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Add remove-index to racket/list#2547

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Le 8000 ifAndersen:remove-index
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Add remove-index to racket/list#2547
LeifAndersen wants to merge 1 commit intoracket:masterfrom
LeifAndersen:remove-index

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@LeifAndersen
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@LeifAndersen LeifAndersen commented Mar 20, 2019

Its surprisingly missing.

(This initial commit doesn't add documents and tests. They should obviously be added before merging.)

@gus-massa
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gus-massa commented Mar 21, 2019

I'm not sure if this is the usual definition somewhere, but I expect (remove-index '(0 1 2 3) 99), similar to the error generated by (list-ref '(0 1 2 3) 99). (I'm not 100% sure what is better here.)

Also, I expect that the tail of the list is preserved, so

(define l (list 0 1 2 3 4))
(eq? (cdr (remove-index l 1))
     (cddr l) )                ; ==> #t

@AlexKnauth
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AlexKnauth commented Apr 4, 2019

A definition of remove-index that meets @gus-massa's suggestions could be something like this:

;; remove-index : [Listof X] Nat -> [Listof X]
;; Removes the element at the given index in the list, such that
;; (take (remove-index ls idx) idx) stays equal? to (take ls idx), and
;; (drop (remove-index ls idx) idx) is eq? to (drop ls (add1 idx)).
(define (remove-index ls idx)
  (unless (list? ls)
    (raise-argument-error 'remove-index "list?" 0 ls idx))
  (unless (exact-nonnegative-integer? idx)
    (raise-argument-error 'remove-index "exact-nonnegative-integer?" 1 ls idx))
  (let loop ([lst ls] [i idx])
    (unless (pair? lst)
      (raise-arguments-error 'remove-index
                             "index is too big for list"
                             "list" ls
                             "index" idx))
    (cond [(zero? i) (cdr lst)]
          [else      (cons (car lst) (loop (cdr lst) (sub1 i)))])))

(module+ test
  (require rackunit)
  (define l (list 0 1 2 3 4))
  (check-eq? (cdr (remove-index l 1))
             (cddr l))
  (check-equal? (take (remove-index l 3) 3)
                (take l 3))
  (check-eq? (drop (remove-index l 2) 2)
             (drop l (add1 2)))
  (check-eq? (drop (remove-index l 3) 3)
             (drop l (add1 3)))
  (check-exn #rx"index is too big for list.*index: *99"
             (λ () (remove-index '(0 1 2 3) 99))))

@shhyou shhyou added the api design Related to the design of core libraries and language features label Nov 17, 2023
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@LeifAndersen @gus-massa @AlexKnauth @shhyou
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