Implement dict-filter.

dictionary.scm

@@ -32,6 +32,8 @@
   dict-has-key? dict-ref
   dict-remove dict-set
   dict-keys
+  dict-map
+  dict-filter
   dictionary-tests!
   )
 
@@ -102,6 +104,19 @@
 	  (cons k (if both? (proc k v) (proc v)))))
       d)))
 
+ ;; Returns a dictionary containing only kv pairs matching the
+ ;; predicate which must accept two arguments. Unlike list filter,
+ ;; does not perform final reverse on the result.
+ (define (dict-filter pred? d)
+   (let loop ((d d)
+	      (r '()))
+     (if (null? d)
+	 r
+	 (loop (cdr d)
+	       (if (pred? (caar d) (cdar d))
+		   (cons (car d) r)
+		   r)))))
+
  ;; Performs self-tests of the dictionary module.
  (define (dictionary-tests!)
    (run-tests
@@ -124,6 +139,11 @@
 				     (b . 2)))
 		'((a . 2)
 		  (b . 4)))
+   (test-equal? dict-filter (dict-filter (lambda (k v)
+					   (odd? v))
+					 '((a . 1)
+					   (b . 2)))
+		'((a . 1)))
    ))
 
  )