f_groups.f90 File Reference

Demonstrates NetCDF-4 hierarchical groups, nested groups, and dimension visibility (Fortran) More...

Functions/Subroutines
program	f_groups
subroutine	validate_groups (ncid, grp1_id, grp2_id, nested_id)

Detailed Description

Demonstrates NetCDF-4 hierarchical groups, nested groups, and dimension visibility (Fortran)

Fortran equivalent of groups.c, demonstrating NetCDF-4's hierarchical group feature using the Fortran 90 NetCDF API. Groups enable organizing datasets into logical groupings similar to directories in a filesystem, providing namespace isolation for variables while allowing dimensions to be shared across the hierarchy through dimension visibility rules.

The program creates a three-level group hierarchy (root → SubGroup1, root → SubGroup2 → NestedGroup), demonstrates dimension visibility across group boundaries, and showcases all five new NetCDF-4 integer types (NF90_UBYTE, NF90_USHORT, NF90_UINT, NF90_INT64, NF90_UINT64).

Learning Objectives:

• Understand NetCDF-4 hierarchical group structures in Fortran
• Learn to create and navigate nested groups
• Master dimension visibility rules across group boundaries
• Work with all five new NetCDF-4 integer types
• Recognize when groups provide organizational benefits

Key Concepts:

• Hierarchical Groups: Organize datasets into logical groupings (like directories)
• Dimension Visibility: Parent dimensions visible in all child groups
• Variable Scoping: Variables only visible in their defining group
• Group Navigation: Use nf90_inq_grp_ncid() to navigate by name
• New Integer Types: NF90_UBYTE, NF90_USHORT, NF90_UINT, NF90_INT64, NF90_UINT64

NetCDF-4 Group Architecture:

• Groups implemented via NC_GRP_INFO_T structures (libsrc4/libhdf5)
• Dimensions visible in child groups via parent chain lookup
• Variables NOT inherited (scoped to defining group only)
• Requires NF90_NETCDF4 flag (HDF5 backend)
• Not compatible with NF90_CLASSIC_MODEL

Dimension Visibility Rules:

• Dimensions defined in a group are visible in that group and all descendants
• Root dimensions (x, y) visible in SubGroup1, SubGroup2, and NestedGroup
• Local dimensions (z in NestedGroup) only visible in defining group
• Dimension lookup walks parent chain: child → parent → root

Use Cases for Groups:

• Multi-instrument datasets: Group data by instrument or sensor
• Model ensembles: Separate ensemble members into groups
• Quality levels: Organize raw, calibrated, and derived products
• Temporal organization: Group data by year, month, or campaign
• Namespace management: Avoid variable name conflicts

Prerequisites:

• f_simple_nc4.f90 - Basic NetCDF-4 file operations
• f_multi_unlimited.f90 - Advanced NetCDF-4 features

Related Examples:

• groups.c - C equivalent
• f_user_types.f90 - User-defined types in groups

Compilation:

gfortran -o f_groups f_groups.f90 -lnetcdff -lnetcdf

Usage:

./f_groups
ncdump f_groups.nc
ncdump -h f_groups.nc  # header only

Expected Output: Creates f_groups.nc in NetCDF-4/HDF5 format containing:

• Root group with dimensions x(3), y(4) and variable ubyte_var(NF90_UBYTE)
• SubGroup1 with variable ushort_var(NF90_USHORT)
• SubGroup2 with variable uint_var(NF90_UINT)
• NestedGroup (under SubGroup2) with dimension z(2) and variables:
  • int64_var(NF90_INT64, 2D: x, y)
  • uint64_var(NF90_UINT64, 3D: x, y, z)

Author

Edward Hartnett, Intelligent Data Design, Inc.

Date

2026

Function/Subroutine Documentation

f_groups()

program f_groups

validate_groups()

subroutine f_groups::validate_groups	(	integer, intent(in)	ncid,
		integer, intent(out)	grp1_id,
		integer, intent(out)	grp2_id,
		integer, intent(out)	nested_id
	)