PPT Slide

The Lisp if statement provides an if - then - else facility.

( if <test> <thenform> <elseform> )

The if returns the value of <thenform> if the test is non NIL, otherwise <elseform>.

( if ( > X 10 ) ( - X 10 ) 0 )

when, unless

These forms may be used instead of if when either the <thenform> or the <elseform> is NIL.

( when <test> <thenform> )

else NIL is understood

( unless <test> <elseform> )

then NIL is understood

There’s also a case form.