The ant simulation

[rob....@gmail.com]

In the ant simulation the world function looks like this

(def world
(apply vector
(map (fn [_]
(apply vector (map (fn [_] (ref (struct cell 0 0)))
(range dim))))
(range dim))))

I'm not sure why the calls to '(apply vector' are required. I have two
theories:

* It is to work round the lazy nature of map and force the creation of
all of the refs. If this is the case, can anyone enlighten me as to
why this might be necessary?
* It is a performance optimisation to make lookups of faster later.

A third theory is that neither of the above theories is correct.

Can anyone provide me with some insight?

Thanks,


Rob Lally.

[Parth Malwankar]

vector takes individual elements as arguments and map
returns a list. If we simply (vector (map ..)) the entire
list returned by map is seen as a single element. Hence
'apply'.

user=> (vector 1 2 3)
[1 2 3]
user=> (map inc [1 2 3])
(2 3 4)
user=> (vector (map inc [1 2 3]))
[(2 3 4)]
user=> (apply vector (map inc [1 2 3]))
[2 3 4]
user=>

Parth

>
> Thanks,
>
> Rob Lally.

[Rich Hickey]

On Aug 30, 10:52 am, "rob.la...@gmail.com" <rob.la...@gmail.com>
wrote:

> In the ant simulation the world function looks like this
>
> (def world
> (apply vector
> (map (fn [_]
> (apply vector (map (fn [_] (ref (struct cell 0 0)))
> (range dim))))
> (range dim))))
>
> I'm not sure why the calls to '(apply vector' are required. I have two
> theories:
>
> * It is to work round the lazy nature of map and force the creation of
> all of the refs. If this is the case, can anyone enlighten me as to
> why this might be necessary?
> * It is a performance optimisation to make lookups of faster later.

Yes, we want to look up by index, and so use a vector.

Note that (apply vector s) is never needed now, use (vec s) instead.

Rich

[Robert Lally]

2008/8/30 Parth Malwankar <parth.m...@gmail.com>

vector takes individual elements as arguments and map
returns a list. If we simply (vector (map ..)) the entire
list returned by map is seen as a single element. Hence
'apply'.

user=> (vector 1 2 3)
[1 2 3]
user=> (map inc [1 2 3])
(2 3 4)
user=> (vector (map inc [1 2 3]))
[(2 3 4)]
user=> (apply vector (map inc [1 2 3]))
[2 3 4]
user=>

Parth

Thanks for taking the time to reply Parth, it was the point of the vector call rather than the apply that was confusing me.

R.

[Robert Lally]

Thanks Rich.

2008/8/30 Rich Hickey <richh...@gmail.com>