Topic: mapcar or foreach

[Mark]

Can you tell me when it's appropriate to use either mapcar or foreach?

[LE]

I use;

- MAPCAR when I need to get LIST.
- FOREACH when I want to do several things to each item on a LIST and I do not require any output and simple I want to process ALL of them to certain function.

HTMSS

[JohnK]

^^ Same for me.

Here is a quick demo.

Command:

Command: (setq starterlist '(1 2 3 4 5 6 7 8 9))
(1 2 3 4 5 6 7 8 9)

Command: (setq mylist starterlist)
(1 2 3 4 5 6 7 8 9)

Command: (repeat 7 (setq mylist (append starterlist mylist)))
(1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3
4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9)

Command: (setq mapcarlist (mapcar '(lambda (x) (1+ x)) mylist))
(2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2
3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10)

Command: (setq foreachlist (foreach x mylist (1+ x)))
10

Command: !mapcarlist
(2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2
3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10)

Command: !foreachlist
10

Command:

[MP]

Lile Luis said / implied, generally speaking -- when I want the result of an operation on a list to return a list I use mapcar, otherwise foreach.

Interestingly, mapcar is frequently used when foreach is actually more appropriate.

Classic example --

(start_list "key")
(mapcar 'add_list items)
(end_list)

Result is discarded.

I suspect it's the brevity of mapcar that frequently inspires it's use.

The foreach equivalent just for completeness --

(start_list "key")
(foreach item items (add_list item))
(end_list)

[LE]

It is somehow what happens to the function REPEAT that [according to my impression] not many are using it anymore [including me] or maybe very little.

[MP]

I use repeat often --

(if (setq ss (ssget))
    (repeat (setq i (sslength ss))
        (foo (ssname ss (setq i (1- i))))
    )
)

[Joe Burke]

In addition to what's been said, the other key difference between foreach and mapcar is mapcar accepts multiple list arguments. Foreach only takes one list argument.

As Michael said, which one to use given a single list argument is sometimes a toss up.

[Mark]

How about some examples of working with an entity list?

[MP]

Context please.

[Mark]

May not be the best example.

Code: [Select]

(setq ss (ssget "X" '((0 . "TEXT")))
      cntr 0
      )

(while (setq ent (ssname ss cntr))
  ;
  ; pull out significant parts, DXF values 1, 8, 40, 410, etc.
  ;
  (setq cntr (1+ cntr))
  )

[LE]

In example change the ename's color to RED

Code: [Select]

(foreach
       ename  (list ent1 ent2 ent3)
  (setq elist (entget ename))
  ;; if they are bylayer change to color 1
  (if (not (cdr (assoc 62 elist)))
    (entmod (append elist (list (cons 62 1))))))


Code: [Select]

(mapcar '(lambda (obj) (vla-put-color obj acRed))
(mapcar 'vlax-ename->vla-object (list ent1 ent2 ent3)))


[MP]

Code: [Select]

(if (setq ss (ssget "x" '((0 . "text"))))
    (repeat (setq i (sslength ss))
        (setq data (entget (ssname ss (setq i (1- i)))))
        ;;
        ;;  pull out significant parts, DXF values 1, 8, 40, 410, etc.
        ;;
        (print
            (mapcar
               '(lambda (key) (assoc key data))
               '(1 8 40 410)
            )
        )
        (princ)
    )
)

[MP]

Sorry don't have time to pen something pretty but perhaps this will illuminate ...

Code: [Select]

(defun c:CatSkinning ( / ss i keys data )

    (setq keys '(8 40 1))

    (if (setq ss (ssget "x" '((0 . "text"))))

        (repeat (setq i (sslength ss))

            (setq data (entget (ssname ss (setq i (1- i)))))

            ;;  mapcar 1

            (princ "\nmapcar 1 ...")

            (print
                (mapcar
                   '(lambda (key) (assoc key data))
                    keys
                )
            )

            ;;  mapcar 2 (not desirable, but for
            ;;  demonstration purposes included)

            (princ "\nmapcar 2 ...")

            (print
                (apply 'append
                    (mapcar
                       '(lambda (pair)
                            (if (member (car pair) keys)
                                (list pair)
                            )
                        )
                        data
                    )
                )
            )   

            ;;  vl-remove-if-not

            (princ "\nvl-remove-if-not ...")

            (print
                (vl-remove-if-not
                   '(lambda (pair) (member (car pair) keys))
                    data
                )
            )

            ;;  foreach

            (princ "\nforeach ...")

            (print
                (   (lambda ( / result )
                        (foreach pair data
                            (if (member (car pair) keys)
                                (setq result
                                    (cons
                                        pair
                                        result
                                    )
                                )
                            )
                        )
                        (reverse result)
                    )
                )
            )

            (princ "\n\n")

        )
    )

    (princ)

)

[paulmcz]

"foreach" can speed things up significantly. Compare these two ways of drawing an ellipse:

This one with use of "foreach"

Code: [Select]

(defun c:lll (/ osn a b x ip clv clh p1 p2 cpy cpx e1 e2 z plp cc)
; draws ellipse out of polyline
  (setq osn (getvar "OSMODE"))
  (setvar "cmdecho" 0)
;====================================================================
  (if aa
    ()
    (setq aa 1.0)
  )
  (princ "\n A radius < ") ; short radius
  (princ aa)
  (princ " > ?? : ")
  (initget 6)
  (setq a (getdist))
  (if (= a nil)
    (setq a aa)
  )
  (setq aa a)
;==================================================================
  (if bb
    ()
    (setq bb 2.0)
  )
  (princ "\n B radius < ")
  (princ bb)
  (princ " > ?? : ")
  (initget 6)
  (setq b (getdist))
  (if (= b nil)
    (setq b bb)
  )
  (setq bb b)
;==================================================================

  (setq x (/ pi 90))

;====================================================================
  (setq ip (getpoint "\n Insert : "))

; calc

  (setvar "osmode" 0)
  (command "line"
   (polar ip (* pi 0.5) (* a 1.125))
   (polar ip (* pi 1.5) (* a 1.125))
   ""
  )
  (setq clv (entlast))
  (command "change"
   clv
   ""
   "p"
   "c"
   3
   "lt"
   "center"
   "s"
   (/ a 4)
   ""
  )
  (command "line"
   (polar ip 0 (* b 1.125))
   (polar ip pi (* b 1.125))
   ""
  )
  (setq clh (entlast))
  (command "change"
   clh
   ""
   "p"
   "c"
   3
   "lt"
   "center"
   "s"
   (/ a 4)
   ""
  )
  (setq p1 (polar ip (* pi 1.5) a))
  (setq cpy (polar ip (* pi 1.5) (* a (cos x))))
  (setq cpx (polar ip 0 (* b (sin x))))
  (setq p2 (list (car cpx) (cadr cpy)))
  (command "pline" p1 p2 "")
  (setq e1 (entlast))
  (setq z (* 2 x))
  (while (< z (+ (* 2 pi) (/ z 2)))
    (setq plp (cons p2 plp))
    (setq cpy (polar ip (* pi 1.5) (* a (cos z))))
    (setq cpx (polar ip 0 (* a (sin z) (/ b a))))
    (setq p2 (list (car cpx) (cadr cpy)))
    (setq z (+ z x))
  )
  (setq plp (reverse plp))
  (command "pline")
  (foreach cc plp (command cc))
  (command "")
  (setq e2 (entlast))
  (command "pedit" e1 "j" e2 "" "")


  (prompt "\nRotation Angle: ")
  (setvar "osmode" osn)
  (command "ROTATE" e1 clv clh "" IP pause)

  (princ)
)
(prompt "\n Type < LLL > for ellipse")
... and this one stepping through segments of polyline:

Code: [Select]

(defun c:lll (/ osn a b x ip clv clh p1 p2 cpy cpx e1 e2 z) ; draws ellipse out of polyline
  (setq osn (getvar "OSMODE"))
  (setvar "cmdecho" 0)
;====================================================================
  (if aa
    ()
    (setq aa 1.0)
  )
  (princ "\n A radius < ") ; short radius
  (princ aa)
  (princ " > ?? : ")
  (initget 6)
  (setq a (getdist))
  (if (= a nil)
    (setq a aa)
  )
  (setq aa a)
;==================================================================
  (if bb
    ()
    (setq bb 2.0)
  )
  (princ "\n B radius < ")
  (princ bb)
  (princ " > ?? : ")
  (initget 6)
  (setq b (getdist))
  (if (= b nil)
    (setq b bb)
  )
  (setq bb b)
;==================================================================
  ;|(if xp ()(setq xp 2))
  (princ "\n Precision in degrees [1 - 10] < ")
  (princ xp)
  (princ " > ?? : ")
  (setq x (getint))
  (if (> x 10)(setq x 10))
  (if (or (= x 3) (= x 4))(setq x 2))
  (if (or (= x 7) (= x 8) (= x 9))(setq x 6))
  (if (= x nil)(setq x (/ (* xp pi) 180))(setq x (/ (* x pi) 180)))
  (setq xp (/ (* x 180) pi))|;
  (setq x (/ pi 90))

;====================================================================
  (setq ip (getpoint "\n Insert : "))

; calc

  (setvar "osmode" 0)
  (command "line"
   (polar ip (* pi 0.5) (* a 1.125))
   (polar ip (* pi 1.5) (* a 1.125))
   ""
  )
  (setq clv (entlast))
  (command "change"
   clv
   ""
   "p"
   "c"
   3
   "lt"
   "center"
   "s"
   (/ a 4)
   ""
  )
  (command "line"
   (polar ip 0 (* b 1.125))
   (polar ip pi (* b 1.125))
   ""
  )
  (setq clh (entlast))
  (command "change"
   clh
   ""
   "p"
   "c"
   3
   "lt"
   "center"
   "s"
   (/ a 4)
   ""
  )
  (setq p1 (polar ip (* pi 1.5) a))
  (setq cpy (polar ip (* pi 1.5) (* a (cos x))))
  (setq cpx (polar ip 0 (* b (sin x))))
  (setq p2 (list (car cpx) (cadr cpy)))
  (command "pline" p1 "w" 0 0 p2 "")
  (setq e1 (entlast))
  (setq z (* 2 x))
  (while (< z (* 2 pi))
    (setq cpy (polar ip (* pi 1.5) (* a (cos z))))
    (setq cpx (polar ip 0 (* a (sin z) (/ b a))))
    (setq p2 (list (car cpx) (cadr cpy)))
    (command "pline" "" "w" 0 0 p2 "")
    (setq e2 (entlast))
    (command "pedit" e1 "j" e2 "" "")
    (setq z (+ z x))
  )

  (prompt "\nRotation Angle: ")
  (setvar "osmode" osn)
  (command "ROTATE" e1 clv clh "" IP pause)
 
  (princ)
)
(prompt "\n Type < LLL > for ellipse")

[whdjr]

Foreach his own, but alas for me is mapcar.

As MP stated I use mapcar more for its brevity.


[sidebar]
MP,  it's rather ironic that you used 'ssget' to show that you use 'repeat' quite often when 'foreach' and 'mapcar' can't get 'ssget' entities.
[/sidebar]