【SICP练习】111 练习3.24

练习3-24

原文

Exercise 3.24. In the table implementations above, the keys are tested for equality using equal? (called by assoc). This is not always the appropriate test. For instance, we might have a table with numeric keys in which we don’t need an exact match to the number we’re looking up, but only a number within some tolerance of it. Design a table constructor make-table that takes as an argument a same-key? procedure that will be used to test “equality” of keys. Make-table should return a dispatch procedure that can be used to access appropriate lookup and insert! procedures for a local table.

代码

(define (make-table same-key?)
    (let ((local-table (list '*table*)))
       (define (lookup key-1 key-2)
          (let ((subtable (assoc key-1 (cdr local-table))))
             (if subtable
                 (let ((record (assoc key-2 (cdr subtable)))
                    (if record
                        (cdr record)
                        false))
                false)))
      (define (insert! key-1 key-2 value)
          (let ((subtable (assoc key-1 (cdr local-table))))
             (if subtable
                 (let ((record (assoc key-2 (cdr subtable))))
                      (if record
                          (set-cdr! record value)
                          (set-cdr! subtable
                                    (cons (key-2 value)
                                          (cdr subtable)))))
                    (set-cdr! local-table
                              (cons (list key-1
                                          (cons key-2 value))
                                    (cdr local-table)))))
            'ok)
        (define (assoc key records)
            (cond ((null? records)
                    false)
                  ((same-key? key (caar records))  
                    (car records))
                  (else
                    (assoc key (cdr records)))))      
        (define (dispatch m)
            (cond ((eq? m 'lookup-proc) lookup)
                  ((eq? m 'insert-proc!) insert!)
                  (else 
                    (error "Unknown operation -- TABLE" m))))
        dispatch))

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