Re: All subsets of a vector

[neveu]

On Nov 9, 2:47 pm, Shoeb Bhinderwala shoeb.bhinderw...@gmail.com
wrote:
 ([a] [a b] [a b c] [a b c d])

It should be pointed out that this result -- while it may be what you
actually want -- is not all subsets of [a b c d]. For that
you might want to use combinatorics/subsets:

user= (C/subsets [a b c d])
(() (a) (b) (c) (d) (a b) (a c) (a d) (b c)
(b d) (c d) (a b c) (a b d) (a c d) (b c
d) (a b c d))

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loop/recur stack overflow

[neveu]

I implemented the Flavius Josephus algorithm from Programming Praxis
in Clojure using loop/recur. My first version looked like this:

(defn rotate-left
  ([ln n m]
 (take n (drop (dec m) (cycle ln
  ([ln m] (rotate-left ln (count ln) m)))

(defn josephus3 [N M] ;; execute every mth soldier
  (loop [soldiers (range N)
 m M
 deceased '()]
(let [n (count soldiers)]
  (cond
 (zero? n) deceased
 true (let [r (rotate-left soldiers n m)
dead (cons (first r) deceased)
survivors (rest r)]
(recur survivors m dead))

This works fine, but I don't really need to count the soldiers every
pass, it always decreases by 1. So I wrote this version in which n is
one of the loop parameters:

(defn josephus4 [N M] ;; execute every mth soldier
  (loop [soldiers (range N)
 n N
 m M
 deceased '()]
(cond
 (zero? n) deceased
 true (let [r (rotate-left soldiers n m)
dead (cons (first r) deceased)
survivors (rest r)]
(recur survivors (dec n)  m dead)

Both work for small N (e.g. 41) but the second one fails for large
(41000) N with a stack overflow.
What am I missing here?

Thanks,
Charles

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