Music 220b: Winter 2001
Fernando Lopez-Lezcano, instructor
Christopher Burns, teaching assistant
Tamara Smyth, teaching assistant

Week 2: logic - (if) and (cond)

We often want our lisp programs to make decisions. For instance, we might generate (with-sound) calls using a list of frequencies, and use different instruments depending upon the frequency. Or, we might analyze a soundfile, and take different actions depending upon the maximum amplitudes in a particular region.

In order to do these kinds of things, we need logical tests. Lisp provides two commands for this purpose: (if) and (cond).

• (if)

  The body of an if statement has three sections, quickly summarized as "if," "then," and "else." In full syntax:

  (if (test-clause)
      (then-clause)
      (else-clause))

  For instance, "if the current frequency is greater than 440, generate a sound from fm-violin; else generate a sound from grani." Try -

  CM(1): :cl /zap/v
  CM(2): :cl /zap/grani
  CM(3): (setf frequency 450)
  CM(4): (with-sound (:statistics t :srate 44100) 
           (if (> frequency 440)
               (fm-violin 0 1 frequency 0.3)
             (grani 0 1 0.3 "/zap/cencerros-1.snd")))
  

  For a more simple example, "if the frequency list has more than five elements, print a warning, otherwise return t [true]":

  CM(8): (setf frequency-list (list 440 220 660 110 880 330))
  CM(9): (if (> (length frequency-list) 5)
             (format t "frequency list is too long....")
           t)
  frequency list is too long....
  NIL
  
  CM(13): (setf frequency-list (list 440 220))
  CM(14): (if (> (length frequency-list) 5)
              (format t "frequency list is too long....")
            t)
  T
  

  This kind of test is often usefully expressed as a function:

  (defun check-frequency-list-length (frequency-list)
     (if (> (length frequency-list) 5)
         (format t "frequency list is too long....")
       t))
  

• (cond)

  Cond is a more generic logic test than if; it allows you to specify many different conditions, instead of just one. This first example squares positive numbers, and returns the negative square of a negative number. In any other case it returns zero. ("t" automatically evaluates to true -- the equivalent of our else condition in the (if) statement).

  (defun cond-example (number)
     (cond ((> number 0) (* number number))
  	 ((< number 0) (- (* number number)))
  	 (t 0)))
  
  CM(18): (cond-example 3)
  9
  CM(19): (cond-example -3)
  -9
  CM(20): (cond-example 0)
  0

  In case the example doesn't make it clear, the cond syntax works like this:

  (cond ((test-1) (do-this-if-test-1-is-true))
        ((test-2) (do-this-if-test-2-is-true))
        ...arbitrary number of tests...
        ((test-n) (do-this-if-test-n-is-true))
        (t (do-this-if-all-tests-fail)))
  

  Note that the tests are evaluated sequentially -- the interpreter tries test-2 only if test-1 fails. This means that the order of the tests are important.

So far we've only seen the simplest applications of if and cond; but they are the building blocks of far more complicated programs. Let your imagination run wild....

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