Function fieldnames - structure functions:
The function fieldnames will return the names of the fields which are contained in the structure variable.
>> pack_fields = fieldnames(package)
As the name of the fields is of different lengths, the function fieldnames returns a cell array with the names of the fields.
To refer to the elements the curly braces are used, as pack_fields is a cell array.
For illustration, we can refer to the length of one of the strings as shown below: