User Group used to get one of the package maintainers from
nearby Mentor Graphics (Wilsonville). He was adding complex numbers to the
gmpy mix, and was in touch with Alex Martelli. I enjoyed our little chats.
Without having all the higher level math it might take to actually prove
some identity, and while starting to learn of those identities nonetheless,
extended precision would seem especially relevant.
"Seriously, I can generate Pi with that summation series?" Lets check.
Just writing the program is a great workout.
Some subscribers may recall a contest here on edu-sig, for Pi Day (3/14),
to generate same to a 1001 places, given one of Ramanujan's convergence
expressions for the algorithm.
https://github.com/4dsolutions/Python5/blob/master/Pi%20Day%20Fun.ipynb
http://mathforum.org/kb/thread.jspa?threadID=2246748
I had an adventure in gmpy2 just recently when my friend David Koski shared
the following:
from math import atan as arctan
Ø = (1 + rt2(5))/2
arctan( Ø ** 1) - arctan( Ø ** -1) == arctan(1/2)
arctan( Ø **-1) - arctan( Ø ** -3) == arctan(1/3)
arctan( Ø **-3) - arctan( Ø ** -5) == arctan(1/7)
arctan( Ø **-5) - arctan( Ø ** -7) == arctan(1/18)
arctan( Ø **-7) - arctan( Ø ** -9) == arctan(1/47)
arctan( Ø **-9) - arctan( Ø **-11) == arctan(1/123)
...
Without offering any kind of algebraic proof, I just wanted to see if the
arithmetic panned out.
These denominators on the right turned out to be a "bisection of the Lucas
numbers" (like Fibonaccis with a different seed). That's a great job for a
Python generator, and only integers are involved.
But am I really confined to verification (not proof) using floating
points? Not at all. At first it seemed that way thought, because whereas
Decimal supports powering and roots, it's not equipped with atan.
Then I remembered gmpy2 and could all of a sudden push the number of
verifying decimals out past 92. Definitely a calculator couldn't do this.
https://github.com/4dsolutions/Python5/blob/master/VerifyArctan.ipynb
Math students the world over, if free of the tyranny of calculators -- or
at least allowed to aid and abet (supplement) with Raspberry Pi etc. -- get
to dabble in these industrial strength algorithms, a part of their
inheritance.
Indeed lets use Python as a calculator sometimes, in accordance with said
Python.org tutorial.
A much stronger calculator, with a bigger screen, more keys, and a lot more
horsepower.
Kirby
PS: I ran into this problem with gmpy2.atan: the above convergence wasn't
verifying beyond like 19 digits. When I switched to gmpy2.atan2 istead,
everything started to work out. The question being, why should atan(x) vs
atan2(y,x) differ except in terms of API? gmpy2.atan2(y,x) computes the
arctan of y/x but in some other way apparently. Food for thought. I did
do some digging in the docs.
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