Re: [Haskell-beginners] Haskell laziness and data structures.

Ford writes:

I am trying to implement ant colony optimization algorithm (or any other similar simulation) . In procedural language I would keep the world map in 2D matrix that provides O(1) collision check and O(1) ant position change.

I do not know that particular algorithm, but most algorithms are even written down imperatively. First check if odd, iff so do foo and then repeat first step with next item else repeat step 1 with next item without doing foo. In FP the world is working in a slightly different way. FP would say the above roughly like “for all odd items in a list do foo” Maybe the difference is not that obvious right now. But trying to repostulate the wording of an algorithm from imperative style to declarative style, does help a lot very often. As I already said, when I think to need mutable datastructures, most of the time my algorithm is not declarative enough. Try to reach for a different algorithm which has the same goal, or at least try to rethink the one you currently have.

(The memory usage is also important.)

It is always, but it is also very hard to reason about memory usage in GC'd languages. You might be able to reason about pure allocations while assuming that garbage is collected instantly, but this is just not true. Sometimes even immutable stuff is changed in place, just because the runtime knows that the old value will never be used again. And this does massively depends on the optimisation, the compiler and the runtime used.

Thank you for any tips.

King regards,

Ford

Odesláno z BlueMail

7. 7. 2016 11:40 AM, 11:40 AM, Norbert Melzer napsal/a:

...

OxFord writes:

...

Hello,

Why is there no default O(1) random access list data structure in haskell (for example clojure has [] vector). I would expect that this kind of data structure is used very often, so you shouldn't need to import one yourself.

Whenever I try to reach for something with O(1) random access, then I have used imperative languages for too long ;) In most cases I am able to rewrite my algorithms to work by iterating instead of random access. And if I do not come up with such an algorithm, I then do realize most of the time, that I do want to have a Map or Set with a certain Key-Type, which is very unlikely to be Num.

...

Is it safe to assume that ' reverse [1, 2, 3, 4, 5] ' is O(1) ?

It is not. Reversing in constant time is impossible without dooing nasty hacks, and make it necessary to not only save distinct elements, but its reading direction in addition.

Also to make this actually happen you need something in the spirit of a C Array or a doubly linked list.

Also you will get in trouble as soon as you want to change an element in the reversed collection, but not in the original one.

As soon as you want distinct containers you need to copy, and copying is O(n) at least.

...

Is it safe to assume that main = x = [1, 2, 3, 4, 5] x = reverse reverse x won't use more space than for the initial [1, 2, 3, 4, 5] list? (No copy of x)

According to my observations it is not safe to assume anything about memory in GC'd languages.

...

Why is array indexeded by ! and list by !!. Shouldn't they be both instances of something like Indexable?

Because indexed access is always something you should think about, and for lists you should think about it even twice. Thats just a guess though. _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners