diff options
| -rw-r--r-- | racket.html.markdown | 90 |
1 files changed, 63 insertions, 27 deletions
diff --git a/racket.html.markdown b/racket.html.markdown index 006d6b86..4b5465d9 100644 --- a/racket.html.markdown +++ b/racket.html.markdown @@ -324,40 +324,30 @@ m ; => '#hash((b . 2) (a . 1) (c . 3)) <-- no `d' ;;; Loops -;; looping can be done through recursion +;; Looping can be done through (tail-) recursion (define (loop i) (when (< i 10) - (printf "i:~a\n" i) + (printf "i=~a\n" i) (loop (add1 i)))) +(loop 5) ; => i=5, i=6, ... -(loop 5) ; => i:5 i:6 ... - -;; similarly, with a named let +;; Similarly, with a named let (let loop ((i 0)) (when (< i 10) - (printf "i:~a\n" i) - (loop (add1 i)))) ; => i:0 i:1 ... - -;;; Comprehensions - -(for/list ([i '(1 2 3)]) - (add1 i)) ; => '(2 3 4) - -(for/list ([i '(1 2 3)] #:when (even? i)) - i) ; => '(2) - -(for/hash ([i '(1 2 3)]) - (values i (number->string i))) -; => '#hash((1 . "1") (2 . "2") (3 . "3")) - -;; To combine iteration results, use `for/fold' -(for/fold ([sum 0]) ([i '(1 2 3 4)]) - (+ sum i)) ; => 10 - -;;; Sequences - -;; `for' allows iteration over sequences: + (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, ... + +;;; 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)) @@ -373,6 +363,52 @@ m ; => '#hash((b . 2) (a . 1) (c . 3)) <-- no `d' (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 |