Clojure - Fibonacci using loop recur - comes to infinite loop

Question

I'm trying to get a Fibonacci numbers using loop-recur construction:

(defn fibo-looprecur [x]
  (loop [current 0
         next 1
         result x]
       (if (= x 0) current (recur next (+' current next) (dec result))))
)

When I run it, it works OK with 0, but when I put 1 to fibo-looprecur, it goes to an infinite loop. What could cause that behaviour?

Every time you recur back into the loop, current, next, and result take on new values. More importantly, x will always have its same original value --- if you call the function with x set to 1, it will always be 1 and never 0. Hence the loop will never exit. — jas
– jas, Commented May 15, 2018 at 9:19

status203 · Accepted Answer · 2018-05-15 10:43:30Z

5

Clojure works with values not reference types; in other words x and result do not point to the same value but are separate bindings. When you decrement result it has no effect on x, and therefore you never hit your base case of x = 0.

If you change your condition to check result instead of x you'll get the expected results.

(defn fibo-looprecur [x]
  (loop [current 0
         next 1
         result x]
    (if (= result 0) current (recur next (+' current next) (dec result)))))

(map fibo-looprecur (range 10))
=> (0 1 1 2 3 5 8 13 21 34)

answered May 15, 2018 at 10:43

status203

8766 silver badges11 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Boba_Fett Over a year ago

Indeed! Now I get it, I was trying to compare wrong part of binding. Thanks for that clue!

Collectives™ on Stack Overflow

Clojure - Fibonacci using loop recur - comes to infinite loop

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related