variable.(key) what is?

[shivax]

hi, i see in example code the .()  after variable 

Example:

function v = mod09_05_state(key, val)
  persistent state;
  if (nargin == 0)  
    % Return the entire struct
     printf("0 \n" );
    v = state;
  elseif (nargin == 1)
    % Return a particular field from the struct
    printf("1 \n" );
    v = state.(key);
  elseif (nargin == 2)
      printf("2 \n" );
    % Set a key value (key must be a string)
    state.(key) = val;
    v = val;
  end
end

what is  .(key)? i dont find in help octave variable



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