Question

I am writing a function in lisp and I am not getting a result.
The function is to count the number of atoms in a expression.

(defun count-atoms(exp)
'Return the total number of non-nil atoms in the expression'
(cond((null exp) 0)
     ((atom exp) 1)
     ( t (+ (count-atoms (first exp))
            (count-atoms (rest exp))))))

When I run in clisp, all I get is the following without the result.

[3]> (count-atoms '(a b c))
(COND ((NULL EXP) 0) ((ATOM EXP) 1)
(T (+ (COUNT-ATOMS (FIRST EXP)) (COUNT-ATOMS (REST EXP)))))

Can anyone help?

Answer 1

I’m surprised that you get a result at all. I get an error:

*** - PROGN: variable THE has no value
The following restarts are available:
USE-VALUE      :R1      Input a value to be used instead of THE.
STORE-VALUE    :R2      Input a new value for THE.
ABORT          :R3      Abort main loop

The reason is that strings in Common Lisp must be in double quotes: "Return ...". Single quotes are only used to prevent evaluation.