f_opendap_subset.f90 File Reference

OPeNDAP client-side subsetting example in Fortran. More...

Functions/Subroutines
program	f_opendap_subset

Detailed Description

OPeNDAP client-side subsetting example in Fortran.

This is the Fortran equivalent of opendap_subset.c, demonstrating client-side subsetting using start/count arrays.

What this example does:

1. Opens the full remote dataset without constraints
2. Demonstrates four different subset access patterns:
   • Single time slice: All lat/lon for time step 1 (using 1-based)
   • Time series: All time points at lat=46, lon=91
   • Regional subset: 3x10x10 cube using fixed-size array
   • Multiple scattered reads: Loop showing multiple requests
3. Closes the dataset

Key differences from constraint expressions:

• Full dataset metadata is available (original dimension sizes)
• Can make multiple different subset requests without reopening
• More flexible for data exploration and iterative analysis
• Network transfer includes only requested subset data

Fortran-specific notes:

• nf90_get_var uses 1-based start indices: (/1, 1, 1/)
• allocatable arrays handle dynamic sizes based on dimensions

Learning Objectives:

• Open a full remote dataset without constraint expressions from Fortran
• Use start/count arrays (1-based) for multiple targeted data requests
• Implement four different access patterns (time slice, time series, regional, scattered)
• Understand the trade-offs between client-side and server-side subsetting
• Recognize the overhead of many small requests vs fewer large requests

Key Concepts:

• Client-Side Subsetting: Open the full dataset once, then use start/count in nf90_get_var() calls to select different subregions on each read
• Time Slice Access: Read all spatial points for one time step (start=(/1,1,1/), count=(/lon_len,lat_len,1/) in Fortran column-major)
• Time Series Access: Read all time points at one spatial location (start=(/lon,lat,1/), count=(/1,1,time_len/))
• Regional Subset: Read a small 3D cube from the full dataset
• Multiple Requests: One nf90_open() supports unlimited nf90_get_var() calls with different start/count values — no need to reopen for each subset
• 1-Based Indexing: Fortran start arrays begin at 1, unlike C's 0-based indexing

Prerequisites:

• f_opendap_simple.f90 - Basic OPeNDAP access from Fortran
• opendap_subset.c - C equivalent for comparison
• f_simple_2D.f90 - Understanding start/count subsetting for local files

Related Examples:

• opendap_subset.c - C equivalent
• f_opendap_simple.f90 - Simplest OPeNDAP access (single subset)
• f_opendap_constraint.f90 - Server-side subsetting via URL constraints

Compilation:

gfortran -o f_opendap_subset f_opendap_subset.f90 -lnetcdff -lnetcdf

Usage:

./f_opendap_subset

Expected Output:

• Opens remote SST dataset and prints full dimension sizes
• Demonstrates four access patterns with data summaries: single time slice, time series at a point, regional 3D cube, and multiple scattered requests

Note

Companion code for "The NetCDF Developer's Handbook: The Authoritative Guide to Writing High-Performance Programs for Scientific Data Management, Second Edition" (https://www.amazon.com/dp/B0H7Q1Z75L)

Author

Edward Hartnett

Date

6/15/26

Function/Subroutine Documentation

f_opendap_subset()

program f_opendap_subset