StackOverflow when using Iterate in Clojure

Question

I'm trying to write a function in Clojure that finds the first x prime numbers. I wrote these functions:

(defn isprime? [n]
  (if (empty? (filter #(= 0 (mod n  %)) (range 2 n)))
    n
    0)
  )

(defn listprimes [nums]
    (if (first nums)
      (cons (isprime? (first nums)) (listprimes (rest nums)))
      [])
  )

The first one checks if a given number is prime or not, retruns it if it is or 0 id it isn't. The second gets a vector of numbers and activates the first function on each element. So, if my input to listprimes is [1 2 3 4 5 6] the output will be [1 2 3 0 5 0].

I was planning to use filter in the following way (for any x):

(take x (filter #(== 0 %) (listprimes(iterate inc 0)))

But I get a StackOverflow.. Any ideas what am I doing wrong?

Thanks!

Possible duplicate of Recursive function causing a stack overflow — bereal
– bereal, Commented Jan 31, 2016 at 12:02
@bereal Not the same. This one fails for lack of laziness. The question you refer to fails for too much! — Thumbnail
– Thumbnail, Commented Jan 31, 2016 at 12:57

Thumbnail · Accepted Answer · 2016-01-31 17:06:28Z

3

listprimes is recursive and not lazy. Supplying it with the endless (iterate inc 0) is bound to overflow the stack, as each number nests another function call.

A lazy version of listprimes is ...

(defn listprimes [nums]
  (map isprime? nums))

Then, for example,

(take 20 (filter #(== 0 %) (listprimes (iterate inc 0))))

yields ...

;(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0)

... not informative, but at least it terminates.

By the way,

isprime? does too much for its name. Peel it of the if form, leaving ...

(defn isprime? [n]
  (empty? (filter #(= 0 (mod n  %)) (range 2 n))))

For example,

(filter isprime? (range 2 20))
;(2 3 5 7 11 13 17 19)

Then the sort of result you want is better expressed thus:

(take 20 (remove isprime? (iterate inc 0)))
;(4 6 8 9 10 12 14 15 16 18 20 21 22 24 25 26 27 28 30 32)

edited Jan 31, 2016 at 17:06

answered Jan 31, 2016 at 12:13

Thumbnail

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2 Comments

shakedzy Over a year ago

Nope.. changes nothing.

John Szakmeister Over a year ago

@shakedzy Are you still trying to use your listprimes function? I believe Thumbnail is arguing that you shouldn't. Just use filter to get the list of primes. Doing so does change the behavior for me. You're version SOs on (range 2 10000), while I have no issues with this version. And it does get rid of the recursion introduced is listprimes. If you're keen on having similar output of listprimes, you could redefine it to be: (defn listprimes [nums] (map #(if (isprime? %) % 0) nums)) with this version of isprime?.

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