CODEWITHSUNDEEP
for-looprecursionscheme
Janghyup Lee
Janghyup Lee

Reputation: 167

Retuning values using recursion in Scheme

I want to make for-loop using recursion in scheme.

I wrote recursion code which stops at 2, but it doesn't return a

number.

If I write the code in JAVA,

It should be look like this.

 global varialbe int i;

public int decreasingNumber( int n)
{
 if (n == 2) 
  return -1
 else 
 {
  i = n
  return decreasingNumber(n - 1)
 }
}

So that I can get changed i, until function reaches 2.

In scheme, I tried

(define (IT n)
  (let ((i n ))

(if (= i 2) "done"


  (IT(- n 1))

   )    
  )
 )

Scheme doesn't let this code work 

(+ i 0)

before the recursion.

Upvotes: 0

Views: 53

Answers (1)

Sylwester
Sylwester

Reputation: 48745

You are making a global variable in Java and a local in Scheme. If you were to do this:

public static int decreasingNumber(int n) {
    if (n == 2) {
        return -1;
    } else { 
        int i = n; // dead code
        return decreasingNumber(n - 1);
    }
}

int minusOne = decreasingNumber(10);
System.out.println(i + 0); // compilation error!!! i doesn't exist

So you see why i wouldn't exist after calling decreasibngNumber? If you want IT do return something else than the string "done" you shoudl perhaps replace "done" with whatever you want it to return.. eg. 

(define (count-down-to-two n)
  (if (= n 2)
      -1                             ; return -1 (why?)
      (count-down-to-two (- n 1))))  ; return the result of this recursion

However seeing this can be simplified makes me want to write this instead:

; simplified version
(define (count-down-to-two n)
  -1)

Setting global variables (or lexical when closer) can be done with set!:

(set! i n) ; like i = n in java

Often you can get away by just make the procedure return whateveryou need and make your code functional and easier to test. Go through a book or tutorial to learn how to do it idiomatic rather than trying to use ideas from a totally different family of programming languages (algol family) in the this new language (in the lisp family).

Upvotes: 1

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