shortcut for for comprehension

[Michael Hunger]

Is it possible to use :while to shortcut a for macro when a certain number of yiels have happened?

e.g. (for [x (range 1000) :when (= (rem x) 1) :while (number of yields <= 10)]

so i want only the first 10 results.

Or should I just use (take 10 ) on the for ?

Michael

[Timothy Pratley]

As far as I know the number of yields is not available for testing, so
you have to use take
user=> (take 10 (for [x (range 1000) :when (= (rem x 2) 1)] x))
(1 3 5 7 9 11 13 15 17 19)

[Christopher Taylor]

On 21.04.2009, at 00:19, Michael Hunger wrote:

>
> Is it possible to use :while to shortcut a for macro when a certain
> number of yiels have happened?
>
> e.g. (for [x (range 1000) :when (= (rem x) 1) :while (number of
> yields <= 10)]
>
> so i want only the first 10 results.

you could:
(def zip (partial map vector))

and then use:

user> (for [[i x] (zip (iterate inc 0) (range 10 -10 -1))
:while (< i 5)]
x)
(10 9 8 7 6)

> Or should I just use (take 10 ) on the for ?

that's probably more readable and works just as well ;).

hth,
--Chris

[Alvaro Vilanova Vidal]

I am a newbie,  but seems that "for comprehension" are lazy lists:

user> (take 5 (for [x (range 20) :when (do (printf "*%d* " x) (= (rem x 2) 1))] x))
(*0* *1* *2* *3* 1 *4* *5* 3 *6* *7* 5 *8* *9* 7 9)

So, I think that you should use take :)

2009/4/21 Michael Hunger <clo...@jexp.de>

[David Sletten]

On Apr 20, 2009, at 12:19 PM, Michael Hunger wrote:

>
> Is it possible to use :while to shortcut a for macro when a certain
> number of yiels have happened?
>
> e.g. (for [x (range 1000) :when (= (rem x) 1) :while (number of
> yields <= 10)]
>
> so i want only the first 10 results.

Is it possible? Yes...
(let [yields (ref 0)]
(for [x (range 1000) :when (when (odd? x) (dosync (ref-set yields
(inc @yields))) true) :while (<= @yields 10)] x)) =>

(1 3 5 7 9 11 13 15 17 19)

Is it advisable? No.

>
> Or should I just use (take 10 ) on the for ?

Definitely yes.

Aloha,
David Sletten