28 lines
1.2 KiB
Scheme
28 lines
1.2 KiB
Scheme
; Merge the given alists. Prefer the items from the "update" alist.
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; Returns a new list; the argument lists are not modified.
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(define merge-alists
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(lambda (base update)
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(let ((merged (list-copy update))
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(remains (list-copy base)))
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(let loop ((shadowing merged))
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(if (null? shadowing)
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(append merged remains)
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(begin (set! remains (remp (lambda (pair) (equal? (car pair) (caar shadowing)))
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remains))
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(loop (cdr shadowing))))))))
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; Test...
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(printf "~%Merge using defined merge-alists procedure...~%")
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; The original base and update alists.
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(let ((base '(("name" . "Rocket Skates") ("price" . 12.75) ("color" . "yellow" )))
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(update '(("price" . 15.25) ("color" . "red") ("year" . 1974))))
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; Merge using the defined merge-alists procedure.
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(let ((merged (merge-alists base update)))
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; Show that everything worked.
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(printf "Merged alist:~%~s~%" merged)
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(printf "Values from merged alist:~%")
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(let loop ((keys '("name" "price" "color" "year")))
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(unless (null? keys)
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(printf "~s -> ~s~%" (car keys) (cdr (assoc (car keys) merged)))
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(loop (cdr keys))))))
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