This article seems rather un-Zen
>>> import this
... though rules are made to be broken
How To Write A Calculator in 70 Python Lines
11–20 of 34 posts
Re: How To Write A Calculator in 70 Python Lines
#12I wonder if there's a way to add algebraic equation solving to this, or something like it, easily.
Re: How To Write A Calculator in 70 Python Lines
#13Very cool. Thanks for the cool post. I am not sure I got the difference between the LL and LR parser. Is what you have above an LR parser? Also, why did you choose to represent both + and - as "ADD" tokens (and * and / as "MUL") is this to enforce evaluation priority? It would be interesting to see if you can add or ^ as an exponent for this calculator. Maybe you intended this post strictly as as an educational post,…
An LR-parser tries to reduce the input over and over again into rules, eventually ending with the 'start' rule. So a+b+c+d becomes [add]+c+d -> [add]+d -> [add] -> [start]
An LL-parser tries to expand the initial rule into a more complex rule structure, until it matches the input. So to match a+b+c+d it will do [start] -> [add] -> [add] + [num] -> [add] + [num] + [num] -> etc.
What I wrote is an LL-parser, simply because it's much much simpler to write and to understand.
Yes, both ADD and MUL are used for precedence. Since any list of +- or of */ will evaluate correctly if reduced from left to right, I didn't mind grouping them together and making my life easier (and shorter).
It was strictly educational, and also a shtick; a short code hack. If I was to write an actual parser (and I don't think I would ever try to), it would look very different!
Re: How To Write A Calculator in 70 Python Lines
#14This article seems rather un-Zen >>> import this ... though rules are made to be broken
Why is it un-zen?
Re: How To Write A Calculator in 70 Python Lines
#15I wonder if there's a way to add algebraic equation solving to this, or something like it, easily.
Not easily, and not in 70 lines. But there's the Python library "sympy" which can do symbolic algebraic equation solving (and many other things):
from sympy import *
var('x,b,c')
print solve(a*x**2+b*x+c,x)
[(-b + (-4*a*c + b**2)**(1/2))/(2*a), -(b + (-4*a*c + b**2)**(1/2))/(2*a)]Re: How To Write A Calculator in 70 Python Lines
#16How to Write a Calculator in 1 Python Line print input() > 25*4-50 > 50
Q. How to write code?
A. I don't need to; other people will write it for me.
That's fine, I guess. I prefer to understand.
Re: How To Write A Calculator in 70 Python Lines
#17Re: How To Write A Calculator in 70 Python Lines
#18This article seems rather un-Zen >>> import this ... though rules are made to be broken
Why is it un-zen?
I think it is not too unzen in its aim and method but if you ask there many little tricks that I'd say are unzen. First, give all the code a pass of autopep8 comb. Second, avoid things like "lambda (op,num): (num, -num)[...], just use "x if y else z"
Then you wouldn't be too far from Norvig's little educational gems.
Re: How To Write A Calculator in 70 Python Lines
#19Earlier quoted context omitted.
Why is it un-zen?
I think it is not too unzen in its aim and method but if you ask there many little tricks that I'd say are unzen. First, give all the code a pass of autopep8 comb. Second, avoid things like "lambda (op,num): (num, -num)[...], just use "x if y else z" Then you wouldn't be too far from Norvig's little educational gems.
I suppose I got carried away with the lambdas :)
Thank you, you gave me a great compliment. Norvig's short spellchecker was very inspirational for me.