Wednesday, 30 April 2008

Enough of C++ !

I used to like C++ a lot but the more I use this language the less I like it.

I think I really definitively made up my opinion about this language when I started to look at the boost libraries.
First I though all those crazy meta programing stuffs were genius (and indeed the boost libraries are an impressive piece of work), but then I realize that if C++ didn't have so much drawback, the boost code would be quite simple.

All the boost libraries do is find tricky ways to do things that C++ hasn't been made for in the first place.

For me it is a clear sign that C++ is going nowhere. And the new C++0x doesn't look so good to me. They make the language very complicated, I wonder if people are actually going to learn all the new things they add to the language. Yet one of the most important feature (garbage collector) is still not planed.

I am getting very interested by the D language, I even made a small video game in D.
At first sight D looks similar to C++, cause it has the same syntax (I would have preferred a python like syntax though), but it corrects all the things that made me not want to write C++ again :
  • a garbage collector (you can disable it if you want more control on memory)
  • a foreach loop
  • dynamic arrays part of the core language
  • automatic type deduction (like "auto x = 10")
  • simpler and more powerful template system.
Well, and many other good stuffs.

So far the language is still new, and unfortunately not so many libraries have been created for it, but for me D may well be the future of compiled language.

How to compute "y = a * x + y"

I recently tried to write a numerical computation software that needed to perform a lot of vector operations. My first idea was to use python, and the numpy library.
But then I found that this simple operation :

y = a * x + y

Where x and y are large numpy arrays, and a is a scalar,
was much slower than the same thing coded in C, like this :

void func(float x[N], float y[N], float a) {
for(size_t i = 0; i < N; ++i) {
y[i] += a * x[i];

The reason is that numpy has to create a temporary object to store the result of the multiplication, and then also create a new object to store the result of the addition. When the size of the arrays becomes large - I was dealing with several Megabytes data arrays - it can make a lot of difference.

But I don't like the C code either, because of the very ugly loop.

So what can I do ? Well, first I could use the C blas library, then the operation is replaced by a function call, like this :

cblas_daxpy(N, a, (double*)x, 1, (double*)y, 1);

But it still looks over complicated.

OK maybe some other languages can be more suitable for this task. What about C++ ? With the boost::ublas library, it looks already much better :

using namespace boost::numeric::ublas;
void func(const vector& x, vector& y, double a) {
y += a * x;

Well, almost what I wanted, I could even use template parameters to make the function accepts any kind of input data.

But since at the end I want to be able to call this function from python, C++ is not the easiest choice (beside, I don't like C++ that much). Finally I used a language I really don't like, but that appeared to be the best for this particular task : Fortran90

Fortran90 is already much better than the horrible Fortran77 (That nobody should ever use). and most important, it has vector operations included in the language.

So now my simple operation looks like this :

subroutine func(x, y, a)
implicit none
REAL, dimension(:), intent(in) :: x
REAL, dimension(:), intent(inout) :: y
REAL, intent(in) :: a

y = a * x + y

end subroutine func

(Note : in Fortran90, you DON'T need to start every lines at the 7th character !)

I can call this fortran function from python quite easily, thanks to F2PY, and voilà. Then I can extend my function to perform all sort of numerical operations.

Conclusion : Fortran90 is not as bad as I though it was, combined with python it can even be quite useful. But I still don't like this language, I am looking forward for the D language to have good numerical operations library, as well as python wrapping (it already exists, but there are still a few issues.)

Wednesday, 23 April 2008

New job at openmoko

In a few weeks I will start to work with the great people of the OpenMoko project.

The goal of this project is to produce a totally open source mobile phone that everyone can modify at will.

You can learn more from the web site, download the sources, and even the CAO files !

My job will start in a few weeks, for the moment I am still working for Taiwan university, but I start to have a look at the huge mailing list archives of OpenMoko to get a better idea of the project and the people involved.

I'll post more on the subject when I will actually start working. I guess this will be a great experience for me, and I will do my best to contribute to the project.

Welcome to my new blog

Welcome to Charlie137 (aka Guillaume Chereau) new blog.
I start this new blog in addition to my other blog.

This blog will be more tech oriented. I will talk about the things I like in computer science, and also about my new job for OpenMoko.
So get ready to hear a lot about linux, OpenMoko, Python, D, Vala, Stellarium (among other things.)

I'll keep updating my old blog with drawings and video games stuffs when I'll get the time. I have been pretty busy (and sick) recently so I didn't take time to post anything.