| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

C-ORM: docs, API.

Last 100 entries

Not The Onion: Google Fireside Chat w Kissinger; Bicycle Wheels, Inertia, and Energy; Another Tax Fraud; Google's Borg; A Verion That Redirects To Local HTTP Server; Spanish Accents For Idiots; Aluminium Cans; Advice on Spray Painting; Female View of Online Chat From a Male; UX Reading List; S4 Subgroups - Geometric Interpretation; Fucking Email; The SQM Affair For Idiots; Using Kolmogorov Complexity; Oblique Strategies in bash; Curses Tools; Markov Chain Monte Carlo Without all the Bullshit; Email Para Matias Godoy Mercado; The Penta Affair For Idiots; Example Code To Create numpy Array in C; Good Article on Bias in Graphic Design (NYTimes); Do You Backup github?; Data Mining Books; SimpleDateFormat should be synchronized; British Words; Chinese Govt Intercepts External Web To DDOS github; Numbering Permutations; Teenage Engineering - Low Price Synths; GCHQ Can Do Whatever It Wants; Dublinesque; A Cryptographic SAT Solver; Security Challenges; Word Lists for Crosswords; 3D Printing and Speaker Design; Searchable Snowden Archive; XCode Backdoored; Derived Apps Have Malware (CIA); Rowhammer - Hacking Software Via Hardware (DRAM) Bugs; Immutable SQL Database (Kinda); Tor GPS Tracker; That PyCon Dongle Mess...; ASCII Fluid Dynamics; Brandalism; Table of Shifter, Cassette and Derailleur Compatability; Lenovo Demonstrates How Bad HTTPS Is; Telegraph Owned by HSBC; Smaptop - Sunrise (Music); Equation Group (NSA); UK Torture in NI; And - A Natural Extension To Regexps; This Is The Future Of Religion; The Shazam (Music Matching) Algorithm; Tributes To Lesbian Community From AIDS Survivors; Nice Rust Summary; List of Good Fiction Books; Constructing JSON From Postgres (Part 2); Constructing JSON From Postgres (Part 1); Postgres in Docker; Why Poor Places Are More Diverse; Smart Writing on Graceland; Satire in France; Free Speech in France; MTB Cornering - Where Should We Point Our Thrusters?; Secure Secure Shell; Java Generics over Primitives; 2014 (Charlie Brooker); How I am 7; Neural Nets Applied to Go; Programming, Business, Social Contracts; Distributed Systems for Fun and Profit; XML and Scheme; Internet Radio Stations (Curated List); Solid Data About Placebos; Half of Americans Think Climate Change Is a Sign of the Apocalypse; Saturday Surf Sessions With Juvenile Delinquents; Ssh, tty, stdout and stderr; Feathers falling in a vacuum; Santiago 30m Bike Route; Mapa de Ciclovias en Santiago; How Unreliable is UDP?; SE Santiago 20m Bike Route; Cameron's Rap; Configuring libxml with Eclipse; Reducing Combinatorial Complexity With Occam - AI; Sentidos Comunes (Chilean Online Magazine); Hilary Mantel: The Assassination of Margaret Thatcher - August 6th 1983; NSA Interceptng Gmail During Delivery; General IIR Filters; What's happening with Scala?; Interesting (But Largely Illegible) Typeface; Retiring Essentialism; Poorest in UK, Poorest in N Europe; I Want To Be A Redneck!; Reverse Racism; The Lost Art Of Nomography; IBM Data Center (Photo); Interesting Account Of Gamma Hack; The Most Interesting Audiophile In The World; How did the first world war actually end?; Ky - Restaurant Santiago; The Black Dork Lives!; The UN Requires Unaninmous Decisions

© 2006-2015 Andrew Cooke (site) / post authors (content).

Cute Piece of Clojure Code

From: andrew cooke <andrew@...>

Date: Sun, 15 Apr 2012 22:35:58 -0300

This is related to the SO question http://stackoverflow.com/questions/10161971
which is playing around with those puzzles where you need to make some number
form a series of digits and various mathematical operators.

The problem being that the problem assumes infix maths, which is a pain if
you're working with a prefix language like lisp.

So what this code does is take two lists - one of operators, and one of
digits, and generates an ast that evaluates correctly, given the precedence of
the operators.

(def default-priorities
  {'+ 1, '- 1, '* 2, '/ 2, '& 3})

(defn- extend-tree [tree priorities operator value]
  (if (seq? tree)
    (let [[op left right] tree
          [old new] (map priorities [op operator])]
      (if (> new old)
        (list op left (extend-tree right priorities operator value))
        (list operator tree value)))
    (list operator tree value)))

(defn priority-tree
  ([operators values] (priority-tree operators values default-priorities))
  ([operators values priorities] (priority-tree operators values priorities
  nil))
  ([operators values priorities tree]
    (if-let [operators (seq operators)]
      (if tree
        (recur
          (rest operators) (rest values) priorities
          (extend-tree tree priorities (first operators) (first values)))
        (let [[v1 v2 & values] values]
          (recur (rest operators) values priorities 
            (list (first operators) v1 v2))))
      tree)))

; [] [+ & *] [1 2 3 4] 1+23*4
; [+ 1 2] [& *] [3 4] - initial tree
; [+ 1 [& 2 3]] [*] [4] - binds more strongly than + so replace right-most
; node
; [+ 1 [* [& 2 3] 4]] [] [] - descend until do not bind more tightly, and
; extend

(println (priority-tree ['+ '& '*] [1 2 3 4])) ; 1+23*4
(println (priority-tree ['& '- '* '+ '&] [1 2 3 4 5 6])) ; 12 - 3*4 + 56

The output from the last two lines is:

(+ 1 (* (& 2 3) 4))
(+ (- (& 1 2) (* 3 4)) (& 5 6))

(I should have explained earlier that "&" is the pseudo-operator to combine
two digits into a number, so (& 1 2) is 12).

I know it's not ground-breaking or super-exciting, but it was fun to write...

Andrew

Re: Cute Piece of Clojure Code

From: andrew cooke <andrew@...>

Date: Mon, 16 Apr 2012 07:07:24 -0300

Thanks - fixed now.

Andrew

Cute Piece of Clojure Code

From: Tuom Larsen <tuom.larsen@...>

Date: Mon, 16 Apr 2012 11:07:36 +0200

Hello,

are you sure about the link in your article? As it point to:
http://stackoverflow.com/questions/10161917/changing-name-of-uploaded-images

Kind regard,

Tuom

Wrong SO question?

From: Ashok Bakthavathsalam <ashok@...>

Date: Mon, 23 Apr 2012 04:30:38 +0000

Andrew,

Thank you very much for your answer for the SO question, and also for your =
blog article explaining the mathematical puzzle (http://www.acooke.org/cute=
/CutePieceo0.html). By the way, I think you may have the wrong link to the =
SO question in your write-up. Please check.

http://stackoverflow.com/questions/10161971/improving-performance-of-racket=
-code-and-error-when-trying-to-byte-compile versus http://stackoverflow.com=
/questions/10161917

I have been pleasantly surprised to get contributions from you (Chile) and =
Lopez (Colombia). What are all you software guns upto in S America? Would b=
e great to have you come and teach students/professionals in India about Cl=
ojure and Scheme, and programming in general.

Thanks,

  % ashok

Comment on this post