[Haskell-beginners] Arrays in Haskell
Lorenzo Isella
lorenzo.isella at gmail.com
Sat Sep 11 10:09:09 EDT 2010
Dear All,
The recent feedback I got on the mailing list led me to think about the
data structure I need for my computations and array manipulations
(loosely speaking, let us say that I need indexing and slicing tables of
data).
Coming from Python, I am a bit confused: let me say that in my Python
scripts I almost never use lists, but rather NumPy arrays.
In that case, it is an easy choice (almost every decent software for
numerics/visualization etc... in Python relies explicitly or implicitly
on NumPy). On top of that, NumPy is fast, has a lot of inbuilt functions
and interfaces nicely with SciPy, matplotlib, Mayavi2 etc...
It seems to me (please correct me if I am mistaken) that the situation
in Haskell is a bit more 'open' to choices.
At least I think so when I look at
http://en.wikibooks.org/wiki/Haskell/Hierarchical_libraries/Arrays
I may want to drop lists at some point for performance reasons but also
because in my work I really have tables of numbers I find convenient to
think of as arrays/matrices (again, loosely speaking, I mean matrices as
arrays + linear algebra like taking the inverse, the determinant and so on).
Bottom line, I would need a data type that
(1) is decently fast (OK, there is more than performance to scripting,
but you see my point)
(2) allows slicing/indexing (e.g. take 3rd row, second column, flatten
it out, take every element larger than 34 in a row, find its unique
elements, sort them etc...) without having to re-invent the wheel
myself. This is more on the manipulation side than the linear algebra.
As you can see, I would like to be able to find something similar to the
very useful functions in Data.List for an array.
(3) it would be nice if these data type could have either integers of
real numbers as entries. If the original data is a made up of integer
numbers, conversion to real number is always suspicious (I have horrible
memories (at least in other languages) of numbers that were equal as
integers and no longer when read as real numbers because of one last
digit changing...which can give you a headache in some cases.
(4) again it would be nice if I could feed these arrays/vectors to tools
that take integrals, derivatives, inverse etc...
I am considering for this reason several possibilities
http://www.haskell.org/tutorial/arrays.html
http://users.skynet.be/jyp/html/base/Data-Array.html
http://code.haskell.org/hmatrix/hmatrix.pdf
hmatrix is probably what I am looking for (linear algebra, interface to
gnuplot for plotting, ODE solver etc...) but there is no possibility of
using arrays of integers, so I am concerned about using it to read and
compare data files filled with integers where I check if certain entries
are equal or not. Also I wonder if I can find any extra documentation
other than the well written tutorial (which explains a lot, but cannot
do everything in less than 30 pages and I would have plenty of questions
about array manipulations there).
Any suggestion/clarification is more than welcome.
Cheers
Lorenzo
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