How to resolve the algorithm Last Friday of each month step by step in the Common Lisp programming language

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

How to resolve the algorithm Last Friday of each month step by step in the Common Lisp programming language

Table of Contents

Problem Statement

Write a program or a script that returns the date of the last Fridays of each month of a given year. The year may be given through any simple input method in your language (command line, std in, etc).

Example of an expected output:

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Last Friday of each month step by step in the Common Lisp programming language

Source code in the common programming language

(defun friday-before (year month day)
 (let* 
  ((timestamp (encode-universal-time 0 0 12 day month year))
   (weekday (nth 6 (multiple-value-list (decode-universal-time timestamp))))
   (fri (- timestamp (* (+ (mod (+ weekday 2) 7) 1) 86400))))
    (multiple-value-bind (_ _ _ d m y) (decode-universal-time fri)
     (list y m d))))
    
(defun last-fridays (year) 
  (append (loop for month from 2 to 12 collecting (friday-before year month 1))
          (list (friday-before (1+ year) 1 1))))

(let* ((year (read-from-string (car *args*))))
  (format t "~{~{~a-~2,'0d-~2,'0d~}~%~}" (last-fridays year)))

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