70 lines
2.2 KiB
Racket
70 lines
2.2 KiB
Racket
;;;
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;;; <delete.rkt> ---- List deletion functions
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;;; Time-stamp: <02/03/01 07:26:12 noel>
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;;;
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;;; Copyright (C) 2002 by Noel Welsh.
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;;;
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;;; This file is part of SRFI-1.
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;;; This SRFI-1 implementation is distributed under the same terms as
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;;; Racket.
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;;; Author: Noel Welsh <noelwelsh@yahoo.com>
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;; Commentary:
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;; Based on the reference implementation by Olin Shiver and hence:
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;; Copyright (c) 1998, 1999 by Olin Shivers. You may do as you please with
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;; this code as long as you do not remove this copyright notice or
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;; hold me liable for its use. Please send bug reports to shivers@ai.mit.edu.
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;; -Olin
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;; Olin Shivers verified that he is fine with redistributing this code
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;; under the LGPL. (Verified personally by Eli Barzilay.)
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#lang scheme/base
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(require srfi/optional "predicate.rkt")
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(provide delete (rename-out [delete delete!])
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delete-duplicates (rename-out [delete-duplicates delete-duplicates!]))
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(define (delete x lis [= equal?])
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(filter (lambda (y) (not (= x y))) lis))
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#; ; lists are immutable
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(define delete! (x lis [= equal?])
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(filter! (lambda (y) (not (= x y))) lis))
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;; right-duplicate deletion
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; delete-duplicates delete-duplicates!
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;;
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;; Beware -- these are N^2 algorithms. To efficiently remove duplicates
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;; in long lists, sort the list to bring duplicates together, then use a
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;; linear-time algorithm to kill the dups. Or use an algorithm based on
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;; element-marking. The former gives you O(n lg n), the latter is linear.
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(define (delete-duplicates lis [elt= equal?])
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(check-arg procedure? elt= 'delete-duplicates)
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(let recur ((lis lis))
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(if (null-list? lis) lis
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(let* ((x (car lis))
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(tail (cdr lis))
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(new-tail (recur (delete x tail elt=))))
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(if (eq? tail new-tail) lis (cons x new-tail))))))
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#; ; lists are immutable
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(define (delete-duplicates! lis [elt= equal?])
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(check-arg procedure? elt= 'delete-duplicates!)
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(let recur ((lis lis))
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(if (null-list? lis) lis
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(let* ((x (car lis))
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(tail (cdr lis))
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(new-tail (recur (delete! x tail elt=))))
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(if (eq? tail new-tail) lis (cons x new-tail))))))
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;;; delete.rkt ends here
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