SICP Exercise 1.17 Aborting!: maximum recursion depth exceeded

Question

I solve it by php，and it works.but I try to use scheme,I receive the " Aborting!: maximum recursion depth exceeded" error. I use MIT/GNU Scheme microcode 15.3 .Here is the code.

php

function cc($a,$b)
{
    if($b==1){
        return $a;
    }elseif($b%2!==0){
        return $a+cc($a,$b-1);
    }else{
        return cc(double1($a),halve($b));
    }
}
function double1($i)
{
    return 2*$i;
}
function halve($i)
{
    return $i/2;
}

scheme

(define (cc a b)
  (cond ((= b 1) a))
  ((odd? b) (+ a (cc a (- b 1))))
  (else (cc (double a) (halve b)))
  )
(define (double n)
  (+ n n)
  )
(define (halve n)
  (/ n 2)
  )

Answer 1

Your Scheme version isn't quite correct. It's more like this PHP version:

function cc($a, $b){
  if ($b === 1) { return $a; }

  call_user_func($b%2!==0, 
                 $a + cc($a, $b-1) );

  return else(cc(double1($a), halve($b)) );
}

Perhaps this is a better version:

(define (cc a b)
  (cond ((= b 1) a)
        ((odd? b) (+ a (cc a (- b 1))))
        (else (cc (double a) (halve b)))))

Notice the identation reflects the move of ) from the first line of cond to the very last.