## Nesting level of a list

Q&A's, tips, howto's
cameyo
Posts: 101
Joined: Sun Mar 27, 2011 3:07 pm

### Nesting level of a list

I use the following function to calculate the maximum nesting deep of a list:

Code: Select all

``````(define (nesting lst)
(cond ((null? lst) 0)
((atom? lst) 0)
(true (max (+ 1 (nesting (first lst)))
(nesting (rest lst))))
)
)

(nesting '(a (((b c (d)))) (e) ((f)) g))
;-> 5``````
Is there a better/faster way to do this?
Thanks.

rickyboy
Posts: 595
Joined: Fri Apr 08, 2005 7:13 pm
Location: Front Royal, Virginia

### Re: Nesting level of a list

Here's another way to do it.

Code: Select all

``````(define (nesting lst)
(if (null? lst) 0
(atom? lst) 0
(+ 1 (apply max (map nesting lst)))))
``````
You be the judge if it's "better". Beware though that, although the code length is shorter, timing tests show that it is slightly *slower* than the original. map has to create more cells, so the slowdown is probably due to that -- which also means increased space overhead! So, maybe you should stick with the original. :)
(λx. x x) (λx. x x)

cameyo
Posts: 101
Joined: Sun Mar 27, 2011 3:07 pm

### Re: Nesting level of a list

The magic of "map" :-)

fdb
Posts: 49
Joined: Sat Nov 09, 2013 8:49 pm

### Re: Nesting level of a list

Not very elegant but maybe faster (for short lists I presume)

Code: Select all

``````
(define (nesting lst prev (t 0))
(if (= lst prev)
t
(nesting (flat lst 1) lst (inc t))))

> (nesting '(a (((b c (d)))) (e) ((f)) g))
5``````

rickyboy
Posts: 595
Joined: Fri Apr 08, 2005 7:13 pm
Location: Front Royal, Virginia

### Re: Nesting level of a list

Excellent, fdb! Faster than mine by about a factor of 3 (on the sample input)! 2.5 times faster than the original.
(λx. x x) (λx. x x)

cameyo
Posts: 101
Joined: Sun Mar 27, 2011 3:07 pm

Wow!!!
Thanks fdb