L I S P T U T O R

BASIC OPERATIONS

A LISP program consist of functions applied to its arguments. Those functions return LISP objects and are in general either predicates or commands.

WHAT IS A PREDICATE?

A predicate is a function that tests for some condition involving its arguments and returns Nil if the condition is FALSE, where Nil stands for FALSE, and anything else stands for TRUE. Also TRUE is often symbolized with the special LISP variable T.

WHAT IS A COMMAND?

A Command performs operations involving its arguments and returns a LISP S-expression.

GENERAL FORMAT

(< Command > Arg1 Arg2 .. Arg n)

Where Command is the name of a predefined function or user defined function and arg1 .. arg n are the arguments of this function.

A common mistake is to try to define a list without the quotation like:
(the white house)
the compiler will try to find a function called "the", because everything after the left parentheses has to be a function if it is not begun with quotes.

O P E R A T I O N S

(CAR list)
Return the first element of list; list can not be the null-list.
Example:
(CAR '((the white) house) ) = (the white)
(CAR '(white house) ) = white
TRY:
(CAR ') = .
(CDR list)
Return the remaining part of the list after the deletion of the first element; list can not be the null-list.
Example:
(CDR '((the white) house) ) = (house)
(CDR '(the white house) ) = (white house)
TRY:
= .
Multiple CAR and CDR functions
Usually, functions that factor out expressions that combine CAR and CDR invocations, like (CAAR l), (CADR l) and others and are provided to have an equivalent to (CAR (CAR l)) or (CAR (CDR l)) and so on.
TRY: Write the equivalent function for
(CAR (CDR (CDR (quote (a b c d e)))))
= .
(CONS S-expression list)
It takes two arguments the first of which is an S-expression (a literal atom, numeric atom, or list), and the second of which is a list. CONS works by constructing a new list out of the two arguments so that the first item in the list is the S-expression and the remainder of the list is the list.
Example:
(CONS 'the '(white house) ) = (the white house)
(CONS '(the) '(white house) ) = ((the) white house)
TRY:
= .
(ATOM S-expression)
Returns True if the argument is an atom; Nil otherwise.
Example:
(ATOM '(white house) ) = Nil
(ATOM 'white ) = T
TRY:
= .
(NULL S-expression)
Returns True if the argument is the null-list (); Nil otherwise.
Example:
(NULL '(white house) ) = Nil
(NULL 'white ) = Nil
(NULL '() ) = T
TRY:
= .
(QUOTE Anything)
Returns the value of the argument as a valid S-expression; it is useful to create new or temporary S-expressions.
Example:
(QUOTE (white house) ) = (white house)
(QUOTE white ) = white
(QUOTE () ) = ()
TRY:
= .