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Finding Elements in One Array but Not Another

Problem

You want to find elements that are in one list but not another.

Solution

You want to find elements in al list that aren't in bl list. Build a hash of the keys of bl to use as a lookup table. Then check each element in al to see whether it is in bl.

(require (lib "1.ss" "srfi"))

;; To find elements that are in the al list but not in the bl list
(define (list-minus al bl)
  (let ((h (make-hash-table 'equal)))
    (for-each (lambda (v)
                (hash-table-put! h v #f))
              bl)
    (filter (lambda (v) 
              (hash-table-get h v (lambda () #t)))
            al)))

Discussion

Another way is to use the lset-difference function shipped in the SRFI 1, but this function does not use hashes at all, and therefore it is slower.

Comments about this recipe

Contributors

-- IvanHernandez - 19 May 2004