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hash-table-count

hash-table-count Function

Syntax:

hash-table-count hash-table → count

Arguments and Values:

hash-table—a hash table.

count—a non-negative integer .

Description:

Returns the number of entries in the hash-table. If hash-table has just been created or newly cleared (see clrhash) the entry count is 0.

Examples:

(setq table (make-hash-table)) → #<HASH-TABLE EQL 0/120 32115135> 
(hash-table-count table) → 0 
(setf (gethash 57 table) "fifty-seven") → "fifty-seven" 
(hash-table-count table) → 1 
(dotimes (i 100) (setf (gethash i table) i)) → NIL 
(hash-table-count table) → 100

Affected By:

clrhash, remhash, setf of gethash

See Also:

hash-table-size

Notes:

The following relationships are functionally correct, although in practice using hash-table-count is probably much faster:

(hash-table-count table) ≡

(loop for value being the hash-values of table count t) ≡

(let ((total 0))

(maphash #’(lambda (key value)

(declare (ignore key value))

(incf total))

table)

total)

Expanded Reference: hash-table-count​

Basic Usage​

hash-table-count returns the number of entries currently in the hash table.

(let ((ht (make-hash-table)))
  (hash-table-count ht))
=> 0

(let ((ht (make-hash-table)))
  (setf (gethash 'a ht) 1)
  (setf (gethash 'b ht) 2)
  (hash-table-count ht))
=> 2

Count Reflects Insertions and Removals​

(let ((ht (make-hash-table)))
  (setf (gethash 'x ht) 10)
  (setf (gethash 'y ht) 20)
  (setf (gethash 'z ht) 30)
  (remhash 'y ht)
  (hash-table-count ht))
=> 2

Updating an Existing Key Does Not Change Count​

(let ((ht (make-hash-table)))
  (setf (gethash 'a ht) 1)
  (setf (gethash 'a ht) 999)
  (hash-table-count ht))
=> 1

Count After clrhash​

(let ((ht (make-hash-table)))
  (dotimes (i 50)
    (setf (gethash i ht) (* i i)))
  (let ((before (hash-table-count ht)))
    (clrhash ht)
    (values before (hash-table-count ht))))
=> 50
=> 0

Count vs. Size​

hash-table-count returns the number of stored entries, while hash-table-size returns the allocated capacity. Count is always less than or equal to size.

(let ((ht (make-hash-table :size 100)))
  (dotimes (i 10)
    (setf (gethash i ht) t))
  (values (hash-table-count ht)
          (<= (hash-table-count ht) (hash-table-size ht))))
=> 10
=> T