On Nov 22, 4:59 pm, "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>
wrote:
> On Nov 22, 9:48 pm, Rich Hickey <[EMAIL PROTECTED]> wrote:
>
> > map and filter don't modify anything. What does it mean to filter a
> > vector?
>
> Yes yes, I know that. Still in English its sometimes easier to be
> sloppy and pretend that I modify something even though I get another
> vector.
>
> > In any case, the vector itself is immutable, so calling vec on the
> > return value of filter is fine.
>
> Well yes its fine but what happens is that filter returns a (lazy) seq
> and calling vec on that seq will allocate a new array of the same
> length and copy every item over.
>
I still don't understand your expectation here. If the filter does any
filtering, it won't return everything in the vector, so the new result
will have fewer items. Do you want a vector with fewer items? Then
you'll have a new vector.
> > As far as map, if you have a function that will only affect a few of
> > the items in the vector, you might want to reduce, making 'changes' as
> > you go.
>
> Thanks. That sounds like it would work out nicely. Still in general my
> issue seems to be that data strucutres turn into seqs and this can
> cause subtle issues as for instance the different behaviour of conj
> and a simple rule such as calling vec on the seq seems to be to
> costly.
Nothing turns anything into seqs - a seq is a view on a collection.
If you are 'modifying' every element, then you'll have to have an
entirely new vector. Calling a mapping function and pouring the result
into a new vector will be more efficient than pretending to modify
each element in place with assoc.
In general, it can be efficient to perform a series of transformations
in the seq space and then realize a vector at the end if you want. The
lazy seqs just act as a cursor over the data, and a stream of results,
and building a new vector from scratch by appending, as vec or (into
[] ...) would do, is more efficient than element 'replacement'.
If you don't expect a copy when mapping across an entire vector, and
you want to use the immutable data structures, what alternative do you
imagine?
Rich
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