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Copying The Contents of a Hash Table

Problem

You want to copy the contents of a hash table from one hash table to another, preserving both keys and values.

Solution

Using the recipe given in HashRecipeTraverse hash-table-copy! iterates over the source hash, copying all keys and values to the target hash:

;; hash-table-copy! : hash-table hash-table -> void
(define (hash-table-copy! source target)
  (hash-table-for-each
    source
    (lambda (key value)
      (hash-table-put! target key value))))

Discussion

An example usage, using write-hash-table from HashReadAndWrite, is:

> (define new-hash-table (make-hash-table))
> (hash-table-copy! 
    (make-immutable-hash-table '((a . 1) (b . 2)))
    new-hash-table)
> (write-hash-table new-hash-table (current-output-port))
#hasheq((b . 2) (a . 1))

Any existing values in the target hash table will be overwritten.

This function is useful to convert an immutable hash (for example, one read by HashReadAndWrite) to a mutable hash.

Comments about this recipe

Contributors

-- NoelWelsh - 17 Sep 2004