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.
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| (define (make-table same-key?) | |
| (let | |
| ((table (list '*table*))) | |
| (define (lookup key) | |
| (let | |
| ((record (assoc key (cdr table)))) | |
| (if | |
| record | |
| ;; (cdr record) | |
| record | |
| false))) | |
| (define (assoc key records) | |
| (cond | |
| ((null? records) | |
| false) | |
| ((same-key? | |
| key | |
| (caar records)) | |
| (car records) | |
| (newline) | |
| (display (list records (car records)))) | |
| (else | |
| (assoc key (cdr records))))) | |
| (define (insert! key value) | |
| (let ((record (assoc key (cdr table)))) | |
| (if | |
| record | |
| (set-cdr! record value) | |
| (set-cdr! | |
| table | |
| (cons (cons key value) (cdr table))))) | |
| 'ok) | |
| (define (print-table) | |
| table) | |
| (define (dispatch m) | |
| (cond | |
| ((eq? m 'lookup) lookup) | |
| ((eq? m 'insert!) insert!) | |
| ((eq? m 'print-table) (print-table)) | |
| (else | |
| (error "Bad request" m)))) | |
| dispatch)) | |
| (define t1 (make-table eq?)) | |
| ((t1 'insert!) 'a 1) | |
| ((t1 'insert!) 'b 2) | |
| ((t1 'insert!) 'c 3) | |
| (t1 'print-table) | |
| ((t1 'lookup) 'a) | |
| ((t1 'lookup) 'b) | |
| ((t1 'lookup) 'c) | |