What is the use of mapcar & lamda?

[ESan]

About 2 weeks into my use of AutoLISP. I understand that mapcar evaluates each item seperately, and lamda is used as an undefined function, but:

 

In what situation would you need to use mapcar and lamda? Why would you need an output of a string of values other than presenting the output to the user?

[Emmanuel Delay]

Lee Mac wrote a web page on this topic

https://www.lee-mac.com/mapcarlambda.html

[GLAVCVS]

Basically, 'mapcar' allows you to process each member of a list and return a value for each, grouped equally in another list.
How does 'mapcar' process that list? You can do so using a simple language function (for example, you could apply  '-  to a list of integers to change the sign of each one and obtain the resulting list), or you could apply a more elaborate function that could be implemented using 'lambda', which is nothing more than a temporary substitute for 'defun' that allows you to execute your code without it remaining in memory after execution.
It would be conceptually similar to implementing a local defun within a public defun: the local defun only exists during the execution of the public defun.

[Steven P]

I like mapcar - LISP is a list processing language and mapcar is great for lists. A couple of examples

 

(setq Answer (mapcar 'Modifier' 'List to process 1' 'List to process 2' )) format

 

(setq Answer (mapcar '* '(1 1 0) '(15 20 25) )) ;;modifier is multiply list 1 by list 2 - list 2 is a coordinate, here set z to 0... flatten in
---> Answer: (15 20 0)


(setq Answer (mapcar '+ '(15 0 0) '(15 20 25) )) ;;modifier is multiply list 1 by list 2 - list 2 is a coordinate, offset X direction
---> Answer: (30 20 25)

(setq Answer (mapcar 'strcase '("adam" "ben" "claire" "david"))) ;; Modifier is a function, strcase applied to all in the list
---> Answer: ("ADAM" "BEN" "CLAIRE" "DAVID")

 

That last example was shamelessly taken from Lee Macs tutorial: https://lee-mac.com/mapcarlambda.html

[GLAVCVS]

Although it may complicate the explanation a bit, as StevenP shows in his examples, it is also possible to apply the modifier code to multiple lists, but with two important considerations in mind:
- all lists should have the same number of elements because the result of the operations will stop at the last element of the shortest list
- the result of the operations will be performed, each time, on the same ordinal elements of each list, returning a result for each operation and adding them to a result list that will be returned at the end of all operations

[rlx]

(mapcar '(lambda (x y) (/ (float x)(float y))) (list 2 4 6 8) (list 1 3 7 9))
(mapcar (function (lambda (x y) (/ (float x)(float y)))) (list 2 4 6 8) (list 1 3 7 9))

(defun my-funk (x y)(/ (float x)(float y)))
(mapcar 'my-funk (list 2 4 6 8) (list 1 3 7 9))

 ;;; -> (2.0 1.33333 0.857143 0.888889)

 

[Steven P]

my-funk ?

[ESan]

Quote

3 hours ago, Steven P said:

(setq Answer (mapcar '* '(1 1 0) '(15 20 25) )) ;;modifier is multiply list 1 by list 2 - list 2 is a coordinate, here set z to 0... flatten in
---> Answer: (15 20 0)


(setq Answer (mapcar '+ '(15 0 0) '(15 20 25) )) ;;modifier is multiply list 1 by list 2 - list 2 is a coordinate, offset X direction
---> Answer: (30 20 25)

(setq Answer (mapcar 'strcase '("adam" "ben" "claire" "david"))) ;; Modifier is a function, strcase applied to all in the list
---> Answer: ("ADAM" "BEN" "CLAIRE" "DAVID")

 

From the response I've seen, it seems like mapcar is only used to give an output of multiple separate values. I see how lamda would be used in a greater LISP to locally define throw-away functions. However, I still don't fully understand how the return values of mapcar can/would be used in a greater LISP routine. 

 

Say I want to calculate values based on set integers, and a user input integer. Can I use mapcar to:

1. Calculate using the set and user input value, and

2. Use those values in an if or cond function?

 

I may be misunderstanding when this function is most commonly used, so if what I am asking is not making sense, my lack of knowledge would be the reason.

[SLW210]

Another good read ... (mapcar) and (lambda) | AfraLISP

 

A related thread ...