Topic: mapcar or foreach

[curmudgeon]

they both work - I have never made much use of mapcar. but then, I didn't always appreciate the times to use if and when to use a cond statement.

does either one of these have an advantage?

Code: [Select]

(mapcar 'mode_tile '("dr_gr" "dr_ao" "dr_ex" "dr_pd" "jmb_p") '(1 1 1 1 1))
(foreach z '("dr_gr" "dr_ao" "dr_ex" "dr_pd" "jmb_p") (mode_tile z 1))
thanks

[MP]

The second one's intent is far clearer to me, and is less likely to break when adding a new tile key and thus enjoys significant preference over the first.

[curmudgeon]

thank you. I guess I am STILL looking for a place in  my code where mapcar is the best choice.
have to agree with you, and I did not have to state the second list '(1 1 1 1 1) explicitly.

cheers.
getting to enjoy dialog boxes.
 

[MP]

I guess I am STILL looking for a place in  my code where mapcar is the best choice.

When you wish the result of action performed upon a list to be returned as some embellished version of said list. I'm sure others will chime in with examples. Too much on the go; cheers.

[gile]

mapcar may be useful too when you're dealing with more than one list, even you don't want the expression to return a list.

Code - Lisp: [Select]

1. (mapcar
2.   '(lambda (f v)
3.      (eval
4.        (list
5.          'defun
6.          f
7.          ()
8.          (list 'command "_.view" v)
9.          '(princ)
10.        )
11.      )
12.    )
13.   '(c:0 c:1 c:2 c:3 c:4 c:5 c:6 c:7 c:8 c:9)
14.   '("_bottom" "_swiso" "_front" "_seiso" "_left" "_top" "_right" "_nwiso" "_back" "_neiso")
15. )

[MP]

Salient point and practical example. Thanks Gile.

[CAB]

A gold star for gile

More on the subject:
----------  Mapcar  Lambda  Apply   ------------
http://www.theswamp.org/index.php?topic=2953.0   (MP)
http://www.theswamp.org/index.php?topic=340.0   (CAB)
http://www.cadtutor.net/forum/showthread.php?52127-Mapcar-lambda-Description
http://lee-mac.com/mapcarlambda.html

[curmudgeon]

very useful. after reading several versions, it may be sinking in.
need coffee.
thanks again.

[irneb]

Gile's version using foreach instead :

Code - Auto/Visual Lisp: [Select]

1. (foreach item '((c:0 . "_bottom") (c:1 . "_swiso") (c:2 . "_front") (c:3 . "_seiso")...)
2.   (eval (list 'defun (car item) () (list 'command "_.view" (cdr item)) '(princ))))

IMO mapcar is only to be used when you want to modify the list somehow. It's not that it's "wrong" as such, there's 2 reasons I like foreach better in this case:

• You don't need a lambda
• When compiling, foreach optimizes better than mapcar.

Only issue with foreach is it only works on one list. But then you could use mapcar to combine two (or more) lists into one so you can send them to foreach. E.g. modifying the above to use the best parts of mapcar and foreach together:

Code - Auto/Visual Lisp: [Select]

1. (foreach item (mapcar 'cons
2.                       '(c:0 c:1 c:2 c:3 c:4 c:5 c:6 c:7 c:8 c:9)
3.                       '("_bottom" "_swiso" "_front" "_seiso" "_left" "_top" "_right" "_nwiso" "_back" "_neiso"))
4.   (eval (list 'defun (car item) () (list 'command "_.view" (cdr item)) '(princ))))

Notice the mapcar uses cons to combine the items from both lists into a dotted pair as I had in the previous sample.