Re: delete command weirdness

Nathaniel Calloway wrote:
> Anyone have an explanation for the inability of the delete command to
> delete the car from a list (at least in clisp)? I guess this falls
> under the category of "never expect destructive list functions to act
> how you expect."
> 
> 
> CL-USER> (defvar testing)
> TESTING
> CL-USER> (setq testing '(a b c d))
> (A B C D)
> CL-USER> (delete 'a testing)
> (B C D)

The (B C D) you see above is the value returned by delete. Obvious, 
perhaps, but I am just warming up.

> CL-USER> testing
> (A B C D)

If you are familiar at all with programming (I do not know how 
experienced you are) you will realize delete does not see the /variable/ 
testing, the caller pulls a value out of that variable and p***** it to 
delete. ie, delete sees (a b c d), not "testing".

All delete can do is dutifully return a list without A, and whaddaya 
know, it can do just by returning the CDR of the list received (since A 
appears only in the car of the first cons).


> CL-USER> (delete 'b testing)
> (A C D)

Now delete is forced to do some work, and makes the cdr of the first 
cons the cdr of the second cons, effectively discarding the second cons 
from the structure received.

Another way for you to understand this is to try to write a destructive 
version of my-delete that does behave they way you expected. Methinks 
the light will go on before you write a line of code.

kzo

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