[Haskell-cafe] Advice needed on best way to simulate an STL vector
Cale Gibbard
cgibbard at gmail.com
Wed Apr 19 13:07:41 EDT 2006
Oops, replace Array there with DiffArray.
On 19/04/06, Cale Gibbard <cgibbard at gmail.com> wrote:
> Well, you could use something like C++ does for implementing STL
> vectors in terms of arrays -- iirc, it generally doubles the allocated
> size each time applying an operation to the vector would make it
> exceed its capacity (the current allocation size), and keeps track of
> the actual number of elements stored separately. It's important to do
> allocation which is proportional to the existing capacity, or repeated
> insertion will become quadratic time rather than linear. So
> essentially, some data structure like
>
> data Vector a = Vector { store :: Array Int a, end :: Int }
>
> would be a sufficient minimal way to start. When the store needs to be
> increased, you simply create a new array with twice as many elements,
> copy the initial elements in from the old array which is then thrown
> away, and only update end to the position of the actual last element.
> This is analogous to what C++ implementations of the vector class do.
>
> What will bite you is if you try to generalise from indices of type
> Int to instances of Ix -- the Ix operations assume that there are
> lower and upper bounds on indices. The upper bound of course quickly
> becomes a problem. You could however use Enum, which has toEnum and
> fromEnum, which are sufficient to use with the implementation in terms
> of Ints. It could also be claimed that Int isn't always the best
> initial type to use, and indeed I'd still feel safer with Integer
> somehow, but then, fromEnum and toEnum use Int, and if you have more
> than 2 billion elements in your vector at the same time, maybe you
> should be looking at your algorithm more carefully and/or doing your
> own low level memory allocation via FFI. :)
>
> - Cale
>
> On 19/04/06, Brian Hulley <brianh at metamilk.com> wrote:
> > Hi -
> > In C++, STL provides a vector class which behaves as an array except you can
> > insert/delete elements from it. I'm wondering what is the best Haskell data
> > structure to use to simulate this, either mutable or immutable.
> >
> > I've looked at the Array interface, but although there is the // operation
> > to replace some elements, there does not appear to be a simple way to
> > delete/insert elements.
> >
> > Ideally I'd like functions like:
> >
> > -- Insert new elements starting at the specified index, moving the others up
> > to make room
> > insert:: Array i e -> i -> [e] -> Array i e
> >
> > -- Delete a range of elements, moving later elements back down
> > delete:: Array i e -> i -> i -> Array e
> >
> > -- Append a new element to the end of an array
> > push_back :: Array i e -> e -> Array i e
> >
> > Is there an efficient way of implementing these operations in Haskell, based
> > on arrays, or should I be using some other data structure altogether eg a
> > list?
> >
> > Also, for large arrays, am I right in thinking that it is still more
> > efficient to use immutable arrays rather than mutable arrays (because it is
> > easier for gc to always just deal with immutable values)?
> >
> > Thanks, Brian.
> >
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> >
>
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