Format

Format is one of those features in Common Lisp that people seem to have a love/hate relationship with, the other being loop. Both have their own mini languages that are not especially lispy. I'm still not too sure where I stand in this debate but being able to say things like this is still pretty incredible.

(defun print-diamond (str x &optional c a (max (1+ x)) (star "*"))
  (format str "~&~[~:;~:*~vT~v@{~A ~:*~}~3@*~v,v/print-diamond/~:*~v[~:;~@*~vT~v@{~A ~:*~}~%~]~]"
          x (- max x) star max star (1- x)))

Now for an explanation of that format string.
Bear with me this is hairy.

• ~& : Conditional newline (insert newline if we aren't at the start of a line)


• ~[ : Process the nth clause (seperated by ~:). ~:; specifies a default clause. This consumes the argument x. This directive causes print-diamond to stop processing when x is zero.
  


• ~:* : Backup 1 argument (unconsumes the argument x)


• ~vT : Insert arg number of spaces (this reconsumes the first argument ie. x)


• ~v@{ : Iterate v times, v is (- max x).


• ~A : Print 1 element


• ~:* : Backup 1 element (The effects of the last 3 directives is to print star (- max x) times)


• ~} : End loop


• ~3@ : Move to the 3rd argument (star)


• ~v,v/print-diamond/ : The recursive call. invoke print-diamond again with max, star and (1- x), This is equivalent to calling (print-diamond stream (1- x) nil nil max star)


• ~:* : Backup 1 argument (unconsumes (1- x))


• ~v[ : Conditional processing based (1- x).


• ~@* : Go to the start of the argument list.


• ~vT : Insert arg number of spaces (this consumes the first argument again)


• ~v@{ : Iterate v times, v is (- max x).
  
  
  • ~A : Print 1 element
  
  
  • ~:* : Backup 1 element (The effects of the last 3 directives is to print star (- max x) times)
  
  
• ~} : End loop


• ~% : Insert a new line


• ~] : End conditional processing


• ~] End conditional processing

Whew, brain freeze.

now we can do (print-diamond t 10)

         *
        * *
       * * *
      * * * *
     * * * * *
    * * * * * *
   * * * * * * *
  * * * * * * * *
 * * * * * * * * *
* * * * * * * * * *
 * * * * * * * * *
  * * * * * * * *
   * * * * * * *
    * * * * * *
     * * * * *
      * * * *
       * * *
        * *
         *