Skip to main content

remhash

remhash Function

Syntax:

remhash key hash-table → generalized-boolean

Arguments and Values:

key—an object.

hash-table—a hash table.

generalized-boolean—a generalized boolean.

Description:

Removes the entry for key in hash-table, if any. Returns true if there was such an entry, or false otherwise.

Examples:

(setq table (make-hash-table)) → #<HASH-TABLE EQL 0/120 32115666> 
(setf (gethash 100 table) "C") → "C" 
(gethash 100 table) → "C", *true* 
(remhash 100 table) → true 
(gethash 100 table) → NIL, *false* 
(remhash 100 table) → false

Side Effects:

The hash-table is modified.

Hash

maphash

Expanded Reference: remhash​

Basic Removal​

remhash removes an entry by key and returns true if the entry existed, false otherwise.

(let ((ht (make-hash-table)))
  (setf (gethash 'a ht) 1)
  (remhash 'a ht))
=> T

(let ((ht (make-hash-table)))
  ;; Removing a key that does not exist
  (remhash 'nonexistent ht))
=> NIL

Verifying Removal​

(let ((ht (make-hash-table)))
  (setf (gethash 'x ht) 42)
  (gethash 'x ht))
=> 42
=> T

(let ((ht (make-hash-table)))
  (setf (gethash 'x ht) 42)
  (remhash 'x ht)
  (gethash 'x ht))
=> NIL
=> NIL

Effect on Count​

(let ((ht (make-hash-table)))
  (setf (gethash 'a ht) 1)
  (setf (gethash 'b ht) 2)
  (setf (gethash 'c ht) 3)
  (format nil "Before: ~D" (hash-table-count ht)))
=> "Before: 3"

(let ((ht (make-hash-table)))
  (setf (gethash 'a ht) 1)
  (setf (gethash 'b ht) 2)
  (setf (gethash 'c ht) 3)
  (remhash 'b ht)
  (format nil "After: ~D" (hash-table-count ht)))
=> "After: 2"

Idempotent Removal​

Calling remhash twice on the same key is safe; the second call simply returns nil.

(let ((ht (make-hash-table)))
  (setf (gethash 'key ht) "value")
  (list (remhash 'key ht)
        (remhash 'key ht)))
=> (T NIL)

Removing During Iteration with maphash​

It is permitted to use remhash on the current entry during maphash.

;; Remove all entries with even values
(let ((ht (make-hash-table)))
  (dotimes (i 6)
    (setf (gethash i ht) i))
  (maphash (lambda (key value)
             (when (evenp value)
               (remhash key ht)))
           ht)
  (hash-table-count ht))
=> 3