CODEWITHSUNDEEP
functionrecursionlispfunctional-programmingclisp
xboarder
xboarder

Reputation: 11

What is wrong with this Lisp Function?

This function is a CLisp function, this is part of a homework problem, but which is supposed to be written in this different format (the second function).

(defun range (m M) (cond
((> m M) '() )
((= m M) '() )
((< m M) (cons m (range (+ m 1) M ) ) )  
)
)


(define (range m M) (cond
    ((> m M) '() )
    ((= m M) '() )
    ((< m M) (cons m (range (+ m 1) M ) ) )
)
)

These should both take a min value (m) and a max value (M) and return the list of integers from min to max (exluding the max value / M-1)

I have traced this over and over and I can't see why it is just returning NIL it must be a very dumb logic mistake.

(range 1 4)  => result (1 2 3)

m=1 | M=4 ==> return (cons 1 (2 3) )
m=2 | M=4 ==> return (cons 2 (3) )
m=3 | M=4 ==> return (cons 3 () )
m=4 | M=4 ==> return ()
    v         ^
    ---------/

I'm going crazy trying to figure out WHY this is not performing like I trace it.

Again, when I execute the function it results in NIL.

Upvotes: 1

Views: 417

Answers (1)

waveman
waveman

Reputation: 86

I ran this using SBCL and it complained that the variable M appears twice in the parameter list. Lisp is not case-sensitive for variable names.

On changing it to 

(defun range (m MM) 
  (cond
      ((> m MM) '() )
      ((= m MM) '() )
      ((< m MM) (cons m (range (+ m 1) MM)))))

It worked fine.

> (trace range)
> (range 1 4)
  0: (RANGE 1 4)
    1: (RANGE 2 4)
      2: (RANGE 3 4)
        3: (RANGE 4 4)
        3: RANGE returned NIL
      2: RANGE returned (3)
    1: RANGE returned (2 3)
  0: RANGE returned (1 2 3)
-> (1 2 3)

I checked with CLISP. With different variable names it works OK. CLISP does not pick up the error, unlike SBCL. 

<[1]> (defun range (m MM) (cond ((>= m MM) '())((< m MM) (cons m (range (+ m 1) MM )))))
RANGE

[2]> (range 1 4)
(1 2 3)

Here is your version:

[3]> (defun range (m M) (cond ((>= m M) '())((< m M) (cons m (range (+ m 1) M)))))

RANGE

[4]> (range 1 4)
Nil

Upvotes: 5

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