The :while modifier (list comprehensions)
I find the :while modifier non intuitive user= (for [x (range 1 10) y (range 1 10) :while (= y 2)] [x y]) () user= (for [x (range 1 10) y (range 1 10) :while (= x 2)] [x y]) ([2 1] [2 2] [2 3] [2 4] [2 5] [2 6] [2 7] [2 8] [2 9]) My (false) intuition told me that both expressions would have been evaluated to an empty sequence. Could someone explain the rationale behind the :while modifier? --~--~-~--~~~---~--~~ You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~--~~~~--~~--~--~---
Re: The :while modifier (list comprehensions)
On Mon, Aug 3, 2009 at 2:47 AM, Jonasjonas.enl...@gmail.com wrote: I find the :while modifier non intuitive user= (for [x (range 1 10) y (range 1 10) :while (= y 2)] [x y]) () user= (for [x (range 1 10) y (range 1 10) :while (= x 2)] [x y]) ([2 1] [2 2] [2 3] [2 4] [2 5] [2 6] [2 7] [2 8] [2 9]) My (false) intuition told me that both expressions would have been evaluated to an empty sequence. Could someone explain the rationale behind the :while modifier? :while only bails from the loop on which it is placed. In your examples that's the 'y' loop, so the 'x' loop is unaffected by your :while and proceeds through its entire range in both examples. In the first example, the 'y' loop never produces anything so the result is empty. The :while modifier for 'doseq' behaves the same way. This has come up before: http://clojure-log.n01se.net/date/2009-06-12.html#15:02a-15:33 --Chouser --~--~-~--~~~---~--~~ You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~--~~~~--~~--~--~---
Re: The :while modifier (list comprehensions)
Hi, On Aug 3, 8:47 am, Jonas jonas.enl...@gmail.com wrote: I find the :while modifier non intuitive user= (for [x (range 1 10) y (range 1 10) :while (= y 2)] [x y]) () user= (for [x (range 1 10) y (range 1 10) :while (= x 2)] [x y]) ([2 1] [2 2] [2 3] [2 4] [2 5] [2 6] [2 7] [2 8] [2 9]) My (false) intuition told me that both expressions would have been evaluated to an empty sequence. Could someone explain the rationale behind the :while modifier? The :while modifier works on the clause just before it. That means that in your case the :while works on the `y` clause, but there `x` is constant. So you get a result when `x` is equal to two. When you put the :while at the `x` clause you get the expected empty seq. user= (for [x (range 1 10) :while (= x 2) y (range 1 10)] [x y]) () Hope this helps. Sincerely Meikel --~--~-~--~~~---~--~~ You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~--~~~~--~~--~--~---
Re: The :while modifier (list comprehensions)
When you put the :while at the `x` clause you get the expected empty seq. user= (for [x (range 1 10) :while (= x 2) y (range 1 10)] [x y]) () Interesting, I didn't know that. Still, the behavior of :while feels strange. I guess I'll get used to it. In the following example :while and :when are interchangeable, which is often the case when :while is used last in the list comprehension: user= (for [x (range 1 10) y (range 1 10) :while ( (+ x y) 5)] [x y]) ([1 1] [1 2] [1 3] [2 1] [2 2] [3 1]) user= (for [x (range 1 10) y (range 1 10) :when ( (+ x y) 5)] [x y]) ([1 1] [1 2] [1 3] [2 1] [2 2] [3 1]) --~--~-~--~~~---~--~~ You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~--~~~~--~~--~--~---
Re: The :while modifier (list comprehensions)
Hi, Am 03.08.2009 um 16:56 schrieb Jonas Enlund: In the following example :while and :when are interchangeable, which is often the case when :while is used last in the list comprehension: user= (for [x (range 1 10) y (range 1 10) :while ( (+ x y) 5)] [x y]) ([1 1] [1 2] [1 3] [2 1] [2 2] [3 1]) user= (for [x (range 1 10) y (range 1 10) :when ( (+ x y) 5)] [x y]) ([1 1] [1 2] [1 3] [2 1] [2 2] [3 1]) I would suspect, that :while stops earlier than :when. So while the result is the same, I would suspect that the :while has a (even if tiny) performance advantage. Sincerely Meikel smime.p7s Description: S/MIME cryptographic signature
