Skip to main content

set

set Function

Syntax:

set symbol value → value

Arguments and Values:

symbol—a symbol.

value—an object.

Description:

set changes the contents of the value cell of symbol to the given value.

(set symbol value) ≡ (setf (symbol-value symbol) value)

Examples:

(setf (symbol-value ’n) 1) → 1 
(set ’n 2) → 2 
(symbol-value ’n) → 2 
(let ((n 3)) 
  (declare (special n)) 
  (setq n (+ n 1)) 
  (setf (symbol-value ’n) (\* n 10)) 
  (set ’n (+ (symbol-value ’n) n)) 
  n) → 80 
n → 2 
(let ((n 3)) 
  (setq n (+ n 1)) 
  (setf (symbol-value ’n) (\* n 10)) 
  (set ’n (+ (symbol-value ’n) n)) 
  n) → 4 
n → 44 
(defvar \*n\* 2) 
(let ((\*n\* 3)) 
  (setq \*n\* (+ \*n\* 1)) 
  (setf (symbol-value ’\*n\*) (\* \*n\* 10)) 
  (set ’\*n\* (+ (symbol-value ’\*n\*) \*n\*)) 
  \*n\*) → 80 
\*n\* → 2 

(defvar \*even-count\* 0) → \*EVEN-COUNT\* 
(defvar \*odd-count\* 0) → \*ODD-COUNT\* 
(defun tally-list (list) 
  (dolist (element list) 
    (set (if (evenp element) ’\*even-count\* ’\*odd-count\*) 
	 (+ element (if (evenp element) \*even-count\* \*odd-count\*))))) 
(tally-list ’(1 9 4 3 2 7)) → NIL 
\*even-count\* → 6 
\*odd-count\* → 20

Side Effects:

The value of symbol is changed.

See Also:

setq, progv, symbol-value

Notes:

The function set is deprecated.

set cannot change the value of a lexical variable.

Expanded Reference: set​

warning

set is deprecated. Use (setf (symbol-value ...) ...) instead.

Basic usage​

set changes the value cell of a symbol. It is equivalent to (setf (symbol-value symbol) value).

(set 'my-var 42)
(symbol-value 'my-var)
=> 42

set cannot change lexical variables​

Like symbol-value, set only affects the dynamic (special) value of a symbol, not lexical bindings.

(set 'x 10)

(let ((x 20))
  (set 'x 30)
  (values x (symbol-value 'x)))
=> 20
=> 30

set with dynamic (special) variables​

set works with dynamically bound variables as expected.

(defvar *setting* :initial)

(let ((*setting* :temporary))
  (set '*setting* :modified)
  *setting*)
=> :MODIFIED

*setting*
=> :INITIAL

Equivalence with setf of symbol-value​

(set 'equiv-test 'via-set)
(symbol-value 'equiv-test)
=> VIA-SET

(setf (symbol-value 'equiv-test) 'via-setf)
(symbol-value 'equiv-test)
=> VIA-SETF

Using set with a computed symbol​

One advantage of set over setq is that the symbol argument is evaluated, allowing dynamic dispatch.

(defvar *a* 0)
(defvar *b* 0)

(dolist (pair '((*a* 10) (*b* 20)))
  (set (first pair) (second pair)))

(values *a* *b*)
=> 10
=> 20