How do I print a list of numbers on each line in clojure?

Question

how can I print a list of n, say 10, numbers on 10 lines? I just learned about loop and recur, but cannot seem to combine a side-effect (println i) with (recur (+ i 1)) in a loop form. Just to be very clear: I'd like output like this:

when n is 10.

score 16 · Answer 1 · answered Jun 29 '11 at 17:05

16

You can use doseq for this, which is meant to be used when iteration involves side effects,

(doseq [i (range 10)]
   (println i))

You could use map as pointed but that will produce a sequence full of nils which is both not idiomatic and wastes resources also doseq is not lazy so no need to force it with doall.

answered Jun 29 '11 at 17:05

Hamza Yerlikaya

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This prints number 0 to 9; using (inc i) in the println, as shown in the other answers below, gives 1 to 10. – Roger Jun 30 '11 at 07:10
1

instead of inc just do (range 1 11) – Hamza Yerlikaya Jul 01 '11 at 15:32

score 11 · Answer 2 · answered Jun 29 '11 at 17:35

11

I suggest dotimes for this kind of simple loop:

(dotimes [i 10]
  (println (inc i)))

Note that dotimes is non-lazy, so it is good for things like println that cause side effects.

answered Jun 29 '11 at 17:35

mikera

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score 3 · Answer 3 · answered Jun 29 '11 at 12:41

3

With loop/recur:

(loop [i 1]
  (when (<= i 10)
    (println i)
    (recur (inc i))))

However, it's more idiomatic (read: more "Clojuristic") to map the function println over the numbers in 1..10. But because map returns a lazy sequence, you must force its evaluation with doall:

(doall (map println (range 1 (inc 10))))

answered Jun 29 '11 at 12:41

Leonel

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In fact it is more idiomatic to use `doseq` (or `dotimes`) in this case, because it is all about side-effects. `map` should only be used if you are interested in the return value. Otherwise you get easily caught by laziness. – kotarak Jun 29 '11 at 14:52
So that would be (dotimes [i 10] (println i)) Thanks! Will remember that when I get to chapter 14 in 'Practical Clojure' (now finishing up chapter 3) :-) – Roger Jun 29 '11 at 16:38

score 0 · Answer 4 · edited Jul 17 '15 at 00:23

0

If you only want to print the output on the screen, you might also simply put a (println i) before entering your conditional:

(loop [i 0]
  (println i)
  (if (< i 10)
    (recur (inc i))
    (println "done!")))

And the output will be one number per line.

edited Jul 17 '15 at 00:23

josliber

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answered Jul 17 '15 at 00:15

Arthur Ulfeldt · Answer 5 · 2011-06-29T23:16:15.167

0

And just to be comprehensive you can do it with map also:

(doseq (map #(println %) (range 10))

edited Jun 29 '11 at 23:16

answered Jun 29 '11 at 17:34

Arthur Ulfeldt

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How do I print a list of numbers on each line in clojure?

5 Answers5