f_simple_2D.f90 File Reference

Basic example: 2D array creation and reading in NetCDF (Fortran) More...

Functions/Subroutines
program	f_simple_2d
subroutine	handle_err (status)

Detailed Description

Basic example: 2D array creation and reading in NetCDF (Fortran)

This is the Fortran equivalent of simple_2D.c, demonstrating the fundamental workflow for working with NetCDF files using the Fortran 90 NetCDF API. The program creates a 2D integer array, writes it to a NetCDF file with a global attribute ("title") and a variable attribute ("units"), then reopens the file to verify metadata, attributes, and data correctness using nf90_inquire(), nf90_inquire_dimension(), and nf90_inquire_variable().

Learning Objectives:

• Understand Fortran NetCDF API (nf90_* functions)
• Learn Fortran column-major vs C row-major array ordering
• Add global and variable attributes
• Query file metadata with nf90_inquire(), nf90_inquire_dimension(), nf90_inquire_variable()
• Master error handling with nf90_noerr and nf90_strerror()
• Work with Fortran array indexing (1-based vs C's 0-based)
• Verify equivalence with C version (simple_2D.c)

Key Concepts:

• Fortran Column-Major: Arrays stored column-first [i,j] vs C row-first [j][i]
• Dimension Ordering: Fortran reverses dimension order from C
• 1-Based Indexing: Fortran arrays start at 1, C arrays start at 0
• nf90 Module: Fortran 90 NetCDF interface (use netcdf)
• Error Handling: Check retval against nf90_noerr
• Fill Value: Sentinel value returned for unwritten or missing data elements; set with nf90_def_var_fill() during define mode, queried with nf90_inq_var_fill(). Note: f_coord.f90 uses nf90_put_att() with "_FillValue" which is the CF convention for documenting fill values as an attribute — a complementary but separate approach.

Fortran vs C Differences:

• Array Declaration: Fortran data_out(NX, NY) vs C data_out[NY][NX]
• Dimension Order: Fortran dimids(1)=x, dimids(2)=y vs C dimids[0]=y, dimids[1]=x
• Indexing: Fortran 1-based (1 to N) vs C 0-based (0 to N-1)
• API Prefix: Fortran nf90_* vs C nc_*
• Error Handling: Fortran subroutine call vs C macro

Prerequisites:

• simple_2D.c - C equivalent for comparison

Related Examples:

• simple_2D.c - C equivalent of this example
• f_coord_vars.f90 - Adds coordinate variables
• f_simple_nc4.f90 - NetCDF-4 specific features

Compilation:

gfortran -o f_simple_2d f_simple_2d.f90 -lnetcdff -lnetcdf

Usage:

./f_simple_2d
ncdump f_simple_2d.nc

Expected Output: Creates f_simple_2D.nc containing:

• 2 dimensions: x(6), y(12)
• 1 variable: data(x, y) of type int with fill value -9999
• 1 global attribute: title = "Simple 2D Example"
• 1 variable attribute: units = "m/s"
• Data: sequential integers from 0 to 65 (first NY-1 rows); last row = -9999 (fill)
• Output structure identical to simple_2D.c (verified via ncdump)

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, Intelligent Data Design, Inc.

Date

May 1, 2026

Function/Subroutine Documentation

f_simple_2d()

program f_simple_2d

handle_err()

subroutine f_simple_2d::handle_err

(

integer, intent(in)

status

)