9 ms·

How To Write A Calculator in 70 Python Lines

74 points by gklein 14y ago

philfreo 14y ago
How to Write a Calculator in 1 Python Line print input() > 25*4-50 > 50
- edwintorok 14y ago
  It accepts more than just numbers though: >>> print input() "a"+"b" ab
  - Hello71 14y ago
    it's a feature
  - adr_ 14y ago
    try:print(lambda x:eval(x)if all(i in'0123456789.*/+-'for i in x)else'syntax error')(raw_input()) except:print'error'
  - readme 14y ago
    Mathematically, this is wrong.. a + b is not ab.
    - xxbondsxx 14y ago
      It's not symbolic...
      - readme 14y ago
        Of course. I understand that "a" + "b" when taken to mean "concatenate the strings" is "ab" but OP's post was a calculator. It would not have yielded this result.
- BerislavLopac 14y ago
  The. Best. Comment. Ever. :-)
- prezjordan 14y ago
  ...100? Your calculator seems broken :)
  - philfreo 14y ago
    haha thanks, fixed
- waltz 14y ago
  or just 'python'
- erezsh 14y ago
  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.
  - pekk 14y ago
    How to get things done? Use existing tools unless they are not suitable.
- gshubert17 14y ago
  Or, from a bash command line, $ python -c "print input('> ')" > 2+3*5 17
andrewcooke 14y ago
this reminded me of something similar i did (i'm afraid my recursive decent parser is probably more opaque) - differentiating numerical expressions in python. http://www.acooke.org/cute/Differenti0.html http://www.acooke.org/cute/Differenti0.html
ivan_ah 14y ago
Very 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, but I think doing parsing right (and from first principles) is a really cool thing to have. Check out how khan-exercises framework uses to parse math expressions into an AST: https://github.com/Khan/khan-exercises/blob/master/utils/math-model.js https://github.com/Khan/khan-exercises/blob/master/utils/mat...
- erezsh 14y ago
  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!
piqufoh 14y ago
This article seems rather un-Zen >>> import this ... though rules are made to be broken
- erezsh 14y ago
  Why is it un-zen?
  - gbog 14y ago
    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.
    - erezsh 14y ago
      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.
gusgordon 14y ago
I wonder if there's a way to add algebraic equation solving to this, or something like it, easily.
- lutusp 14y ago
  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) Result: [(-b + (-4*a*c + b**2)**(1/2))/(2*a), -(b + (-4*a*c + b**2)**(1/2))/(2*a)]
abecedarius 14y ago
Very nicely done! I think a precedence parser could be even shorter, though I don't seem to have one handy in Python. Pratt parsing is similar, though: https://github.com/darius/sketchbook/blob/master/parsing/pratt.py https://github.com/darius/sketchbook/blob/master/parsing/pra... And here's precedence parsing in C: http://wry.me/~darius/hacks/dcalc.c http://wry.me/~darius/hacks/dcalc.c (I learned this method from Dave Gillespie, author of Gnu Calc.) Finally, using a small parsing library very similar to the scheme in this post: https://github.com/darius/peglet/blob/master/examples/infix.py https://github.com/darius/peglet/blob/master/examples/infix....
coolSCV 14y ago
The calculator seems to miss the point of recursive descent parsing. The parser should follow naturally from the grammar. As an example, here is a 52 line python calculator: https://gist.github.com/ascv/5022712 https://gist.github.com/ascv/5022712 The idea isn't to 1-up the OP but rather to showcase the technique of recursive descent parsing. The wikipedia entry for recursive descent parsing also has a very nice example.
- erezsh 14y ago
  Well, to be fair, your calculator seems to miss the point of being a calculator... > 10/2*2 2.5
  - coolSCV 14y ago
    Clearly a bug. Thanks when I have more time I will fix it. I didn't give much thought to operator precedence, I assumed the user would specify it using parentheses. The idea is what is important though. Recursive descent parsing uses the functions that correspond to types in the grammar to mutually call each other thus parsing the expression recursively in a descending fashion.
  - coolSCV 14y ago
    I decided to simply add to the description, "use parentheses to specify operator precedence."
deleted 14y ago
[deleted]
tiziano88 14y ago
FYI this http://www.imgur.com/5Ge93Xc.png http://www.imgur.com/5Ge93Xc.png is what it looks like on my Nexus 4, perhaps it's worth considering a more mobile-friendly layout ;)
vladoh 14y ago
Here is a similar calculator implemented in 58 languages including in 16 lines of Python code :) http://www.math.bas.bg/bantchev/place/rpn/rpn.impl.html http://www.math.bas.bg/bantchev/place/rpn/rpn.impl.html
- konstruktor 14y ago
  This is an RPN calculator, which means you can basically execute the input using the token stream directly. TFA shows executing a context free language, which requires parsing, explaining the program being an order of magnitude larger.
deleted 14y ago
[deleted]

74 points by gklein 14y ago

philfreo 14y ago

How to Write a Calculator in 1 Python Line print input() > 25*4-50 > 50

edwintorok 14y ago
It accepts more than just numbers though: >>> print input() "a"+"b" ab
- Hello71 14y ago
  it's a feature
- adr_ 14y ago
  try:print(lambda x:eval(x)if all(i in'0123456789.*/+-'for i in x)else'syntax error')(raw_input()) except:print'error'
- readme 14y ago
  Mathematically, this is wrong.. a + b is not ab.
  - xxbondsxx 14y ago
    It's not symbolic...
    - readme 14y ago
      Of course. I understand that "a" + "b" when taken to mean "concatenate the strings" is "ab" but OP's post was a calculator. It would not have yielded this result.
BerislavLopac 14y ago
The. Best. Comment. Ever. :-)
prezjordan 14y ago
...100? Your calculator seems broken :)
- philfreo 14y ago
  haha thanks, fixed
waltz 14y ago
or just 'python'
erezsh 14y ago
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.
- pekk 14y ago
  How to get things done? Use existing tools unless they are not suitable.
gshubert17 14y ago
Or, from a bash command line, $ python -c "print input('> ')" > 2+3*5 17

andrewcooke 14y ago

this reminded me of something similar i did (i'm afraid my recursive decent parser is probably more opaque) - differentiating numerical expressions in python. http://www.acooke.org/cute/Differenti0.html http://www.acooke.org/cute/Differenti0.html

ivan_ah 14y ago

Very 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, but I think doing parsing right (and from first principles) is a really cool thing to have. Check out how khan-exercises framework uses to parse math expressions into an AST: https://github.com/Khan/khan-exercises/blob/master/utils/math-model.js https://github.com/Khan/khan-exercises/blob/master/utils/mat...

erezsh 14y ago
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!

piqufoh 14y ago

This article seems rather un-Zen >>> import this ... though rules are made to be broken

erezsh 14y ago
Why is it un-zen?
- gbog 14y ago
  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.
  - erezsh 14y ago
    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.

gusgordon 14y ago

I wonder if there's a way to add algebraic equation solving to this, or something like it, easily.

lutusp 14y ago
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) Result: [(-b + (-4*a*c + b**2)**(1/2))/(2*a), -(b + (-4*a*c + b**2)**(1/2))/(2*a)]

abecedarius 14y ago

Very nicely done! I think a precedence parser could be even shorter, though I don't seem to have one handy in Python. Pratt parsing is similar, though: https://github.com/darius/sketchbook/blob/master/parsing/pratt.py https://github.com/darius/sketchbook/blob/master/parsing/pra... And here's precedence parsing in C: http://wry.me/~darius/hacks/dcalc.c http://wry.me/~darius/hacks/dcalc.c (I learned this method from Dave Gillespie, author of Gnu Calc.) Finally, using a small parsing library very similar to the scheme in this post: https://github.com/darius/peglet/blob/master/examples/infix.py https://github.com/darius/peglet/blob/master/examples/infix....

coolSCV 14y ago

The calculator seems to miss the point of recursive descent parsing. The parser should follow naturally from the grammar. As an example, here is a 52 line python calculator: https://gist.github.com/ascv/5022712 https://gist.github.com/ascv/5022712 The idea isn't to 1-up the OP but rather to showcase the technique of recursive descent parsing. The wikipedia entry for recursive descent parsing also has a very nice example.

erezsh 14y ago
Well, to be fair, your calculator seems to miss the point of being a calculator... > 10/2*2 2.5
- coolSCV 14y ago
  Clearly a bug. Thanks when I have more time I will fix it. I didn't give much thought to operator precedence, I assumed the user would specify it using parentheses. The idea is what is important though. Recursive descent parsing uses the functions that correspond to types in the grammar to mutually call each other thus parsing the expression recursively in a descending fashion.
- coolSCV 14y ago
  I decided to simply add to the description, "use parentheses to specify operator precedence."

deleted 14y ago

[deleted]

tiziano88 14y ago

FYI this http://www.imgur.com/5Ge93Xc.png http://www.imgur.com/5Ge93Xc.png is what it looks like on my Nexus 4, perhaps it's worth considering a more mobile-friendly layout ;)

vladoh 14y ago

Here is a similar calculator implemented in 58 languages including in 16 lines of Python code :) http://www.math.bas.bg/bantchev/place/rpn/rpn.impl.html http://www.math.bas.bg/bantchev/place/rpn/rpn.impl.html

konstruktor 14y ago
This is an RPN calculator, which means you can basically execute the input using the token stream directly. TFA shows executing a context free language, which requires parsing, explaining the program being an order of magnitude larger.

deleted 14y ago

[deleted]