4
(ns src.helloworld)

(defn fibonacci[a b] (println a b (fibonacci (+ b 1) a + b)))

(fibonacci 0 1)

I'm new to Functional Programming and decided to start learning Clojure as it's very different from C#. I'd like to broaden my horizons.

Here's the error I get:

Clojure 1.2.0
java.lang.IllegalArgumentException:
Wrong number of args (4) passed to:
helloworld$fibonacci
(helloworld.clj:0) 1:1 user=>
#<Namespace src.helloworld> 1:2 src.helloworld=>

Math problems never were my strong suit and I never really made anything that manipulated numbers like this, so I would like any guidance you can give.

Please don't give me the entire solution.

Preferably I would like some good hints and maybe a skeleton of how it should look like.

amalloy
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Only Bolivian Here
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5 Answers5

4
(fibonacci (+ b 1) a + b)

Here you're calling the function fibonacci with four arguments: The result of (+ b 1), the value of the variable a, the function + and the value of the variable b. Since fibonacci was defined to only take two arguments, you get the error you do.

Once you fix this, you'll get a stack overflow error without seeing any output. This is because you put the recursive call to fibonacci as the argument to println. So it will try to execute the recursive call before executing println. Since the recursion is infinite, the call to println will never happen. What you should do is first print the number and then call fibonacci recursively.

Once you do this the programm will print a lot of numbers, but the stack will still overflow eventually. This is because clojure does not optimize away tail-calls, so even when using tail recursion, infinite recursion will cause a stack overflow. To prevent this use the recur form instead of normal recursion.

In addition to these points, your program will print the wrong numbers as you implemented the Fibonacci sequence wrongly.

sepp2k
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3

Here is a hint about why you got the error you did. You passed the following four arguments to fibonacci:

  • (+ b 1)
  • a
  • +
  • b.

That still won't give you a working function, and here's one hint about that: what will ever cause the code to stop executing and return a value to the user?

amalloy
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dfan
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1

The other answers explain to you the error in your function call. After fixing that you should look into using loop/recur which will do tail call optimization and thus not use the stack for each recursive call. It would be pretty hard to give an example without giving you the whole thing though :-/ Here is another thread where they calculate a factorial using recursion in clojure: Factorial

Community
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jberg
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  • It's a lot easier to make a simple-recursive function like `!` into a tail-recursive version than it is to do that for `fibonacci`, which has two self-calls. You can no longer do it mechanically; instead you have to take advantage of domain-specific knowledge about how the fibonacci recursion tree looks. It's possible in this case, but not for every doubly-recursive function. – amalloy Apr 16 '11 at 19:22
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    @amalloy: There is only one recursive call in Sergio's `fibonacci` function. – sepp2k Apr 16 '11 at 19:31
1

When I was going through some similar code katas to learn, I found this site very helpful. Coding Kata. They have the fib sequence as a yellow-belt kata. They also have solutions posted from people in multiple languages (Clojure is one of them). That way, you can compare your answer to others and see if you can pick up any tips or better way of doing things.

carinmeier
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0

When you are done, you may wish to look at Christophe Grand's Fibonacci solution in Programming Clojure by Stu Halloway. It is the most elegant solution I have seen. It is on page 137 in the 1st edition (since I've heard there is a 2nd edition coming).

Julien Chastang
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