Recursives

This weeks examples of DrRacket generated pretty cool images. But then Prof. Heap also uploaded another DrRacket file and inside were codes that he never taught, so I decided to mess around with those and see what the did/

The first thing I did was apply the function like so:
(spiral 4) 

DrRacket then told me that it didn't past the check-expects. I was taken aback at first, but then I looked at the code closely and I noticed, the code was never finished! 
Lesson 1: read carefully before taking action 

To finish the definition, I examined the check-expects:
(check-expect (spiral 0) (vertical-line 10))
(check-expect (spiral 20)
              (beside/align
               "bottom"
               (vertical-line (+ 10 (* 2 20)))
               (rotate 90 (spiral 19))))
(check-expect (spiral 2)
              (beside/align
               "bottom"
               (vertical-line (+ 10 (* 2 2)))
               (rotate 90 (spiral 1))))
Then I modified the definition like so:
(define (spiral n)
     (cond
    [(zero? n) (vertical-line 10)]
    [else
     (beside/align
      "bottom"
      (vertical-line (+ 10 (* 2 20)))
      (rotate 90 (spiral (sub1 n))))]))
...which didn't work out.
So I went back to comparing the two check-expects, then I realized that the last number line should be 'n' instead of '20' after comparing the differences between the two check-expects
After modification
(define (spiral n)
     (cond
    [(zero? n) (vertical-line 10)]
    [else
     (beside/align
      "bottom"
      (vertical-line (+ 10 (* 2 n)))
      (rotate 90 (spiral (sub1 n))))]))
I tested it out and it worked!