The exclamation mark ‘!’ (aka “Bang!”) is a negation operator, just like the tilde ‘~’:
     octave:1> if ! strcmp (program_name, "octave"),
     >   "It's an error"
     > else
     >   "It works!"
     > end
     ans = It works!
   Note however that Matlab uses the ‘!’ operator for shell
escapes, for which Octave requires using the system command.