Structures

Array data elements are all of the same type

Structures allow user defined types in which the elements can be just about anything

Formal definition
```
      struct tag {
        data_type_1 name_1;
        data_type_2 name_2;
        ...
      };
    
```
Data types can vary and can include any known type including other structs. Data names must be unique within the struct. No data initialization allowed in the structure declaration.

Example:

      struct particle_type {
        double pos [3];
        double mass;
        int type;
        char name [80];
      };

      struct particle_type neutron;