Insert into an indexed seq

[Sean Devlin]

Is there a built in to insert a value into an "indexed" seq?

For example:

user=> (insert [:a :b :c :d] 2 :q)
(:a :b :q :c :d)

Not sure if I'm missing something simple...
Sean

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[Sean Devlin]

Also, why does this work:
user=> (assoc [:a :b :c :d] 2 :q)
[:a :b :q :d]

And this doesn't:
user=> (dissoc [:a :b :c :d] 2)
#<CompilerException java.lang.ClassCastException:
clojure.lang.PersistentVector cannot be cast to
clojure.lang.IPersistentMap (NO_SOURCE_FILE:286)>

Annoying.

[Chouser]

On Tue, Apr 27, 2010 at 1:31 PM, Sean Devlin <francoi...@gmail.com> wrote:
> Is there a built in to insert a value into an "indexed" seq?
>
> For example:
>
> user=> (insert [:a :b :c :d] 2 :q)
> (:a :b :q :c :d)
>
> Not sure if I'm missing something simple...

That's a vector, which cannot efficiently splice internally, so
it's not supported directly. However, you can build a new vector
with your value included:

(apply conj [:a :b] :q [:c :d])
;=> [:a :b :q :c :d]

> Also, why does this work:
> user=> (assoc [:a :b :c :d] 2 :q)
> [:a :b :q :d]
>
> And this doesn't:
> user=> (dissoc [:a :b :c :d] 2)
> #<CompilerException java.lang.ClassCastException:
> clojure.lang.PersistentVector cannot be cast to
> clojure.lang.IPersistentMap (NO_SOURCE_FILE:286)>
>
> Annoying.

Again, vectors cannot efficiently insert or remove items except
from the right-hand end, though as you note items can replaced
internally.

(let [v [:a :b :c :d]]
(into (subvec v 0 2) (subvec v 3)))
;=> [:a :b :d]

There are immutable collections that support both numeric-indexed
lookups and internal splicing, they're just not currently included
with Clojure. See for example finger trees:

http://functionaljava.googlecode.com/svn/artifacts/2.21/javadoc/fj/data/Seq.html

--Chouser
http://joyofclojure.com/

[Sean Devlin]

You're right, inserting into a vector is fundamentally slow.
Inserting into a list (must traverse elements) or String (Char Array)
isn't any better. I get why Clojure doesn't include certain
operations on certain data structures (e.g. assoc on a list), because
it's the wrong tool for the job. However, there are still problems
that require me to use an expensive operation.

Maybe I'm too focused on my current project, and wrong about how much
a typical person would use insert. Still, its absence seems like an
oversight.

Sean

On Apr 27, 2:05 pm, Chouser <chou...@gmail.com> wrote:

>        http://functionaljava.googlecode.com/svn/artifacts/2.21/javadoc/fj/da...
>
> --Chouserhttp://joyofclojure.com/

[Mark J. Reed]

I'm a bit surprised that it's not there already, at least in clojure.contrib, but it's not hard to write, at least for vectors:

(defn insert [vec pos item] 

    (apply merge (subvec vec 0 pos) item (subvec vec pos)))

--
Mark J. Reed <mark...@gmail.com>

[Mark J. Reed]

On Tue, Apr 27, 2010 at 3:41 PM, Mark J. Reed <mark...@gmail.com> wrote:

I'm a bit surprised that it's not there already, at least in clojure.contrib, but it's not hard to write, at least for vectors:

(defn insert [vec pos item] 

    (apply merge (subvec vec 0 pos) item (subvec vec pos)))

Er, that should be conj, not merge.  Though, perhaps surprisingly, it works as written.

-- 

[James Reeves]

On Apr 27, 7:45 pm, Sean Devlin <francoisdev...@gmail.com> wrote:
> However, there are still problems
> that require me to use an expensive operation.

Could you give an example of such a problem? There may be another way
of solving it you have not considered.

- James

[Rich Hickey]

On Apr 27, 2010, at 2:45 PM, Sean Devlin wrote:

> You're right, inserting into a vector is fundamentally slow.
> Inserting into a list (must traverse elements) or String (Char Array)
> isn't any better. I get why Clojure doesn't include certain
> operations on certain data structures (e.g. assoc on a list), because
> it's the wrong tool for the job. However, there are still problems
> that require me to use an expensive operation.
>
> Maybe I'm too focused on my current project, and wrong about how much
> a typical person would use insert. Still, its absence seems like an
> oversight.
>

What are missing are finger trees, as Chouser said. Not having insert
on vector is not an oversight.

Rich