Remove Items from a List

[Lee Mac]

Just wondered if there was a better way to approach this than what I have currently:

 

[b][color=RED]([/color][/b][b][color=BLUE]defun[/color][/b] remove_items  [b][color=RED]([/color][/b]lst items[b][color=RED])[/color][/b]
 [b][color=RED]([/color][/b][b][color=BLUE]vl-remove-if[/color][/b]
   [b][color=DARKRED]'[/color][/b][b][color=RED]([/color][/b][b][color=BLUE]lambda[/color][/b] [b][color=RED]([/color][/b]j[b][color=RED])[/color][/b] [b][color=RED]([/color][/b][b][color=BLUE]member[/color][/b] [b][color=RED]([/color][/b][b][color=BLUE]vl-position[/color][/b] j lst[b][color=RED])[/color][/b] items[b][color=RED])[/color][/b][b][color=RED])[/color][/b] lst[b][color=RED])[/color][/b][b][color=RED])[/color][/b]

[David Bethel]

Lee,

 

Here is very old and fairly slow way to delete an atom from a list:

[b][color=BLACK]([/color][/b]defun delatom [b][color=FUCHSIA]([/color][/b]a l / tmp[b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]while l
    [b][color=NAVY]([/color][/b]if [b][color=MAROON]([/color][/b]not [b][color=GREEN]([/color][/b]equal a [b][color=BLUE]([/color][/b]car l[b][color=BLUE])[/color][/b] 1e-11[b][color=GREEN])[/color][/b][b][color=MAROON])[/color][/b]
        [b][color=MAROON]([/color][/b]setq tmp [b][color=GREEN]([/color][/b]cons [b][color=BLUE]([/color][/b]car l[b][color=BLUE])[/color][/b] tmp[b][color=GREEN])[/color][/b][b][color=MAROON])[/color][/b][b][color=NAVY])[/color][/b]
    [b][color=NAVY]([/color][/b]setq l [b][color=MAROON]([/color][/b]cdr l[b][color=MAROON])[/color][/b][b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]reverse tmp[b][color=FUCHSIA])[/color][/b][b][color=BLACK])[/color][/b]

[b][color=BLACK]([/color][/b]setq tl '[b][color=FUCHSIA]([/color][/b][b][color=NAVY]([/color][/b]0 . [color=#2f4f4f]"LINE"[/color][b][color=NAVY])[/color][/b] 234 [color=#2f4f4f]"ABC"[/color] 456.78 sym [b][color=NAVY]([/color][/b]10 0 0 1[b][color=NAVY])[/color][/b] [color=#2f4f4f]"DEF"[/color][b][color=FUCHSIA])[/color][/b][b][color=BLACK])[/color][/b]

[b][color=BLACK]([/color][/b]prin1 [b][color=FUCHSIA]([/color][/b]delatom [color=#2f4f4f]"ABC"[/color] tl[b][color=FUCHSIA])[/color][/b][b][color=BLACK])[/color][/b]

And a time test routine:

 

[b][color=BLACK]([/color][/b]defun Diesel [b][color=FUCHSIA]([/color][/b]c[b][color=FUCHSIA])[/color][/b] [b][color=FUCHSIA]([/color][/b]menucmd [b][color=NAVY]([/color][/b]strcat [color=#2f4f4f]"M="[/color] c[b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b][b][color=BLACK])[/color][/b]

[b][color=BLACK]([/color][/b]defun c:timetest [b][color=FUCHSIA]([/color][/b]/ st ft[b][color=FUCHSIA])[/color][/b]

[color=#8b4513];| Set Any Variables Here |;[/color]
 [b][color=FUCHSIA]([/color][/b]setq st [b][color=NAVY]([/color][/b]getvar [color=#2f4f4f]"DATE"[/color][b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b]

[color=#8b4513];;; Preform all tests in this section[/color]

 [b][color=FUCHSIA]([/color][/b]repeat 1000  [color=#8b4513];|Vary repeat parameter with complexity of test call|;[/color]

  [b][color=NAVY]([/color][/b]delatom [color=#2f4f4f]"ABC"[/color] tl[b][color=NAVY])[/color][/b]

 [b][color=FUCHSIA])[/color][/b][color=#8b4513];End Repeat[/color]

[color=#8b4513];| Have all functions tested ended by here |;[/color]

 [b][color=FUCHSIA]([/color][/b]setq ft [b][color=NAVY]([/color][/b]getvar [color=#2f4f4f]"DATE"[/color][b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]setvar [color=#2f4f4f]"USERR1"[/color] [b][color=NAVY]([/color][/b]- ft st[b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]princ [b][color=NAVY]([/color][/b]diesel [b][color=MAROON]([/color][/b]strcat [color=#2f4f4f]"$[b][color=GREEN]([/color][/b]edtime,$[b][color=BLUE]([/color][/b]getvar,userr1[b][color=BLUE])[/color][/b],MM\"[/color]:\[color=#2f4f4f]"SS\"[/color].\[color=#2f4f4f]"MSEC[b][color=GREEN])[/color][/b]"[/color][b][color=MAROON])[/color][/b][b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]princ 1[b][color=FUCHSIA])[/color][/b][b][color=BLACK])[/color][/b]

-David

[Lee Mac]

Thanks David,

 

I had originally thought of shuffling through the list, using a (setq lst (cdr lst)) method, but I found that this would only be useful for removing a single item, and if there are duplicates in the list, it would remove both instances of the duplicate, instead of the one requested.

 

PS, I like the time-test routine... very novel.

[taner]

(defun remove-lst-items (lst itemlst / i)
 (setq i -1)
 (vl-remove-if
   '(lambda (x)
      (member (setq i (1+ i))
       itemlst
      )
    )
   lst
 )
)

[Lee Mac]

^^ nice Taner, very original

[CAB]

Other methods:

http://www.theswamp.org/index.php?topic=28100.msg336941#msg336941

[Lee Mac]

Wow

 

It seems the swamp has much more advanced methods than here... tons of recursive functions (they make my head hurt).

[Se7en]

Nah, dont let it hurt. Recursion is the simplest loop you can make.

[Lee Mac]

I just find it hard to work out what the function is doing when you are using a recursive process.

[Se7en]

Maybe some code with comments will help enlighten.

 

With recursion we can itterate thru a list. For example we can build a simple recursive procedure to copy a list.

 

Read the comments to better understand the "Flow" of the procedure durring its operation.

 

(defun copy-tree (x)
 ;; Return an exact copy of a given list.
 ;; (nested lists are this procedures speciality.)
 ;;
 ;; EX: (copy-tree '(1 2 (3 (4 5) 6 (nil)) 7 (()))
 ;;  > (1 2 (3 (4 5) 6 (nil)) 7 (())
 (if (atom x)                   ; If "x" is only an item, not a list,
   x                            ; return the item...else,
   (cons (copy-tree (car x))    ; build a list, sending the next item
                                ; back thru itself along with
         (copy-tree (cdr x))    ; the remander of the items,
                                ; via backtrace.
                                ;
                                ; NOTE:
                                ; last fuction `cons' returns assembled
                                ; list of items or exact copy of what
                                ; was passed to this procedure.
         )
   )
 )

 

Using the above we can build a slightly modified version to incriment the numbers in that list.

 

(defun copy-incriment-tree (x)
 ;; Return a list contaning numbers incrimented by one.
 ;; (nested lists are this procedures speciality.)
 ;;
 ;; NOTE: This procedure will only incriment numbers, it will 
 ;;       leave any non number member of the given list alone.
 ;;
 ;; Ex: (copy-incriment-tree '(1 2 (3 (4 5) 6 (nil "a" 1.225)) 7 (()))
 ;; > (2 3 (4 (5 6) 7 (nil "a" 2.225)) 8 ((9)))
 (if (atom x)
    (if (numberp x)
      ;; if its a number, we know we can add one to it.
       (1+ x)
      ;; if the numberp condition fails then its not a number
      ;; so just return the item
       x
       )
   (cons (copy-incriment-tree (car x))
         (copy-incriment-tree (cdr x))
         )
   )
 )

 

Does that help?

[Lee Mac]

That helps a little more.

 

I suppose it acts like a WHILE statement that iterates through a list until there is no list left.

 

Many thanks John, for your time and patience.

 

Cheers

 

Lee

[Se7en]

yeah sorta. However a WHILE (or other looping constructs) puts less strain on the memory stack...but that's a good enough thought process for now.

"Recursion is a simple way to do something to items of a list".

 

No problem, i like to help.