Next: Iteration, Previous: Conditionals, Up: Control Structures [Contents][Index]
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.
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.
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 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.
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
can be expressed in terms of either if
or cond
.
For example:
(and arg1 arg2 arg3) ≡ (if arg1 (if arg2 arg3)) ≡ (cond (arg1 (cond (arg2 arg3))))
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
.
For example, this expression tests whether x
is either 0 or
nil
:
(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 arg1 arg2
arg3)
never evaluates any argument more than once.
Next: Iteration, Previous: Conditionals, Up: Control Structures [Contents][Index]