06_control_flow.rkt

#lang racket ; defines the language we are using

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;; 6. Control Flow
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;;; Conditionals

(if #t               ; test expression
    "this is true"   ; then expression
    "this is false") ; else expression
; => "this is true"

;; In conditionals, all non-#f values are treated as true
(member 'Groucho '(Harpo Groucho Zeppo)) ; => '(Groucho Zeppo)
(if (member 'Groucho '(Harpo Groucho Zeppo))
    'yep
    'nope)
; => 'yep

;; `cond' chains a series of tests to select a result
(cond [(> 2 2) (error "wrong!")]
      [(< 2 2) (error "wrong again!")]
      [else 'ok]) ; => 'ok

;;; Pattern Matching

(define (fizzbuzz? n)
  (match (list (remainder n 3) (remainder n 5))
    [(list 0 0) 'fizzbuzz]
    [(list 0 _) 'fizz]
    [(list _ 0) 'buzz]
    [_          #f]))

(fizzbuzz? 15) ; => 'fizzbuzz
(fizzbuzz? 37) ; => #f

;;; Loops

;; Looping can be done through (tail-) recursion
(define (loop i)
  (when (< i 10)
    (printf "i=~a\n" i)
    (loop (add1 i))))
(loop 5) ; => i=5, i=6, ...

;; Similarly, with a named let
(let loop ([i 0])
  (when (< i 10)
    (printf "i=~a\n" i)
    (loop (add1 i)))) ; => i=0, i=1, ...

;; See below how to add a new `loop' form, but Racket already has a very
;; flexible `for' form for loops:
(for ([i 10])
  (printf "i=~a\n" i)) ; => i=0, i=1, ...
(for ([i (in-range 5 10)])
  (printf "i=~a\n" i)) ; => i=5, i=6, ...

;;; Iteration Over Other Sequences
;; `for' allows iteration over many other kinds of sequences:
;; lists, vectors, strings, sets, hash tables, etc...

(for ([i (in-list '(l i s t))])
  (displayln i))

(for ([i (in-vector #(v e c t o r))])
  (displayln i))

(for ([i (in-string "string")])
  (displayln i))

(for ([i (in-set (set 'x 'y 'z))])
  (displayln i))

(for ([(k v) (in-hash (hash 'a 1 'b 2 'c 3))])
  (printf "key:~a value:~a\n" k v))

;;; More Complex Iterations

;; Parallel scan of multiple sequences (stops on shortest)
(for ([i 10] [j '(x y z)]) 
  (printf "~a:~a\n" i j))
; => 0:x 1:y 2:z

;; Nested loops
(for* ([i 2] [j '(x y z)]) 
  (printf "~a:~a\n" i j))
; => 0:x, 0:y, 0:z, 1:x, 1:y, 1:z

;; Conditions
(for ([i 1000]
      #:when (> i 5)
      #:unless (odd? i)
      #:break (> i 10))
  (printf "i=~a\n" i))
; => i=6, i=8, i=10

;;; Comprehensions
;; Very similar to `for' loops -- just collect the results

(for/list ([i '(1 2 3)])
  (add1 i)) ; => '(2 3 4)

(for/list ([i '(1 2 3)] #:when (even? i))
  i) ; => '(2)

(for/list ([i 10] [j '(x y z)])
  (list i j)) ; => '((0 x) (1 y) (2 z))

(for/list ([i 1000] #:when (> i 5) #:unless (odd? i) #:break (> i 10))
  i) ; => '(6 8 10)

(for/hash ([i '(1 2 3)])
  (values i (number->string i)))
; => '#hash((1 . "1") (2 . "2") (3 . "3"))

;; There are many kinds of other built-in ways to collect loop values:
(for/sum ([i 10]) (* i i)) ; => 285
(for/product ([i (in-range 1 11)]) (* i i)) ; => 13168189440000
(for/and ([i 10] [j (in-range 10 20)]) (< i j)) ; => #t
(for/or ([i 10] [j (in-range 0 20 2)]) (= i j)) ; => #t
;; And to use any arbitrary combination, use `for/fold'
(for/fold ([sum 0]) ([i '(1 2 3 4)]) (+ sum i)) ; => 10
;; (This can often replace common imperative loops)

;;; Exceptions

;; To catch exceptions, use the `with-handlers' form
(with-handlers ([exn:fail? (lambda (exn) 999)])
  (+ 1 "2")) ; => 999
(with-handlers ([exn:break? (lambda (exn) "no time")])
  (sleep 3)
  "phew") ; => "phew", but if you break it => "no time"

;; Use `raise' to throw exceptions or any other value
(with-handlers ([number?    ; catch numeric values raised
                 identity]) ; return them as plain values
  (+ 1 (raise 2))) ; => 2