Another Exercise From HTDP

Another exercise from Chapter 1 of How to Design Programs.

My answer for exercise 9 in Chapter 1.

1
2
3
4
5
6
7
8
9

(define (in expression)
  (cond
    [(string? expression) (string-length expression)]
    [(image? expression) (* (image-height expression) (image-width expression))]
    [(number? expression)
     (if (<= expression 0)
         expression
         (- expression 1))]
    [(boolean? expression) (if expression 10 20)]))

A few more points lest I forget:

1. It’s very handy to format your racket code. Simply use raco to install the fmt package (raco pkg install fmt). Once you do that the command line is something like: raco fmt --width #your_preferred_maxwidth --indent #your_preferred_indentation -i #racket_source_file. In my case it’s this raco fmt --width 80 --indent 3 -i htdp_chapter1_ex9.rkt.

2. Make sure if you do anything in DrRacket that you save your definitions as text. Otherwise the formatter won’t be able to do a thing with the source. Saving as text is under the Save Other menu in DrRacket.