LISP function that takes 2 lists as parameters

Question

After briefly talking about LISP in a past class, I have decided to jump in head first and try to learn CLISP (reading Seibel's PCL chpt 5). My question is in regards to writing a function that takes a set of lists as parameters. The first list is a series of indexes mapped to the second list. I want to pass a sequence of indexes and have it return the corresponding elements.

Here is the outline of my code so far. I wasn't sure if I could use nth and pass a list of arguments to it. I am not sure what the body-form should look like.

sys-info: CLISP 2.49 Win7

(defun get-elements('(nth (x y z) '(a b c)) )  
  "takes a list of arguments that returns corresponding elements from a list."
  (format t "The elements at index ~d are: ~%" x y z)
  ((maybe indexes go here)'(elements go here?))

The list (x y z) are the indexes and the data list (a b c) is some list of arbitrary elements. The evaluation is passed as data to the function get-elements. Am I even on the right track with this line of thinking?

Hints and pointers to relevant topics in LISP education are greatly appreciated.

postmortem: Upon re-examination of chpts 3-4, it would seem that PCL is a bit of a reach for a beginning programmer (at least for me). I can enter the code from the book, but I obviously don't have a deep understanding of the basic structure of the language. I will probably try a few gentler introductions to Lisp before undertaking PCL again.

Can you give an example of what your code is supposed to do? — sds
– sds, Commented Nov 16, 2017 at 16:57
There is a reason why a book is organized in chapters. Starting with chapter 5 of a book about a new language is unlikely to be a good idea. Why don't you start with chapter 1? — sds
– sds, Commented Nov 16, 2017 at 16:59
I should clarify that I have made it to chpt 5. After making the ripped CD db in chpt 3 I was hooked. — cogito tute
– cogito tute, Commented Nov 16, 2017 at 17:03
I would like the code to eventually do something like this (get-elements 0 1 0) -> (a b a) — cogito tute
– cogito tute, Commented Nov 16, 2017 at 17:05
If you did read the previous chapters, how come you have the quote in the lambda list of your function? — sds
– sds, Commented Nov 16, 2017 at 17:21

sds · Accepted Answer · 2017-11-16 22:37:30Z

3

I am not quite sure if this is what you are asking about, but you might want to try:

(defun get-nth (index-list data-list)
  (mapcar (lambda (index)
            (nth index data-list))
          index-list))
(get-nth '(0 1 0 2 0 3) '(a b c d e f))
==> (A B A C A D)

Please take a look at

mapcar
nth
Formatted Output - your format call is broken
Lather, Rinse, Repeat: A Tour of the REPL - you claim to have read it, but you obviously posted your code before pasting it into a REPL, so please review it again.

More gentle introductions to Lisp:

ANSI Common Lisp by Graham

edited Nov 16, 2017 at 22:37

answered Nov 16, 2017 at 17:26

sds

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1 Comment

cogito tute Over a year ago

Thank you, that is more than enough to get me started. fwiw, most of my self-generated code barfed in the repl. I will be more careful when posting code in the future.

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