question about Chapter 4 Land of Lisp p.51

[farmkitteh]

Hello,

I'm working my way through this wonderful book and I hope that someone can explain the function at the top of page 51 in more detail.  I don't understand what is happening with this part:  

(1+ (my-length (cdr list)))

 0))

How is this chomping off each bit until the list is empty?  How do the 1+ and 0 figure in?

Here is the complete example:

>(defun my-length (list)

    (if list

       (1+ (my-length (cdr list)))

       0))

>(my-length (list with four symbols))

4

Thanks.  I know this is really a newbie question. I think I've understood everything up until this point.  I realize that crd takes the second and subsequent items in a list and this is repeating until the list is empty but I don't understand the +1 and 0 in this context. Thanks in advance!

[Purity Control]

I have answered this in a previous post  https://groups.google.com/forum/?fromgroups#!searchin/land-of-lisp/purity$20control/land-of-lisp/HtqlXIa6lp8/NcPjx0fcqEAJ

But here is some of the text cut and pasted for you :-

If you think of lisp expressions as ordering in the same way as 
complex mathematic expressions this may help you. Whatever a function 
returns sits where the function call is. 

(defun my-length (list) 
     (if list 
         (1+ (my-length (cdr list))) 
         0)) 
> (my-length ' (list with four symbols)) 

becomes 
  (if '(list with four variables) 
which evaluate to true so we operate on the line 
  (1+ (my-length (cdr list))) 
which becomes 
  (1+ (my-length '(with four varianbles))) 
so we get to 
  (1+ (if  '(with four variables)......)) 
which again evaluates to true so we take the first line 
  (1+ (1+ (my-length '(four variables)))) 
which becomes 
  (1+ (1+ (if '(four variables).....))) 
again true so 
  (1+ (1+ (1+ (my-length '(variables))))) 
which becomes 
  (1+ (1+ (1+ (if ('variables) ....)))) 
which again avaluates to true so 
  (1+ (1+ (1+ (1+ (my-length '()))))) 
which becomes 
  (1+ (1+ (1+ (1+ (if '() .....))))) 
this evaluates to false and so returns the second line which is zero 
  (1+ (1+ (1+ (1+ 0)))) 
  (1+ (1+ (1+ 1))) 
  (1+ (1+ 2)) 
  (1+ 3) 
  4 

Hope this makes a bit more sense. 

[farmkitteh]

Thanks Purity Control!  I'm sorry I didn't find your original post. I'll look harder next time.  I understand now thanks to you.  I'm glad I asked because I don't think I would have figured that one out.  

Also I see that I have a typo in my post. In the sentence "I realize that crd takes ..." should of course be "I realize that cdr takes..."

Again, thanks Purity Control for your detailed explanation.

[Purity Control]

Glad it all makes sense.

Recursion seems to be a regular stumbling block for people learning but once you get it, its like a eureka moment so stick with it even if it takes a little while.

The book is an absolutely wonderful way to learn lisp. 

I think you will have great fun going through it.