Problem: Determine the Number of Digits in an Integer

 (define num-digits
   (lambda (n)
     (if (< n 10)
         1
         (+ 1
            (num-digits (quotient n 10))))))

Q: How to make it work on negative integers?

A: Return result of calling on (- n)

 (define num-digits
   (lambda (n)
     (if (< n 0)
         (num-digits (- n))
         (if (< n 10)
             1
             (+ 1
                (num-digits (quotient n 10)]