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Bob gilong
Bob gilong

Reputation: 215

How to get dotted pairs in Lisp?

I've searched google and other places for a while but I can't find out how to generate or create dotted pairs. I ask this because, I need to turn a list that looks like this:

(X Y Z)

To a list that is in this format:

((X . 1) (Y . 2) (Z . 3))

Where the numbers represent the index. I have a function that turns the list into the format of

(X 1 Y 2 Z 3)

Here is that function:

  (defun listFormat (l)
     (defun place-index (idx l)
        (if (null l)
          nil
          (append (list (first l)) (list idx)
                  (place-index (+ idx 1) (rest l)))))
     (place-index 1 l))

But I'm not sure how to get dotted pairs. Thanks in advance

Upvotes: 1

Views: 2857

Answers (3)

Charles Lew
Charles Lew

Reputation: 118

And by the way, for the question itself, this code will do:

(defun listFormat (lst)
  (loop for idx from 1
        for item in lst
        collect (cons item idx)))

Upvotes: 1

Rainer Joswig
Rainer Joswig

Reputation: 139411

Your code has a very basic mistake:

(defun listFormat (l)
     (defun place-index (idx l)    ; <<<---- This DEFUN is wrong
        (if (null l)
          nil
          (append (list (first l)) (list idx)
                  (place-index (+ idx 1) (rest l)))))
     (place-index 1 l))

Don't nest DEFUN. That's just wrong. DEFUN defines a global function. Whenever you run listFormat, it redefines the GLOBAL function PLACE-INDEX. You may have seen similarly nested functions in SCHEME using DEFINE. In Common Lisp you should not use DEFUN for a nested local function.

In Lisp local functions are defined with FLET or LABELS (for recursive functions).

(defun listFormat (l)
   (labels ((place-index (idx l)
              (if (null l)
                  nil
                  (append (list (first l)) (list idx)
                          (place-index (+ idx 1) (rest l))))))
       (place-index 1 l)))

Also Stackoverflow is the wrong place to solve your homework. Google searches is also the wrong way to learn Lisp programming.

I propose to use the good old way of reading an introductory book and using a reference.

Here is a basic introductory Lisp book for download: Common Lisp: A Gentle Introduction to Symbolic Computation.

Reference sheets: a small Common Lisp Quick Reference (PDF) and a more detailed Common Lisp Quick Reference.

Dotted pairs are called conses in Lisp.

See the real online reference for Common Lisp, the Common Lisp HyperSpec.

Upvotes: 11

Raph Levien
Raph Levien

Reputation: 5218

You want the else-branch to read:

(cons (cons (first l) idx) (place-index (+ idx 1) (rest l)))

Upvotes: 6

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