Introduction
HDF is a package of QCL commands
which enable you to manipulate Starlink Heirarchical Data System (HDS)
structures and data objects.
HDS is a flexible system for storing and retrieving data and takes
over from a computer's filing system at the level of an individual
file (see figure 1). In fact, there are
many parallels between the hierarchical way HDS stores data within
files and the way that a filing system organises the files
themselves. The advantage of HDS is that it allows many different
kinds of data to be stored in a consistent and logical fashion. It is
also very flexible, in that objects can be added or deleted whilst
retaining the logical structure.
HDS files are known as container files and contain data
objects which will often be referred to simply as
objects. An object is an entity which contains data or other
objects. This is the basis of the hierarchical nature of HDS and is
analogous to the concepts of file and directory. The
primary way of identifying an HDS object is by its name, which
must be unique within its own container object. The maximum length of
an HDS name is 15 characters. HDS objects can be one of two types:
Structure objects contain other objects called components.
Primitive objects contain only numeric, character, or logical
values. Structures are analogous to the directories in a filing system
and primitives are analogous to files. Structure types are
represented by a character string of maximum length 15 characters (a
structure type may also be completely blank). A structure type may not
start with an underscore character "_", so they are easily
distinguished from primitive types, which do. Primitive types are
pre-defined as follows
| HDS type |
FORTRAN type |
Description |
| _INTEGER |
INTEGER |
Integer (signed) |
| _REAL |
REAL |
Single Precision |
| _DOUBLE |
DOUBLE PRECISION |
Double precision |
| _LOGICAL |
LOGICAL |
Logical |
| _CHAR[*n] |
CHARACTER[*n] |
Character string |
Every HDS object has a shape or dimensionality, even
structures although these usually have a dimension of zero.
To summarise:
- A container file name joke.sdf is like a directory
directory dir.
- A structure object name node1.node2 is like a directory
dir1/dir2.
- A primitive object name time_sample is like a file name
data_file.dat.
Different object names can be joined together to form a complete
specification of a data object.
joke.sdf%node1.node2.time_sample
The % character is used to separate the container file name from
the internal structure specification.
HDF also allows access to "slices" of dimensioned
primitive objects in HDS files. This is done by appending a slice
specification to the end of the object name.
array(3:4,5) or data(3,4,5) etc.
The slice specification is enclosed in parentheses and ranges of a
given subscript are indicated by two integers separated by a colon. If
no colons are present then a single cell or element of the object is
refered to.
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