Skip to main content

hash-table-rehash-size

hash-table-rehash-size Function

Syntax:

hash-table-rehash-size hash-table → rehash-size

Arguments and Values:

hash-table—a hash table.

rehash-size—a real of type (or (integer 1 *) (float (1.0) *)).

Description:

Returns the current rehash size of hash-table, suitable for use in a call to make-hash-table in order to produce a hash table with state corresponding to the current state of the hash-table.

Examples:

(setq table (make-hash-table :size 100 :rehash-size 1.4)) 
→ #<HASH-TABLE EQL 0/100 2556371> 
(hash-table-rehash-size table) → 1.4

Exceptional Situations:

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

See Also:

make-hash-table, hash-table-rehash-threshold

Notes:

If the hash table was created with an integer rehash size, the result is an integer , indicating that the rate of growth of the hash-table when rehashed is intended to be additive; otherwise, the result

Hash

is a float, indicating that the rate of growth of the hash-table when rehashed is intended to be multiplicative. However, this value is only advice to the implementation; the actual amount by which the hash-table will grow upon rehash is implementation-dependent.

Expanded Reference: hash-table-rehash-size​

Basic Usage​

hash-table-rehash-size returns the rehash size of a hash table, indicating how the table grows when it needs to be expanded.

;; Default rehash size is implementation-dependent
(let ((ht (make-hash-table)))
  (numberp (hash-table-rehash-size ht)))
=> T

Multiplicative Growth (Float)​

When a float is specified, it represents a multiplicative growth factor. For example, 1.5 means the table grows to 1.5 times its current size.

(let ((ht (make-hash-table :rehash-size 1.5)))
  (hash-table-rehash-size ht))
=> 1.5

Additive Growth (Integer)​

When an integer is specified, it represents the number of entries to add during rehashing.

(let ((ht (make-hash-table :rehash-size 200)))
  (integerp (hash-table-rehash-size ht)))
=> T

Querying for Table Duplication​

The rehash size can be used alongside other parameters to create a copy of a hash table with the same configuration.

(let* ((original (make-hash-table :rehash-size 2.0
                                   :rehash-threshold 0.8
                                   :test #'equal))
       (clone (make-hash-table
                :rehash-size (hash-table-rehash-size original)
                :rehash-threshold (hash-table-rehash-threshold original)
                :test (hash-table-test original))))
  (hash-table-rehash-size clone))
=> 2.0

Float vs. Integer Rehash Size​

;; Float rehash size indicates multiplicative growth
(let ((ht (make-hash-table :rehash-size 1.7)))
  (floatp (hash-table-rehash-size ht)))
=> T

;; Integer rehash size indicates additive growth
(let ((ht (make-hash-table :rehash-size 50)))
  (integerp (hash-table-rehash-size ht)))
=> T