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Conditional control structures choose among alternatives. SXEmacs
Lisp has two conditional forms: if
, which is much the same as in
other languages, and cond
, which is a generalized case
statement.
if
chooses between the then-form and the else-forms
based on the value of condition. If the evaluated condition is
non-nil
, then-form is evaluated and the result returned.
Otherwise, the else-forms are evaluated in textual order, and the
value of the last one is returned. (The else part of if
is
an example of an implicit progn
. See Sequencing.)
If condition has the value nil
, and no else-forms are
given, if
returns nil
.
if
is a special form because the branch that is not selected is
never evaluated—it is ignored. Thus, in the example below,
true
is not printed because print
is never called.
(if nil (print 'true) 'very-false) ⇒ very-false
cond
chooses among an arbitrary number of alternatives. Each
clause in the cond
must be a list. The CAR of this
list is the condition; the remaining elements, if any, the
body-forms. Thus, a clause looks like this:
(condition body-forms…)
cond
tries the clauses in textual order, by evaluating the
condition of each clause. If the value of condition is
non-nil
, the clause “succeeds”; then cond
evaluates its
body-forms, and the value of the last of body-forms becomes
the value of the cond
. The remaining clauses are ignored.
If the value of condition is nil
, the clause “fails”, so
the cond
moves on to the following clause, trying its
condition.
If every condition evaluates to nil
, so that every clause
fails, cond
returns nil
.
A clause may also look like this:
(condition)
Then, if condition is non-nil
when tested, the value of
condition becomes the value of the cond
form.
The following example has four clauses, which test for the cases where
the value of x
is a number, string, buffer and symbol,
respectively:
(cond ((numberp x) x) ((stringp x) x) ((bufferp x) (setq temporary-hack x) ; multiple body-forms (buffer-name x)) ; in one clause ((symbolp x) (symbol-value x)))
Often we want to execute the last clause whenever none of the previous
clauses was successful. To do this, we use t
as the
condition of the last clause, like this: (t
body-forms)
. The form t
evaluates to t
, which is
never nil
, so this clause never fails, provided the cond
gets to it at all.
For example,
(cond ((eq a 'hack) 'foo) (t "default")) ⇒ "default"
This expression is a cond
which returns foo
if the value
of a
is 1, and returns the string "default"
otherwise.
Any conditional construct can be expressed with cond
or with
if
. Therefore, the choice between them is a matter of style.
For example:
(if a b c) ≡ (cond (a b) (t c))
Next: Combining Conditions, Previous: Sequencing, Up: Control Structures [Contents][Index]