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Thomas · Joined: 02 Dec 2005 Posts: 7

what is the most effectiant way?

Jimka · Guest

the easiest way to do this is to build the list in the other
order. ask yourself why is the list in the order it is in, and
can it be built in the other order.

for example if you use push to add elements to a list
then you can use pop to pop them off. this is
very fast and memory efficient.

but in general to remove any element from a list with the
remove function.

x = (list 1 2 3 4 5 6 7)

now x has value (1 2 3 4 5 6 7)

if you want to remove 4 from x
(remove 4 x)

and x has value (1 2 3 5 6 7)

-jim

Thomas · Joined: 02 Dec 2005 Posts: 7

In fact, the last element is useful at first. For some reason, I need to delete it the later use.
Now I use car(last(my_List)) to get the last element and remove it with remd(),
but I think such operation need to traverse the list two times, I want to know whehter a single traversal is enough.

Bernd Fischer · Guest

What about
x = list( 1 2 3 4 5 6 7 )
remove( length( x ) x )

Bernd

Jimka · Guest

How did the element you want to remove get at the end of the list?
Why is it the last element you need to remove rather than the 13th for
example? Sometimes there are very good reasons for operating
on the last element, however it normally means you built the list
in the wrong order to begin with, or that you should be using a
different type of data structure than a list.

How did the element get to the end of the list anyway?
Why don't you already know its value without having to use
car(last(...))?

Rajeswaran M · Guest

It can simply be due to reading an input file, whose last column
may be irrelevent to the function he is working :)

Jimka wrote:

Jimka · Guest

yes you are right, as i said there are certainly very good reasons for
wanting
to do this. However, in my experience it is also quite often that
the programmer is going about the problem wrong, and it is really
much simpler. I.e., if his program put the element at the end of
the list, then simply avoid putting it there and the problem does
not arrise. If he knows what the last element is, he can remove it
with remove (assuming it only appears at the end and not also
somewhere in the middle).
list = ( 1 2 3 1 2 3 1 2 3 1 2 3 1 2)
(remove 2 list) will return ( 1 3 1 3 1 3 1 3 1)
which is probably not what you want.

If you really want to remove the last element of a list without
knowing what it is try this:

(defun remove_last (list)
(let ((new (list nil))) ; allocate a seed for tconc
(foreach map sublist list ; iterate over the cons cells
(when (cdr sublist) ; unless we are at the end of
the list
(tconc new (car sublist)))) ; efficiently append an element
(car new))) ; return the new list

this will also work, but consumes
a lot of memory for large lists.

(defun remove_last (list)
(reverse (cdr (reverse list))))

Thomas · Joined: 02 Dec 2005 Posts: 7

I certainly attempt to avoid dealing with the last element of the list in my program,
but the list is symmetric and the status of the first element is same as the last one, and so on.
All the elements is useful at first, then the first and last one need to be deleted.
I know that deleting one element of a list in other programming language is an trivial operation and a single traversal is enough.
I put forward the question since I am afraid of not reading the documents of SKILL thoroughly.
It has nothing to do with the data structure and code style of my program.

Andrew Beckett · Guest

On 6 Dec 2005 15:32:17 -0800, "Jimka" <jimka@rdrop.com> wrote:

fogh · Guest

Bernd Fischer > wrote:

Jimka · Guest

another way to do it would work if you can always remember
how long the list is. Since you are removing items in pairs,
first and last, the length decreases by 2 each time.

you can use (rplacd (nthcdr N the_list))

(nthcdr 0 ...) returns the list
(nthcdr 1 ...) returns the list after the first element
(nthcdr 2 ...) returns the list after the 2nd element

(sorry, i might be off by 1 in the index....)

so to remove everything after the Nth element (zero
based) you can use (rplacd (nthcdr N the_list))

Bernd Fischer · Guest

I had in mind that 'remove' needs an 'index' and not a
'value' as first arg, SORRY my mistake.

Bernd

fogh wrote:

Andrew Beckett · Guest

Andrew Beckett · Guest

On 7 Dec 2005 11:24:33 -0800, "Jimka" <jimka@rdrop.com> wrote:

fogh · Guest

It can also be argued that you should make 2 lists then. So that you would only
have to deal with 2 cars, not remq(car(last] or such. Then again... using 2 cars
is not a responsible attitude, especially SUVs (or cadence BMWs).

Thomas wrote:

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