if Special Form
A third special form, in addition to defun and let, is the
conditional if. This form is used to instruct the computer to
make decisions. You can write function definitions without using
if, but it is used often enough, and is important enough, to be
included here. It is used, for example, in the code for the
function beginning-of-buffer.
The basic idea behind an if, is that "if a test is true,
then an expression is evaluated." If the test is not true, the
expression is not evaluated. For example, you might make a decision
such as, "if it is warm and sunny, then go to the beach!"
An if expression written in Lisp does not use the word `then';
the test and the action are the second and third elements of the list
whose first element is if. Nonetheless, the test part of an
if expression is often called the if-part and the second
argument is often called the then-part.
Also, when an if expression is written, the true-or-false-test
is usually written on the same line as the symbol if, but the
action to carry out if the test is true, the "then-part", is written
on the second and subsequent lines. This makes the if
expression easier to read.
(if true-or-false-test
action-to-carry-out-if-test-is-true)
The true-or-false-test will be an expression that is evaluated by the Lisp interpreter.
Here is an example that you can evaluate in the usual manner. The test is whether the number 5 is greater than the number 4. Since it is, the message `5 is greater than 4!' will be printed.
(if (> 5 4) ; if-part
(message "5 is greater than 4!")) ; then-part
(The function > tests whether its first argument is greater than
its second argument and returns true if it is.)
Of course, in actual use, the test in an if expression will not
be fixed for all time as it is by the expression (> 5 4).
Instead, at least one of the variables used in the test will be bound to
a value that is not known ahead of time. (If the value were known ahead
of time, we would not need to run the test!)
For example, the value may be bound to an argument of a function
definition. In the following function definition, the character of the
animal is a value that is passed to the function. If the value bound to
characteristic is fierce, then the message, `It's a
tiger!' will be printed; otherwise, nil will be returned.
(defun type-of-animal (characteristic)
"Print message in echo area depending on CHARACTERISTIC.
If the CHARACTERISTIC is the symbol `fierce',
then warn of a tiger."
(if (equal characteristic 'fierce)
(message "It's a tiger!")))
If you are reading this inside of GNU Emacs, you can evaluate the function definition in the usual way to install it in Emacs, and then you can evaluate the following two expressions to see the results:
(type-of-animal 'fierce) (type-of-animal 'zebra)
When you evaluate (type-of-animal 'fierce), you will see the
following message printed in the echo area: "It's a tiger!"; and
when you evaluate (type-of-animal 'zebra) you will see nil
printed in the echo area.
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