[racket] Why does this tail recursive list length function fail at cdr l at end of list?

[Bo Gus]

My tail recursive implementation of length is like this:

(define (length2 l)

  (define (l-iter l count)

    (if (null? 1) count

        (l-iter (cdr l) (+ count 1))))

  (l-iter l 0))

If I call (length2 '(1 2 3)) and step through the code in racket, count increments to 3 but then on the (if (null? l) count line instead of returning out of the function it goes onto the next line (l-iter (cdr l) (+ count l)))) and of course fails at cder of an empty list.

Racket error message is:

 mcdr: contract violation

  expected: mpair?

  given: '()

I am fairly new to scheme.  What am I doing wrong?

Angus

[Pierpaolo Bernardi]

On Wed, Nov 20, 2013 at 12:09 PM, Bo Gus <forum...@gmail.com> wrote:
> My tail recursive implementation of length is like this:
>
> (define (length2 l)
> (define (l-iter l count)
> (if (null? 1) count
> (l-iter (cdr l) (+ count 1))))
> (l-iter l 0))

> I am fairly new to scheme. What am I doing wrong?

You wrote (null? 1), which is always false. Note 1 instead of l.

Cheers
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[Bo Gus]

I noticed that just after posting.

I changed to:

(define (length2 lst)

  (define (l-iter l count)

    (if (null? l) count

        (l-iter (cdr l) (+ count 1))))

  (l-iter lst 0))

Must get better at spotting obvious mistakes...

[Daniel Prager]

Replace the expression (null? 1) with (null? l). You appear to have typed 1 (one) where you meant l (ell).

As a general rule-of-thumb a name such as lst or a-list will reduce the risk of this kind of error.

Dan

[Daniel Prager]

Oops - dup.

Didn't see Pierpaolo's reply.

Dan

--

Daniel Prager

Agile/Lean Coaching, Software Development and Leadership

Twitter: @agilejitsu 

Blog: agile-jitsu.blogspot.com