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.