Archive for June, 2012

Furnace/Heat Exchange Problem

About 7 years ago in one of my master program courses at The University of Texas at Dallas we were assigned a project that involved the simulation of a furnace. The requirement was that it would be solved three different ways, in process, RPC, and forks.

The main crux of the problem though was a simulation of a furnace and the heat dissipation. The problem was down to a grid of cells, each with its own temperature, and the temperature would change based off of the neighbor cells. There was also a burner in the simulation, that would turn on when the temperature of cells would get below a set threshold.

I figured that this would be a good problem to try solving in Clojure, but I can not remember the overall problem description in order to make an attempt at it.

If anybody knows the problem I am describing, please let me know, as I haven’t been able to find the right terms to search against, so now I am asking for help from the Hive-Mind.



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Project Euler in Clojure – Problem 8

This is my solution to Problem 8 of Project Euler. If you would like to track my progress you can follow my repository on to keep track of my progress, which can be found at  Here is my solution to Problem 7.

Problem 8 of Project Euler is described as:

Find the greatest product of five consecutive digits in the 1000-digit number.

(defn digits-of [n]
  (map #(Integer/parseInt (str %)) (str n)))

(defn problem8 []
  (let [n 7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450]
    (apply max (map
                 #(apply * %) (partition 5 1 (digits-of n))))))

The digits-of function takes a number and creates a sequence of the digits of the number, in the same way a string can be treated as a sequence of characters. I then call partition on that stream of digits to get the digits in groups of five. Those groups of five get multiplied together, and we find the max of those results.

Again, I would love comments and suggestions on my solution to this problem, and if there are tweaks to make it more Clojure-ish.

I have posted my solution to Problem 9.


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Smelly matrix diagonals

As I am working on Problem 11 in Project Euler, I am trying to use a matrix and some functions against that matrix to help me solve the problem. I spent a bit of time browsing incanter and its documentation, but did not see anything that would give me what I was looking for.

The particularly nasty function I am encountering is finding all of the diagonals in a matrix. Given a matrix of

 1  2  3  4
 5  6  7  8
 9 10 11 12
13 14 15 16

I am would expect the following sequence of vectors:

[1 6 11 16] [2 7 12] [3 8] [4] [5 10 15] [9 14] [13]

Where I think the nasty-ness is coming from is that function is seeming very imperative, and it seems like there should be a much more elegant solution to this, but I am currently not seeing it. In addition to what seems to be a large smell in the code, what makes me think I am doing something “wrong” for Clojure is having seen the elegant solution to Conway’s Game of Life in Programming Clojure which can be found at Github. In the book they take an imperative implementation from another language into Clojure, and then they show a functional solution which turns the imperative version on its head and spins it like a top.

Here is what I came up with, and I can only assume that others would think there is a large smell to this as well.

(defn diagonals [m]
  (let [rdim (count m)
        cdim (count (first m))]
      (loop [diags []
             roff 0
             coff 0]
          (cond (>= coff cdim) (recur diags (inc roff) 0)
                (>= roff rdim) diags
                :else (recur
                        (concat diags
                                (conj []
                                      (for [i (range 0 (min (- rdim roff) (- cdim coff)))]
                                           (matrix-get m (+ roff i) (+ coff i)))))
                        (if (zero? roff) (inc coff) cdim))))))

Even as I was writing this function, I could tell that it is doing way to much work in it, and that it is not very composable, nor does it seem to take advantage of much composability. As a side note, one of the things that was also a large sign that I was doing something “wrong” for Clojure, was that it was difficult to incrementally develop, modify, and debug in the REPL.

I would love some suggestions on how to give this code a good scrubbing, and clean up this function.

Thanks in advance,


Sieve of Eratosthenes in Clojure using sets

Working through the Project Euler problems, and with the comment from Arcane Sentiment about filtering based off of a set of primes, I was trying to program the Sieve of Eratosthenes. I also realized that the Sieve becomes a set based problem, so I went about trying to solve it using sets.

My first approach was simply using the difference function given in clojure.set.

(defn primes-under [n]
  (loop [sieve (set (cons 2 (range 3 n 2)))
         f 3]
    (if (> (square f) n)
      (recur (difference sieve (range (square f) n f)) (inc f)))))

I start with the set of numbers starting by cons-ing 2 onto the range from 3 to n, skiping every other number, to get the odd numbers up to n.

The new sieve becomes the the difference between the current sieve, and the multiples of the factor f. I use (range (square f) n f) as a subset of the factors of f up to the number n. I start the range at the square of the factor, as any multiples below the square of the factor would already have been sieved out from the factors below it. So for multiples of 5, I can ignore (10, 15, 20) as they are multiples of 2 and 3, the other primes below 5, and as a result I only need to get multiples of 5 starting at 25.

When timing this solution by using:

(time (dorun (primes-under 1000000)))

The timing for the primes under 1,000,000 is around 2.1 seconds, and anywhere from 6 to 10 seconds for the primes under 2,000,000.

I wasn’t happy with this performance, especially after seeing Christophe Grand’s solution and timing on his blog post Everybody loves the Sieve of Eratosthenes.

I decided to try my hand at solving this using a transient set. After many missteps in using a transient set, due to not having used them before, I came up with the following solution:

(defn primes-under2 [n]
  (let [sieve (transient (set (cons 2 (range 3 n 2))))]
    (loop [s sieve
           f 3]
      (cond (> (square f) n) (persistent! s)
            :else (recur (reduce disj! s (range (square f) n f)) (inc f))))))

I define sieve as a transient set, and instead of using difference, I call reduce on the sieve using the function disj! for the ranges of numbers to filter out. I need to use disj! to remove that number from the sieve, as this is now a transient set I am working against.

The timing of this solution is now down to 1.1 seconds for primes under 1,000,000, and primarily in the 3 second range for the primes under 2,000,000.

I am still not quite satisfied with this solution, and would love suggestions as to how to avoid those numbers which have been filtered out of the sieve already.  As I just do a (inc f), I would still get the numbers 4, 6, 8, 9, 25, etc, that are multiples of other primes, but I could not figure out how to skip these numbers.  I tried the functions contains?, get, superset?, subset?, difference, but these were not giving me the result when dealing with the transient set.

If someone could tell me if I have missed anything I should try for filtering out non-prime numbers as factors, I would love to get your suggestions to try it out, and I will post a follow up with the results.


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Project Euler in Clojure – Problem 7

In trying to learn Clojure and wrap my head around good functional programming, and hoping to learn more idiomatic Clojure, I have started working through the Project Euler problems. In doing this, I have also setup a repository on to keep track of my progress, which can be found at  My approach to Problem 6 can be found here.

Problem 7 of Project Euler is described as:

What is the 10 001st prime number?
(defn is-prime? [n]
  (cond (<= n 1) false
        (= n 2) true
        :else (loop [f 2]
                (cond (zero? (rem n f)) false
                      (> f (Math/sqrt n)) true
                      :else (recur (inc f))))))

(defn problem7
  ([] (problem7 10001))
  ([n] (first (skip-n (dec n) (filter is-prime? (iterate inc 1))))))

The is-prime? function check to see if the number is prime by checking if any of the numbers from 2 to the square root of the number is a divisor of the number.

The problem7 function is finds the n-th number in the sequence by skiping n-1 items, and then taking the first item of that sequence. This was before a previous post in which someone pointed out that the drop function was available instead of my home rolled skip-n function, so that can be replaced on the update.

Again, I would love comments and suggestions on my solution to this problem, and if there are tweaks to make it more Clojure-ish.

My solution to Problem 8 has been posted.


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Project Euler in Clojure – Problem 6

In trying to learn Clojure and wrap my head around good functional programming, and hoping to learn more idiomatic Clojure, I have started working through the Project Euler problems. In doing this, I have also setup a repository on to keep track of my progress, which can be found at  My approach to Problem 5 can be found here.

Problem 6 of Project Euler is described as:

Find the difference between the sum of the squares of the
first one hundred natural numbers and the square of the sum.

This is another problem that seems to translate almost directly from the problem statement into the operations that need to be performed.

I find the square of the sums from 1 to the (inc n), where n is bound to 100 for the problem definition, as I want the number that n is bound to to be included in the range. This is coded as:

(Math/pow (reduce + (range 1 (inc n))) 2)

As well as I find the sum of the squares from 1 to the (inc n).

(reduce + (map #(Math/pow % 2) (range 1 (inc n))))

I then pieced these two solutions together, subtracting the two values. When doing this at the REPL, it gave the result as an exponent, which I didn’t like the look of, so I put the function bigint around the subtraction function, giving the full solution as:

(defn problem6
  ([] (problem6 100))
  ([n] (bigint (-
         (Math/pow (reduce + (range 1 (inc n))) 2)
         (reduce + (map #(Math/pow % 2) (range 1 (inc n))))))))

Again, I would love comments and suggestions on my solution to this problem, and if there are tweaks to make it more Clojure-ish.

My solution to Problem 7 can be found here.


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Project Euler in Clojure – Problem 5

In trying to learn Clojure and wrap my head around good functional programming, and hoping to learn more idiomatic Clojure, I have started working through the Project Euler problems. In doing this, I have also setup a repository on to keep track of my progress, which can be found at  My approach to Problem 4 can be found here.

Problem 5 of Project Euler is described as:

What is the smallest positive number that is evenly
divisible by all of the numbers from 1 to 20?

In reading this problem I realized that this is just another way to state:

Find the Least Common Multiple of all of the numbers between 1 and 20.

The solution I came up with is:

(ns project-euler.core
  (:require [clojure.string :as string]

(defn problem5
  ([] (problem5 20))
  ([n] (reduce lcm 1 (range 2 (inc n)))))

I found that previous to Clojure 1.2 there was a math library in Clojure-Contrib, which after 1.2 is now been moved to clojure.math.numeric-tower, so that now gets included in the vector of libraries in the :require of the ns function.

Once I had this, it was now just as simple as calling the reduce function with the function lcm, feeding it a initialization value of 1 to the reduce function and having that operate on the range of numbers from 2 to the number bound to the var n, which when called without parameters will use the value of twenty as specified in the problem definition. By defining the different arities for the function problem5, it allowed me to call the function with different numbers to the results of the function, as the full description gave the solution for the numbers in the range of 1 through 10.

Again, I would love comments and suggestions on my solution to this problem, and if there are tweaks to make it more Clojure-ish.

My solution to Problem 6 has been posted here.


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Setting up PHP on IIS 7.0

Today is the last day of our previous UX developer David Belcher, since his contract is up and my company has hired a permanent UX developer. David had setup a reference site for the different projects he was involved in, but it is currently hosted on one of his consultancy company sites. The content should stay up for as long as we will need it, but as soon as we are going to need to make changes to the reference styles, or any of the mock-ups, it will be out of date, as our UX developer will not have access the site to make changes.

To help ease this transition, yesterday I offered to see if we could get it setup somewhere where it could live and be updated. The catch was that wherever were would put it up would need to be able to run PHP. I found some information on IIS 7.0 being able to host PHP, so we decided to host it locally on an internal development web-server until we can determine a permanent place where we would like it to live.

I did a Google search and found that there is a whole sub-domain for PHP on the site. Right there at the top of the page was a link to a Microsoft Web Platform Installer for PHP. That wound up being a straight forward setup, but one of the pages was throwing an error and not having any PHP experience, other than reading a few PHP files, I wasn’t sure what was going on.

I found a couple of resources directing me to the display_errors setting in the PHP config file php.ini. As this is an internal development box, I felt comfortable enough to turn this flag to On, since to access the server one has to be on our VPN, and the security risk for a internal PHP site that is for reference of styles and workflow would be pretty low risk. So I made the change to the php.ini file and restarted IIS. When I hit the page, I was now getting:

Parse Error: syntax error, unexpected $end in [somefile].php on line [x]

Cool, I was one step closer to figuring out what was happening instead of a generic server error. Now it was time to figure out what was causing the syntax error. After a while of trying modify the file by trying to strategically remove sections, followed by searching for the generic form of my specific message, I came across this post on the site My Digitial Life. After going back in to the php.ini file and setting short_open_tag to On and restarting IIS, the page came to life.

I realized after doing all of this yesterday, that I should document this somewhere, and as we don’t have a good place to track this internally, I would post these steps so that someone else might benefit from them, instead of just being buried in a personal composition book of work related notes.

Hope this can help someone out there when they encounter a similar situation.

For those unfamiliar with PHP, to make sure that it is working, and to check the version you can create a test file that you can hit to get version and deployment info on your PHP setup. Just create a file test.php with the following in it:

<?php  phpinfo(); ?>

Then hit the page test.php and you should see something along the lines of:
test.php result sample


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