PROSAGA码农传奇-lisp-球拍：双冒号（：:)是什么意思？

<div class =“post-text”itemprop =“text”>
  
    这里有一个扩展的切片示例：
  
  
    <a href="https://docs.racket-lang.org/math/array_slicing.html" rel="nofollow noreferrer">
      https://docs.racket-lang.org/math/array_slicing.html
    </A>
  
  
    找不到任何将切片设置为常量的东西。
我继续实施这样的事情见下文。
  
  
    首先让我们尝试一些例子。
  
   <pre>
 <code>
 #lang racket
(require math/array)

;;; 1d array

; xc : vector -> integer
;  get first coordinate (the "x coordinate")
(define (xc v) 
  (vector-ref v 0))

;; Build a 1d array with elements 0, 1, ..., 9
(define A
  (array->mutable-array
   (build-array
    #(10)  ; one axis (the x-axis) range 0, 1, ..., 9
    xc)))  ; use index as-is

;; Show the 1d array
A

;; Let's set the middle part to zero.
;; First we make a slice

(:: 4 7)

;; Then we use it to set elements in the arrau

(array-slice-set! A (list (:: 4 7)) (array #[0 0 0]))
A ; now the tree middle elements are 0

;; Rather than write the zero array our-selves we can use make-array.
(array-slice-set! A (list (:: 4 7)) (make-array #(3) 1))
A

;;; nd array

(define (coordinates v) (vector->list v))

(define B
  (array->mutable-array
   (build-array
    #(4 4 4)  ; three axes of size 4
    coordinates)))

;;; Let set the all entries except the "edge" ones to (x x x)

(array-slice-set! B (list (:: 1 3) (:: 1 3) (:: 1 3))
                  (make-array (vector (- 3 1) (- 3 1) (- 3 1)) '(x x x)))
B

;;; Now to avoid constructing the constant array, we will use the
;;; array-slice-set-constant! see below.

(require "array-slice-set-constant.rkt")
(array-slice-set-constant! B (list (:: 1 3) (:: 1 3) (:: 1 3)) '(y y y))
B

</code>
 </pre>
  
    输出是：
  
   <pre>
 <code>
 Welcome to DrRacket, version 6.12 [3m].
Language: racket, with debugging.
(mutable-array #[0 1 2 3 4 5 6 7 8 9])
(:: 4 7 1)
(mutable-array #[0 1 2 3 0 0 0 7 8 9])
(mutable-array #[0 1 2 3 1 1 1 7 8 9])
(mutable-array
 #[#[#['(0 0 0) '(0 0 1) '(0 0 2) '(0 0 3)]
 #['(0 1 0) '(0 1 1) '(0 1 2) '(0 1 3)]
 #['(0 2 0) '(0 2 1) '(0 2 2) '(0 2 3)]
 #['(0 3 0) '(0 3 1) '(0 3 2) '(0 3 3)]]
 #[#['(1 0 0) '(1 0 1) '(1 0 2) '(1 0 3)]
 #['(1 1 0) '(1 1 1) '(1 1 2) '(1 1 3)]
 #['(1 2 0) '(1 2 1) '(1 2 2) '(1 2 3)]
 #['(1 3 0) '(1 3 1) '(1 3 2) '(1 3 3)]]
 #[#['(2 0 0) '(2 0 1) '(2 0 2) '(2 0 3)]
 #['(2 1 0) '(2 1 1) '(2 1 2) '(2 1 3)]
 #['(2 2 0) '(2 2 1) '(2 2 2) '(2 2 3)]
 #['(2 3 0) '(2 3 1) '(2 3 2) '(2 3 3)]]
 #[#['(3 0 0) '(3 0 1) '(3 0 2) '(3 0 3)]
 #['(3 1 0) '(3 1 1) '(3 1 2) '(3 1 3)]
 #['(3 2 0) '(3 2 1) '(3 2 2) '(3 2 3)]
 #['(3 3 0) '(3 3 1) '(3 3 2) '(3 3 3)]]])
(mutable-array
 #[#[#['(0 0 0) '(0 0 1) '(0 0 2) '(0 0 3)]
 #['(0 1 0) '(0 1 1) '(0 1 2) '(0 1 3)]
 #['(0 2 0) '(0 2 1) '(0 2 2) '(0 2 3)]
 #['(0 3 0) '(0 3 1) '(0 3 2) '(0 3 3)]]
 #[#['(1 0 0) '(1 0 1) '(1 0 2) '(1 0 3)]
 #['(1 1 0) '(x x x) '(x x x) '(1 1 3)]
 #['(1 2 0) '(x x x) '(x x x) '(1 2 3)]
 #['(1 3 0) '(1 3 1) '(1 3 2) '(1 3 3)]]
 #[#['(2 0 0) '(2 0 1) '(2 0 2) '(2 0 3)]
 #['(2 1 0) '(x x x) '(x x x) '(2 1 3)]
 #['(2 2 0) '(x x x) '(x x x) '(2 2 3)]
 #['(2 3 0) '(2 3 1) '(2 3 2) '(2 3 3)]]
 #[#['(3 0 0) '(3 0 1) '(3 0 2) '(3 0 3)]
 #['(3 1 0) '(3 1 1) '(3 1 2) '(3 1 3)]
 #['(3 2 0) '(3 2 1) '(3 2 2) '(3 2 3)]
 #['(3 3 0) '(3 3 1) '(3 3 2) '(3 3 3)]]])
(mutable-array
 #[#[#['(0 0 0) '(0 0 1) '(0 0 2) '(0 0 3)]
 #['(0 1 0) '(0 1 1) '(0 1 2) '(0 1 3)]
 #['(0 2 0) '(0 2 1) '(0 2 2) '(0 2 3)]
 #['(0 3 0) '(0 3 1) '(0 3 2) '(0 3 3)]]
 #[#['(1 0 0) '(1 0 1) '(1 0 2) '(1 0 3)]
 #['(1 1 0) '(y y y) '(y y y) '(1 1 3)]
 #['(1 2 0) '(y y y) '(y y y) '(1 2 3)]
 #['(1 3 0) '(1 3 1) '(1 3 2) '(1 3 3)]]
 #[#['(2 0 0) '(2 0 1) '(2 0 2) '(2 0 3)]
 #['(2 1 0) '(y y y) '(y y y) '(2 1 3)]
 #['(2 2 0) '(y y y) '(y y y) '(2 2 3)]
 #['(2 3 0) '(2 3 1) '(2 3 2) '(2 3 3)]]
 #[#['(3 0 0) '(3 0 1) '(3 0 2) '(3 0 3)]
 #['(3 1 0) '(3 1 1) '(3 1 2) '(3 1 3)]
 #['(3 2 0) '(3 2 1) '(3 2 2) '(3 2 3)]
 #['(3 3 0) '(3 3 1) '(3 3 2) '(3 3 3)]]])

</code>
 </pre>
  
    文件“array-slice-set-constant.rkt”包含：
  
   <pre>
 <code>
 #lang typed/racket/base
(provide array-slice-set-constant!)

(require math/private/array/for-each
         math/private/array/array-struct
         math/private/array/typed-array-indexing
         math/private/array/array-constructors
         math/private/array/for-each
         math/private/array/utils)

(: array-indexes-set-constant! (All (A) ((Settable-Array A) (Array In-Indexes) A -> Void)))
(define (array-indexes-set-constant! arr idxs constant)
  (define ds        (array-shape idxs))
  (define idxs-proc (unsafe-array-proc idxs))
  (for-each-array-index ds (锟斤拷 (js) (array-set! arr (idxs-proc js) constant))))

(: array-slice-set-constant! (All (A) ((Settable-Array A) (Listof Slice-Spec) A -> Void)))
(define (array-slice-set-constant! arr slices val)
  (let ([idxs  (parameterize ([array-strictness #f])
                 (array-slice-ref (indexes-array (array-shape arr)) slices))])
    (array-indexes-set-constant! arr idxs val)))

</code>
 </pre>
  
    我们应该将该功能放入标准库中。
  
</DIV>

<div class =“post-text”itemprop =“text”>
  
     <code>
 ::
 </code>
    <a href="https://docs.racket-lang.org/math/array_slicing.html" rel="nofollow noreferrer">
      似乎
    </A>
     一个
    <a href =“https://docs.racket-lang.org/math/array_indexing.html#%28def._%28%28lib._math%2Farray..rkt%29._~3a~3a%29%29 “rel =”nofollow noreferrer“>
      程序定义于
       <code>
 math/array
 </code>
    </A>
    ：
  
  <BLOCKQUOTE>
    
      创建
      <a href =“https://docs.racket-lang.org/math/array_indexing.html#%28form._%28%28lib._math%2Farray..rkt%29._.Slice%29%29"rel =“nofollow noreferrer”>
         <code>
 Slice
 </code>
      </A>
       使用的对象
      <a href =“https://docs.racket-lang.org/math/array_indexing.html#%28def._%28%28lib._math%2Farray..rkt%29._~3a~3a%29%29 “rel =”nofollow noreferrer“>
         <code>
 ::
 </code>
      </A>
       和
      <a href =“https://docs.racket-lang.org/math/array_indexing.html#%28form._%28%28lib._math%2Farray..rkt%29._.Slice-.New-.Axis ％29％29“rel =”nofollow noreferrer“>
         <code>
 Slice-New-Axis
 </code>
      </A>
       使用的对象
      <a href =“https://docs.racket-lang.org/math/array_indexing.html#%28def._%28%28lib._math%2Farray..rkt%29._~3a~3anew%29%29 “rel =”nofollow noreferrer“>
         <code>
 ::new
 </code>
      </A>
      。只有一个
      <a href =“https://docs.racket-lang.org/math/array_indexing.html#%28form._%28%28lib._math%2Farray..rkt%29._.Slice-.Dots%29% 29“rel =”nofollow noreferrer“>
         <code>
 Slice-Dots
 </code>
      </A>
       对象，即
      <a href =“https：//docs.racket-lang.org/math/array_indexing.html#%28def ._％28％28lib._math％2Farray..rkt％29._〜3a~3a .... ..％29％29“rel =”nofollow noreferrer“>
         <code>
 ::...
 </code>
      </A>
      。
    
  </BLOCKQUOTE>
  
    我不认为它通常适用于Racket的其他部分。
  
</DIV>