In LISP, which predicate returns t (true) if the supplied

Question

Solve this multiple-choice question and choose the correct option.

Curioustab · Accepted Answer

Introduction / Context:LISP lists are built from CONS cells—pairs consisting of a head (car) and a tail (cdr). Recognizing list structure at runtime is a common need in symbolic processing. LISP provides type predicates to test the nature of objects. This question asks for the predicate that checks whether an object is specifically a CONS cell. Given Data / Assumptions: We need a boolean test that distinguishes CONS cells from atoms or other objects. The result should be t for CONS and nil otherwise. Standard Common Lisp predicate names are assumed. Concept / Approach: The predicate consp returns t when its argument is a CONS cell. By contrast, cons constructs a new CONS cell and is not a predicate, and eq tests identity between two objects, not type. The misspelled form ‘‘(cous = )’’ is invalid syntax and not a standard function. Step-by-Step Solution: Recall standard predicates: consp, listp, atom, null, etc.Match the requirement ‘‘true if argument is a CONS’’ to consp.Eliminate constructor and equality functions (cons, eq) and invalid tokens.Select ‘‘(consp )’’ as correct. Verification / Alternative check: In a Common Lisp REPL: (consp '(a b)) → t; (consp 'a) → nil. This confirms the predicate's behavior. Why Other Options Are Wrong: (cons ): Creates a cell; does not test type. (eq ): Compares two arguments for identity; wrong arity and purpose here. (cous = ): Not valid LISP syntax or function. None: Incorrect because consp is the precise predicate. Common Pitfalls: Confusing constructors with predicates; overlooking arity requirements in predicate forms. Final Answer: (consp )

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