• Aedis@lemmy.world
    0·
    23 days ago

    I’m partial to a recursive solution. Lol

    def is_even(number):
    if number < 0 or (number%1) > 0:
        raise ValueError("This impl requires positive integers only") 
    if number < 2:
        return number
    return is_even(number - 2)
    • tetris11@lemmy.ml
      0·
      23 days ago

      I prefer good ole regex test of a binary num

      function isEven(number){
   binary=$(echo "obase=2; $number" | bc)
   if [ "${binary:-1}" = "1" ]; then
         return 255
   fi
   return 0
}
      • balsoft@lemmy.ml
        0·
        23 days ago

        Amateur! I can read and understand that almost right away. Now I present a better solution:

        even() ((($1+1)&1))

        (I mean, it’s funny cause it’s unreadable, but I suspect this is also one of the most efficient bash implementations possible)

        (Actually the obvious one is a slight bit faster. But this impl for odd is the fastest one as far as I can tell odd() (($1&1)))

          • balsoft@lemmy.ml
            0·
            22 days ago

            I don’t think there’s much to codegolf. The “obvious” solution (even() (($1%2))) is both shorter and faster. Don’t think it can be optimized much more.

        • tetris11@lemmy.ml
          0·
          23 days ago

          woah your bash is legit good. I thought numeric pretexts needed $(( blah )), but you’re ommiting the $ like an absolute madman. How is this wizardy possible

            • tetris11@lemmy.ml
              0·
              23 days ago

              Oh I see it, but for some reason I was taught to always use $(( arith )) instead of (( arith )) and I guess I’m just wondering what the difference is

              • balsoft@lemmy.ml
                0·
                23 days ago

                The difference is that (( is a “compound command”, similar to [[ (evaluate conditional expression), while $(( )) is “aritmetic expansion”. They behave in almost exactly the same way but are used in different contexts - the former uses “exit codes” while the latter returns a string, so the former would be used where you would expect a command, while the latter would be used where you expect an expression.

                This is similar to how there is ( compound command (run in a subshell), and $( ) (command substitution). You can actually use the former to define a function too (as it’s a compound command):

                real_exit() { exit 1; }
fake_exit() ( exit 1 )

                Calling real_exit will exit from the shell, while calling fake_exit will do nothing as the exit 1 command is executed in a separate subshell. Notice how you can also do the same in a command substition (because it runs in a subshell):

                echo $(echo foo; exit 1)

                Will run successfully and output foo.

                It is another one of those unknown, very rarely useful features of bash.