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hash-table-rehash-threshold

hash-table-rehash-threshold Function

Syntax:

hash-table-rehash-threshold hash-table → rehash-threshold

Arguments and Values:

hash-table—a hash table.

rehash-threshold—a real of type (real 0 1).

Description:

Returns the current rehash threshold of hash-table, which is 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-threshold 0.5)) 
→ #<HASH-TABLE EQL 0/100 2562446> 
(hash-table-rehash-threshold table) → 0.5

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-size

Expanded Reference: hash-table-rehash-threshold​

Basic Usage​

hash-table-rehash-threshold returns the current rehash threshold, a value between 0 and 1 indicating how full the table can get before it grows.

(let ((ht (make-hash-table :rehash-threshold 0.7)))
  (hash-table-rehash-threshold ht))
; Implementation-dependent, but close to 0.7

Default Threshold​

;; Default rehash threshold is implementation-dependent
(let ((ht (make-hash-table)))
  (let ((threshold (hash-table-rehash-threshold ht)))
    (and (<= 0 threshold) (<= threshold 1))))
=> T

Effect of Different Thresholds​

A lower threshold means the table rehashes sooner (more memory, potentially fewer collisions). A higher threshold means the table fills more before growing.

;; Low threshold: eager rehashing
(let ((ht (make-hash-table :rehash-threshold 0.3)))
  (realp (hash-table-rehash-threshold ht)))
=> T

;; High threshold: compact storage, later rehashing
(let ((ht (make-hash-table :rehash-threshold 0.9)))
  (realp (hash-table-rehash-threshold ht)))
=> T

Preserving Configuration When Copying​

(let* ((ht1 (make-hash-table :rehash-threshold 0.5 :test #'equal))
       (ht2 (make-hash-table
               :rehash-threshold (hash-table-rehash-threshold ht1)
               :test (hash-table-test ht1))))
  (maphash (lambda (k v) (setf (gethash k ht2) v)) ht1)
  (hash-table-test ht2))
=> EQUAL