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<HTML><HEAD>
<TITLE>HDF5 Tutorial - Iterating over Group Members
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<H1>
<BIG><BIG><BIG><FONT COLOR="#c101cd">Iterating over Group Members</FONT>
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<H2>Contents:</H2>
<UL>
<LI><A HREF="#def">How to Iterate over Group Members Using C </A>
<LI><A HREF="#deff">How to Iterate over Group Members Using FORTRAN</A>
<LI>Programming Example
<UL>
<LI> <A HREF="#desc">Description</A>
<LI> <A HREF="#remc">Remarks for C Example</A>
<LI> <A HREF="#remf">Remarks for FORTRAN Example</A>
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<A NAME="def">
<H2>How to Iterate over Group Members Using C</H2>
This section discusses how to find names and object types of HDF5 group
members using C.
<P>
The HDF5 Group interface includes the <CODE>H5Giterate</CODE> function,
which iterates over the group members.
<P>
Operations on each group member can be performed during the iteration process
by passing the operator function and its data to the iterator as parameters.
There are no restrictions on what kind of operations can be performed on
group members during the iteration procedure.
<P>
The following steps are involved:
<OL>
<LI> Write an operator function which will be used during the iteration process.
The HDF5 library defines the operator function signature and return values.
<LI> Open the group to iterate through.
<LI> Iterate through the group or just a few members of the group.
</OL>
<A NAME="deff">
<H2>How to Iterate Over Group Members using FORTRAN</H2>
There is no FORTRAN call to iterate over group members.
Instead, this functionality is provided by two FORTRAN calls:
<ul>
<li><CODE>hgn_members_f</CODE> returns the number of group members.
<li><CODE>h5gget_obj_info_idx_f</CODE> returns the name and type of the
group member, which is identified by its index.
</ul>
<P>
<H2> Programming Example</H2>
<A NAME="desc">
<H3><U>Description</U></H3>
In this example we iterate through the members of the root group.
<UL>
[ <A HREF="examples/h5_iterate.c">C example</A> ]
- <code>h5_iterate.c</code><BR>
[ <A HREF="examples/grpit.f90">FORTRAN example</A> ]
- <code>grpit.f90</code>
</UL>
<B>NOTE:</B> To download a tar file of the examples, including a Makefile,
please go to the <A HREF="references.html">References</A> page.
<P>
Following is the output from these examples:
<P>
<I><U>Output from C Example</U></I>
<PRE>
Objects in the root group are:
Object with name Dataset1 is a dataset
Object with name Datatype1 is a named datatype
Object with name Group1 is a group
</PRE>
<I><U>Output from FORTRAN Example</U></I>
<PRE>
Number of root group member is 1
MyGroup 1
Number of group MyGroup member is 2
Group_A 1
dset1 2
Number of group MyGroup/Group_A member is 1
dset2 2
</PRE>
<A NAME="remc">
<H3><U>Remarks for C Example</U></H3>
<P>
<UL>
<LI> The operator function in this example is called <I>file_info</I>.
The signature of the operator function is as follows:
<PRE>
herr_t *(H5G_operator_t) (hid group_id, const char* name,
void *operator_data)
</PRE>
<UL>
<LI>The <em>group_id</em> parameter is a group identifier for the
group being iterated over.
It is passed to the operator by the iterator function,
<CODE>H5Giterate</CODE>.
<P>
<LI>The <em>name</em> parameter is the name of the current object.
The name is passed to the operator function by the HDF5 library.
<P>
<LI> The <em>operator_data</em> parameter is the operator data.
It is passed to and from
the operator by the iterator, <CODE>H5Giterate</CODE>.
</UL>
<P>
The operator function in this example simply prints the name and type
of the current object and then exits.
This information can also be used to open the object and perform
different operations or queries. For example a named datatype object's
name can be used to open the datatype and query its properties.
<P>
The operator return value defines the behavior of the iterator.
<UL>
<P>
<LI>A zero return value causes the iterator to continue, returning
zero when all group members have been processed.
<P>
<LI>A positive value causes the iterator to immediately return that
value, indicating a short-circuit success. The iterator can be restarted
at the next group member.
<P>
<LI>A negative value causes the iterator to immediately return that value,
indicating failure. The iterator can be restarted at the next group
member.
</UL>
<P>
In this example the operator function returns 0, which causes the iterator
to continue and go through all group members.
<P>
<LI>The function <CODE>H5Gget_objinfo</CODE> is used to determine the type of the object.
It also returns the modification time, number of hard links, and some
other information.
<P>
The signature of this function is as follows:
<PRE>
herr_t H5Gget_objinfo (hid_t loc_id, const char * name,
hbool_t follow_link,
H5G_stat_t *statbuf)
</PRE>
<UL>
<LI>The <em>loc_id</em> and <em>name</em> arguments
specify the object by its location and name.
This example uses the group identifier and name relative to the group
to specify the object.
<P>
<LI>The <em>follow_link</em> argument is a flag which indicates
whether a symbolic link should be followed. A zero value indicates
that information should be returned for the link itself, but not
about the object it points to.
<P>
The root group in this example does not have objects that are
links, so this flag is not important for our example.
<P>
<LI>The <em>statbuf</em> argument is the buffer in which to return
information.
Type information is returned into the field <em>type</em> of the
<CODE>H5G_stat_t</CODE> data structure (<code>statbuf.type</code>).
Valid values are
<CODE>H5G_GROUP</CODE>, <CODE>H5G_DATASET</CODE>,
<CODE>H5G_TYPE</CODE>, and <CODE>H5G_LINK</CODE>.
</UL>
<P>
<LI> The <CODE>H5Giterate</CODE> function has the following signature:
<PRE>
int H5Giterate (hid_t loc_id, const char *name , int *idx,
H5G_operator_t operator, void * operator_data)
</PRE>
<UL>
<LI> The <em>loc_id</em> parameter is the group identifier for the
group being iterated over.
<LI> The <em>name</em> parameter is the group name.
<LI> The <em>idx</em> parameter is an index specifying that iteration
begins with the <em>idx</em>-th object in the group.
Upon the function's return, the index of the next element
to be processed is returned in <I>idx</I>. In our example, NULL is
used to start at the first group member. Since no stopping point
is returned in this case, the iterator cannot be restarted if one
of the calls to its operator returns a non-zero value.
<LI> The <em>operator</em> parameter is the operator function.
<LI> The <em>operator_data</em> argument is the operator data.
We used NULL since no data was passed to or from the operator.
</UL>
</UL>
<A NAME="remf">
<H3><U>Remarks for FORTRAN Example</U></H3>
<P>
<UL>
<LI>This program creates an HDF5 file with groups in it and
then uses <CODE>h5gn_members_f</CODE> to get the number of members in
each group and <CODE>h5gget_obj_idx_f</CODE> to obtain the group member's
name and type.
<P>
<LI>The number of members in a group are obtained with the
<CODE>h5gn_members_f</CODE> call:
<PRE>
h5gn_members_f (loc_id, name, nmembers, hdferr)
loc_id IN: INTEGER (HID_T)
name IN: CHARACTER (LEN=*)
nmembers OUT: INTEGER
hdferr OUT: INTEGER
</PRE>
<UL>
<LI>The <I>loc_id</I> parameter is the file or group identifier.
<LI>The <I>name</I> parameter is the name of the group to obtain the number
of members in.
<LI>The number of members in the group is returned in <I>nmembers</I>.
<LI>The <I>hdferr</I> parameter contains the return code from the
call: 0 if successful and -1 otherwise.
</UL>
<P>
<LI>The name of each group and its type are obtained with the
<CODE>h5gget_obj_info_idx_f</CODE> call:
<PRE>
h5gget_obj_info_idx_f (loc_id, name, idx, &
obj_name, obj_type, hdferr)
loc_id IN: INTEGER (HID_T)
name IN: CHARACTER (LEN=*)
idx IN: INTEGER
obj_name OUT: CHARACTER (LEN=*)
obj_type OUT: INTEGER
hdferr OUT: INTEGER
</PRE>
<UL>
<LI>The <I>loc_id</I> parameter is the file or group identifier.
<LI>The <I>name</I> parameter is the name of the group.
<LI>The <I>idx</I> parameter is the index of the member object.
<LI>The <I>obj_name</I> parameter is the name of the object that gets returned.
<LI>The <I>obj_type</I> parameter is the object type that gets returned.
Valid values are as follows:
<PRE>
H5G_LINK_F
H5G_GROUP_F
H5G_DATASET_F
H5G_TYPE_F
</PRE>
<LI>The <I>hdferr</I> parameter contains the return code from the
call: 0 if successful and -1 otherwise.
</UL>
</UL>
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