add dots

#!/usr/bin/env tclsh
            
if { $argc == 1 } {
   set num [lindex $argv 0]
} else {    
   puts "nope"
   exit     
}           
            
proc add_dots {num} {
   set length [string length $num]
   set out ""
   set o 0  
             
   for {set i [expr $length]} {$i > 0} {incr i -1} {
      incr o
      set char [string index $num $i-1]

      if { ![expr $o % 3] && $o != $length } {
         set out ".$char$out"
      } {
         set out "$char$out"
      }
   }

   return "$out"
}

set withdots [add_dots $num]
puts "$withdots"