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| ANSI Common Lisp 18 Hash Tables 18.2 Dictionary of Hash Tables
18.2.11 maphash | Function
  Syntax: maphash function hash-table nil Arguments and Values: function - a designator for a function of two arguments, the key and the value. hash-table - a hash table. Description: Iterates over all entries in the hash-table. For each entry, the function is called with two arguments--the key and the value of that entry. The consequences are unspecified if any attempt is made to add or remove an entry from the hash-table while a maphash is in progress, with two exceptions: the function can use can use setf of gethash to change the value part of the entry currently being processed, or it can use remhash to remove that entry. Examples:
       (setq table (make-hash-table))  #<HASH-TABLE EQL 0/120 32304110>
       (dotimes (i 10) (setf (gethash i table) i))   NIL
       (let ((sum-of-squares 0))
          (maphash #'(lambda (key val)
                       (let ((square (* val val)))
                         (incf sum-of-squares square)
                         (setf (gethash key table) square)))
                   table)
          sum-of-squares)  285
       (hash-table-count table)  10
       (maphash #'(lambda (key val)
                     (when (oddp val) (remhash key table)))
                 table)   NIL
       (hash-table-count table)  5
       (maphash #'(lambda (k v) (print (list k v))) table)
      (0 0)
      (8 64)
      (2 4)
      (6 36)
      (4 16)
        NIL
Side Effects: None, other than any which might be done by the function. See Also: loop, with-hash-table-iterator, Section 3.6 Traversal Rules and Side Effects Allegro CL Implementation Details: None.

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