Info Catalog eintr: if eintr: type-of-animal in detail

eintr: if in more detail

 
 ‘if’ in more detail
 -------------------
 
 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 is 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 string \"fierce\",
      then warn of a tiger."
        (if (equal characteristic "fierce")
            (message "It is 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 "striped")
 
 
 When you evaluate ‘(type-of-animal "fierce")’, you will see the
 following message printed in the echo area: ‘"It is a tiger!"’; and when
 you evaluate ‘(type-of-animal "striped")’ you will see ‘nil’ printed in
 the echo area.
 

Info Catalog eintr: if eintr: type-of-animal in detail