How to resolve the algorithm Self-describing numbers step by step in the Common Lisp programming language

Published on 12 May 2024 09:40 PM
#Common lisp

How to resolve the algorithm Self-describing numbers step by step in the Common Lisp programming language

Table of Contents

Problem Statement

There are several so-called "self-describing" or "self-descriptive" integers. An integer is said to be "self-describing" if it has the property that, when digit positions are labeled 0 to N-1, the digit in each position is equal to the number of times that that digit appears in the number. For example,   2020   is a four-digit self describing number:

Self-describing numbers < 100.000.000  are:     1210,   2020,   21200,   3211000,   42101000.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Self-describing numbers step by step in the Common Lisp programming language

Source code in the common programming language

(defun to-ascii (str) (mapcar #'char-code (coerce str 'list)))

(defun to-digits (n)
  (mapcar #'(lambda(v) (- v 48)) (to-ascii  (princ-to-string n))))

(defun count-digits (n)
  (do
      ((counts (make-array '(10) :initial-contents '(0 0 0 0 0 0 0 0 0 0)))
       (curlist (to-digits n) (cdr curlist)))
      ((null curlist) counts)
    (setf (aref counts (car curlist)) (+ 1 (aref counts (car curlist)))))))
    
(defun self-described-p (n)
  (if (not (numberp n))
      nil
  (do ((counts (count-digits n))
       (ipos 0 (+ 1 ipos))
       (digits (to-digits n) (cdr digits)))
      ((null digits) t)
    (if (not (eql (car digits) (aref counts ipos))) (return nil)))))


(loop for i from 1 to 4000000 do (if (self-described-p i) (print i)))

1210 
2020 
21200 
3211000 
NIL

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