LISP - Logical Operators


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Common LISP provides three logical operators: and, or, and not that operates on Boolean values. Assume A has value nil and B has value 5, then −

Operator Description Example
and It takes any number of arguments. The arguments are evaluated left to right. If all arguments evaluate to non-nil, then the value of the last argument is returned. Otherwise nil is returned. (and A B) will return NIL.
or It takes any number of arguments. The arguments are evaluated left to right until one evaluates to non-nil, in such case the argument value is returned, otherwise it returns nil. (or A B) will return 5.
not It takes one argument and returns t if the argument evaluates to nil. (not A) will return T.

Example

Create a new source code file named main.lisp and type the following code in it.

(setq a 10)
(setq b 20)

(format t "~% A and B is ~a" (and a b))
(format t "~% A or B is ~a" (or a b))
(format t "~% not A is ~a" (not a))

(terpri)
(setq a nil)
(setq b 5)

(format t "~% A and B is ~a" (and a b))
(format t "~% A or B is ~a" (or a b))
(format t "~% not A is ~a" (not a))

(terpri)
(setq a nil)
(setq b 0)

(format t "~% A and B is ~a" (and a b))
(format t "~% A or B is ~a" (or a b))
(format t "~% not A is ~a" (not a))

(terpri)
(setq a 10)
(setq b 0)
(setq c 30)
(setq d 40)

(format t "~% Result of and operation on 10, 0, 30, 40 is ~a" (and a b c d))
(format t "~% Result of and operation on 10, 0, 30, 40 is ~a" (or a b c d))

(terpri)
(setq a 10)
(setq b 20)
(setq c nil)
(setq d 40)

(format t "~% Result of and operation on 10, 20, nil, 40 is ~a" (and a b c d))
(format t "~% Result of and operation on 10, 20, nil, 40 is ~a" (or a b c d))

When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result returned is −

A and B is 20
A or B is 10
not A is NIL

A and B is NIL
A or B is 5
not A is T

A and B is NIL
A or B is 0
not A is T

Result of and operation on 10, 0, 30, 40 is 40
Result of and operation on 10, 0, 30, 40 is 10

Result of and operation on 10, 20, nil, 40 is NIL
Result of and operation on 10, 20, nil, 40 is 10

Please note that the logical operations work on Boolean values and secondly, numeric zero and NIL are not same.

lisp_operators.htm
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