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hash-table-test

hash-table-test Function

Syntax:

hash-table-test hash-table → test

Arguments and Values:

hash-table—a hash table.

test—a function designator . For the four standardized hash table test functions (see make-hash-table), the test value returned is always a symbol. If an implementation per mits additional tests, it is implementation-dependent whether such tests are returned as function objects or function names.

Description:

Returns the test used for comparing keys in hash-table.

Exceptional Situations:

Should signal an error of type type-error if hash-table is not a hash table.

See Also:

make-hash-table

Hash

Expanded Reference: hash-table-test​

Basic Usage​

hash-table-test returns the test function used for comparing keys in a hash table. For the four standard tests, it returns a symbol.

(hash-table-test (make-hash-table))
=> EQL

(hash-table-test (make-hash-table :test #'equal))
=> EQUAL

(hash-table-test (make-hash-table :test #'equalp))
=> EQUALP

(hash-table-test (make-hash-table :test #'eq))
=> EQ

Using the Test for Comparison​

The returned test can be used to compare keys in user code.

(let* ((ht (make-hash-table :test #'equal))
       (test (hash-table-test ht)))
  (funcall test "hello" "hello"))
=> T

Cloning a Hash Table with the Same Test​

(let* ((original (make-hash-table :test #'equalp)))
  (setf (gethash "Key" original) "value")
  (let ((clone (make-hash-table :test (hash-table-test original))))
    ;; Clone uses the same test
    (setf (gethash "KEY" clone) "cloned-value")
    (gethash "key" clone)))
=> "cloned-value"
=> T

Conditional Logic Based on Test​

(defun describe-hash-table (ht)
  "Return a string describing the hash table's test."
  (format nil "Hash table uses ~A test with ~D entries"
          (hash-table-test ht)
          (hash-table-count ht)))

(describe-hash-table (make-hash-table :test #'equal))
=> "Hash table uses EQUAL test with 0 entries"

Verifying Test After Creation​

;; Whether you pass a symbol or function designator, the result is a symbol
(eq (hash-table-test (make-hash-table :test 'equal))
    (hash-table-test (make-hash-table :test #'equal)))
=> T