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This section describes three constructs that are often used together
with `if`

and `cond`

to express complicated conditions. The
constructs `and`

and `or`

can also be used individually as
kinds of multiple conditional constructs.

- Function:
`not`

*condition* This function tests for the falsehood of

`condition`. It returns`t`if`condition`is`nil`

, and`nil`

otherwise. The function`not`

is identical to`null`

, and we recommend using the name`null`

if you are testing for an empty list.

- Special Form:
`and`

*conditions…* The

`and`

special form tests whether all the`conditions`are true. It works by evaluating the`conditions`one by one in the order written.If any of the

`conditions`evaluates to`nil`

, then the result of the`and`

must be`nil`

regardless of the remaining`conditions`; so`and`

returns`nil`

right away, ignoring the remaining`conditions`.If all the

`conditions`turn out non-`nil`

, then the value of the last of them becomes the value of the`and`

form. Just`(and)`

, with no`conditions`, returns`t`, appropriate because all the`conditions`turned out non-`nil`

. (Think about it; which one did not?)Here is an example. The first condition returns the integer 1, which is not

`nil`

. Similarly, the second condition returns the integer 2, which is not`nil`

. The third condition is`nil`

, so the remaining condition is never evaluated.(and (print 1) (print 2) nil (print 3)) -| 1 -| 2 ⇒ nil

Here is a more realistic example of using

`and`

:(if (and (consp foo) (eq (car foo) 'x)) (message "foo is a list starting with x"))

Note that

`(car foo)`

is not executed if`(consp foo)`

returns`nil`

, thus avoiding an error.`and`

expressions can also be written using either`if`

or`cond`

. Here’s how:(and

`arg1``arg2``arg3`) ≡ (if`arg1`(if`arg2``arg3`)) ≡ (cond (`arg1`(cond (`arg2``arg3`))))

- Special Form:
`or`

*conditions…* The

`or`

special form tests whether at least one of the`conditions`is true. It works by evaluating all the`conditions`one by one in the order written.If any of the

`conditions`evaluates to a non-`nil`

value, then the result of the`or`

must be non-`nil`

; so`or`

returns right away, ignoring the remaining`conditions`. The value it returns is the non-`nil`

value of the condition just evaluated.If all the

`conditions`turn out`nil`

, then the`or`

expression returns`nil`

. Just`(or)`

, with no`conditions`, returns`nil`

, appropriate because all the`conditions`turned out`nil`

. (Think about it; which one did not?)For example, this expression tests whether

`x`

is either`nil`

or the integer zero:(or (eq x nil) (eq x 0))

Like the

`and`

construct,`or`

can be written in terms of`cond`

. For example:(or

`arg1``arg2``arg3`) ≡ (cond (`arg1`) (`arg2`) (`arg3`))You could almost write

`or`

in terms of`if`

, but not quite:(if

`arg1``arg1`(if`arg2``arg2``arg3`))This is not completely equivalent because it can evaluate

`arg1`or`arg2`twice. By contrast,`(or`

never evaluates any argument more than once.`arg1``arg2``arg3`)

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