HDS recognises a selection of primitive data types which correspond to Fortran data types but have names prefixed by an underscore. The correspondence between Fortran types and HDS data types is as follows:
| HDS Type | VAX FORTRAN Type |
| _INTEGER | INTEGER |
| _REAL | REAL |
| _DOUBLE | DOUBLE PRECISION |
| _LOGICAL | LOGICAL |
| _CHAR[*n] | CHARACTER*n |
| _UBYTE | BYTE |
| _BYTE | BYTE |
| _UWORD | INTEGER*2 |
| _WORD | INTEGER*2 |
For example, a variable declared as REAL in a program has HDS type ’_REAL’. It is necessary to
appreciate that if the data type is an argument in a routine then that argument should be ’_REAL’
rather than ’REAL’. For example:
N.B. HDS structures also have a type, although this is purely descriptive. For example, the type of an
axis structure in an NDF is AXIS. The only restriction on the names of structure types is
that they must not begin with an underscore (to distinguish them from primitive data
types).