Thursday, February 24, 2011

Bubble sort for prolog

Hello again,
this time I've found a version of bubble sort here. I wanted to provide my version, which is less iterative and, I think, more intuitive. What it does is, simply, bubble until it's sorted:

bubblesort(L1, L2) :- bubblesort2(L1,L2,unsorted),!.
bubblesort2(L1,L2,unsorted) :- bubble(L1,L3,C), bubblesort2(L3,L2,C).
bubble([X,Y|L], [X|L1], C) :- X <= Y, bubble([Y|L],L1,C).
bubble([X,Y|L], [Y|L1], unsorted) :- X > Y, bubble([X|L],L1,_).

Yes, the exam is tomorrow so I will finally stop annoying you readers ;)

Saturday, February 19, 2011

Aptitude string for downgradable packages

I'm lately doing some tests with Debian experimental packages thus I often upgrade some packages to experimental and downgrade them back to unstable.
WARNING: Downgrading in Debian is not supported etc.

aptitude search "?narrow(?installed,?archive(experimental))" -F %p|\
's,\([^ ]*\),\1/unstable,'|xargs echo

This will give you a list of experimental packages installed on your system each concatenated with "/unstable". The output can go straight to "aptitude install". I don't directly use "xargs aptitude install" because it's not interactive.

Tuesday, February 15, 2011

Matrix transpose with Prolog

an exam exercise requires me to write a matrix transpose method. I've written one and it took a little before I was able to define it completely in 4 rules.
I'm curious then I've found this on stackoverflow: the approach is to calculate first transposed column, then shift by one column and calculate the transpose of that new matrix.
This was one of the first solutions I've thought but I haven't realized it because I'm too lazy to create a rule for calculating the shifted matrix.

My approach is iterative thus less intuitive:
trans(M1, M2) :- trans2(M1, M1, [], M2, 0), !.

trans2([A|_], _, _, [], N) :- length(A, N).
trans2(M, [], H1, [H1|R1], N) :- N1 is N+1, trans2(M, M, [], R1, N1).
trans2(M, [H|R], L, [H1|R1], N) :- nth0(N, H, X),
append(L, [X], L1), trans2(M, R, L1, [H1|R1], N).

Ok, apart the fact that I haven't got the time to beautify it, the code will iterate columns and for each column it calculates a row of the transposed matrix (yes, exactly what you expect a transpose method to do :P). The key is "passing" around the nth column we're looking at.
After we finish calculating a row, we restart from the first row but looking at the nth+1 column. Recursion ends when there are no more resulting rows, i.e. when we reached the end of the columns.

Wednesday, February 09, 2011

Bluetooth simple one-line device connection pairing with Bluez

I've written a simple Python script using the Bluez (version 4.66) stack (thanks to with this usage:


Snippet code for is here. If the device is paired, it will be removed and unmounted.

Disconnection is as easy with:


Snippet code for is here.

Feel free to use the code also for other services, in this case my primary concern was to mount the file system.

Rubik's Cube 3D Game in Vala/Clutter

I've written a simple program for playing with a Rubik's Cube using Vala and Clutter.

It features:
  • high simplicity in rotating cube slices and rotating the cube itself in a very natural manner
  • shuffle the cube
  • autosave the game

Download and more usage information at the homepage... have fun :)