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

Sim's answer copies a hash table, but there are two other features of hash tables thst 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))))