Copy Hash Table in Lisp

Question

I have recently been working with hash tables in Common Lisp. I have been wondering how to make a separate copy of a hash table containing all the same values as the first. Is there an official way to do this? If not, can you give me an example using maphash?

Answer 1

As clhs does not list a copy table function I'd assume that maphash is the way to go.

(defun copy-table (table)
  (let ((new-table (make-hash-table
                    :test (hash-table-test table)
                    :size (hash-table-size table))))
    (maphash #'(lambda(key value)
                 (setf (gethash key new-table) value))
             table)
    new-table))

(let ((table (make-hash-table)))
  (mapcar #'(lambda(arg argg)
              (setf (gethash arg table) argg))
          '(1 2 3 4) '(a b c d))
  (format t "~a~%" table)
  (format t "~a~%" (copy-table table)))

#<HASH-TABLE :TEST EQL :COUNT 4 {10063C7F13}>
#<HASH-TABLE :TEST EQL :COUNT 4 {10063C86D3}>

This function however does not take special configurations of the hashtable into account, but it should suffice as an example.

Answer 2

Sim's answer copies a hash table, but there are two other features of hash tables taht might be good to copy for efficient population of the table. Here's a version that preserves that information, and also showcases loop's ability to work with hash tables (as an alternative to maphash):

(defun copy-hash-table (hash-table)
  (let ((ht (make-hash-table 
             :test (hash-table-test hash-table)
             :rehash-size (hash-table-rehash-size hash-table)
             :rehash-threshold (hash-table-rehash-threshold hash-table)
             :size (hash-table-size hash-table))))
    (loop for key being each hash-key of hash-table
       using (hash-value value)
       do (setf (gethash key ht) value)
       finally (return ht))))

Answer 3

Don't reinvent the wheel, use copy-hash-table from Alexandria.