Tuesday, March 30, 2010

Upgrading a Laptop

Upgrading is never easy. No matter how much preparation you make.


We decided to upgrade all the developers laptops on the team by increasing the RAM and moving from Windows XP to Windows 7 64 bit. We started by getting more RAM and increasing to 4 GB. Of course, because we were running Windows XP 32 bit, so we only got a usable 3.48 GB of RAM. While that is an improvement over the 2GB we had before we wanted more.

So we are now attempting to upgrade to Windows 7. I spent the better part of the last two days backing up and installing. Backing up took all night despite the use of some helpful migration tools available from Microsoft.

The installation went quite smoothly and so far the new system is working nicely. Unfortunately, I still have only 3.48 GB of RAM. It turns out that I installed the 32 bit version. Hmm. Now I will have to get the 64 bit cd and do it all over again. At least this time I have everything backed up so if the installation runs smoothly then I will not have  to spend too much time on the operation.

Saturday, March 27, 2010

Mastermind in SML

Functional languages are making a comeback. For a while they were in the exclusive domain of academia but recently there has been increased interest in functional programming in the wider industry. I will leave it up to WikiPedia to explain if you never heard of this before.

Here is a simple program written in SML that implements a Mastermind game.

(*
This program implements a mastermind game.
The computer is the board master and the user
has to make guesses until...

The tokens "g", "r", "y", "b" are used to represent

green, red, yellow and blue respectively.

Author: Vincent Ramdhanie
Date: 11 February 2010
*)


(*Start by opening the TextIO library for input and output of text data.*)
open TextIO;

(*Next we need a random number generator*)
val ran = Random.rand(1, 20);
fun random() = Random.randRange(1, 4) ran;

(* member returns true if c is a member of the list, false otherwise. Of course there is a built in member function. But we are trying to learn the language here are we not?*)

fun member(c, []) = false |
    member(c, h::t) = c = h orelse member(c, t);
(*inPosition returns true is c is in the ith position ina given list*)
fun inPosition(c, i, []) = false |
    inPosition(c, 0, h::_) = c = h |
    inPosition(c, i, _::t) = inPosition(c, i - 1, t);


(*We need a datatype to track character correctness*)
datatype correct = A | B | C;


(*This function takes a character and a position and returns a correct value*)
fun charCorrect(c, i, L) =
    if member(c, L) then
        if inPosition(c, i, L) then
            A
        else
            B
    else
        C;

(*Remove all newLine characters from a char list*)
fun removeNewLine([]) = [] |
    removeNewLine(#"\n"::_) = [] |
    removeNewLine(h::t) = h::removeNewLine(t);

(*Remove newline characters from a string *)
fun trimString(s) = implode(removeNewLine(explode(s)));

(*Ensure that only the characters #"g", #"r", #"b" and #"y" are in a string*)
local val chars = [#"g", #"r", #"b", #"y"] in
fun limitChars([]) = true |
    limitChars(h::t) = member(h, chars) andalso limitChars(t)
end;

fun limitString(s) = limitChars(explode(s));

(* Generate Random characters*)
fun randomChar() =
    let
        val r = random()
    in
        case r of
            1 => #"g" |
            2 => #"b" |
            3 => #"r" |
            4 => #"y"
    end;

(*Generate just n characters*)
fun nRandomChars(0) = [] |
    nRandomChars(n) = randomChar()::nRandomChars(n - 1);

(*Create a string of 4 characters*)
fun randomCharString() = implode(nRandomChars(4));

(*Some user interaction functions*)

(*This function gets an input line from the user*)
fun inLine() = valOf(inputLine(stdIn));

(*Prompt the user for input*)
fun prompt(msg) = (print(msg ^ "\r\n");inLine())

(*The Yes No datatype*)
datatype answer = Yes | No;

(*The Valid Invalid datatype*)
datatype valids = Valid | Invalid;

(*Prompt the user for a Yes No value*)
fun yesNoPrompt(msg) =
    let
        val ans = prompt(msg)
    in
        if ans = "Yes\n" then
            Yes
        else
            No
    end;

(* Get a cleaned up guess*)
fun guessPrompt() =
    let
        val guess = prompt("Please enter 4 character guess using the characters g, r, b and y")
    in
        trimString(guess)
    end;


(* Validate Guess*)
fun getValidGuess() =
    let
        val inp = guessPrompt()
        val lim = limitString(inp)
    in
        if (size(inp)  = 4) andalso lim then
            inp
        else
            (print("Sorry, That input is invalid. Please try again.\n\n");getValidGuess())
    end;

(*  Print a report*)
fun printResult([]) = print("_______________________\n") |
    printResult(h::t) =
    if h = A then
        (print("Correct Position\n");printResult(t))
    else
        if h = B then
            (print("Incorrect Position\n");printResult(t))
        else
            (print("Incorrect Token\n");printResult(t))

(*Determine the quality of a guess*)
fun processGuessAux(original, _, []) = [] |
    processGuessAux(original,i, h::t) =  charCorrect(h, i, original)::processGuessAux(original,i + 1, t);

fun processGuess(original, guess) =
    if original = guess then
        true
    else
        (printResult(processGuessAux(explode(original), 0, explode(guess)));false)
   

(*A run of the Game*)
fun gameRun(original) =
    let
        val guess = getValidGuess()
                val result = processGuess(original, guess)
    in
        if result then
            print("Congratulations. You Win!")
        else
            gameRun(original)
    end;

(*Game Control*)
fun playAgain() =
    let
        val ans = yesNoPrompt("Do you want to play again? (Yes/No):\n")
    in
        if ans = Yes then
            play()
        else
            print("Thank You for playing.")
    end
and
   play() =
    let
        val original = randomCharString()
    in
        (gameRun(original);playAgain())
    end;

Friday, March 05, 2010

Collatz Conjecture

In today's xkcd comic, the topic was Collatz Conjecture. This is the first I have ever heard of this so of course I looked it up on Wikipedia. Turns out it was proposed by Lothar Collatz in 1937.

 
 Simply put, given any natural number n. If n is even, we halve it (n / 2), otherwise we multiply by 3 and add 1 to obtain 3n + 1. Rinse, repeat. The conjecture is that for all numbers this process converges to 1.

The Wikipedia article gives several implementations of this algorithm so I opened up my editor and tried out an SML implementation. Without exhaustive testing, it seems to be working so far.
Here is the code:

(*
  Implements Collatz Conjecture.
  Accepts: an integer n.
  Returns: The list of integers produced by following collatz sequence until the sequence converges to 1.
  Pre: n >= 1
*)
fun collatz(1) = [1] |
    collatz(n) =
        let
            val even = n mod 2 = 0
        in
           if even then
              n::collatz(n div 2)
           else
              n::collatz(3*n + 1)
        end;