Basic Flow of FORTRAN (77, 90, and 95) body code and subroutines:

  1. Name your body code and subroutine;
  2. Declare variables;
  3. Write the actual code;
  4. Signal an end of your body code and subroutine.

Walkthrough of sample code : learn_f.f

Overview:
Body code - myfirstprogram
Subroutine - writefile
Function - circlearea

Declaration of variables:
Format: Variable Type Var Name 1, Var Name 2 ......
Basic variable types: Character (string), real, integer, logical, complex
There are also "long format" for real and complex numbers
All string declaration: character*n myword (n = length of string)
Parameter = fixed value, cannot be changed
Declaration of arrays: Variable Type (n0:nf, m0:mf, ......)
Initialize your variables! (Otherwise variables contain garbled information)

Basic I/O to/from screen and keyboard:
Write (*,*) -> write to screen without formatting
Read (*,*) -> read keyboard input without formatting

Do-loop:
do [label, i.e. 50] n=1,5
50 continue
Label: Used in loops and formatting.

Format statement:
Used to format text, numbers
Usage:[Label] format (format statements)
Common format statements:
fnn.m = a real number with a total of nn digits (including decimal points) with m-1 digits after decimal points inn = integer with a total nn digits
ann = string with a total nn characters
Complex numbers are "formed" by 2 real numbers (a+bi); to format them, you need a compound format statement like 2(f8.2)

Basic math operations and intrinsic (build-in) functions:
* = multiply, / = division, + = addition, - = subtraction, ** = power
The order of calculations follows basic math rules: ** > * & / < +/- unless specified by brackets "( .... )" In sample code, I used the intrinsic function sqrt() (square root) and complex () (creates a complex number). Other common functions:
sin (), cos (), tan (), exp(), real (), int(), ...
Fortran uses radians.

Functions
[var type] function functionname (variable names passed...)
Calling a function: Variable = functionname (variables passed)
The name of variables passed does not have to same as the ones specified in the functions, but the order must agree!

Subroutines
Subroutine subroutinename (variables passed)
Calling a subroutine: call subroutinename (variables passed)

Basic file I/O
Opening a ascii file:
open (unit=n,file=filenumber)
Closing a ascii file:
close (unit=n)
Writing to a file:
write (unit number, format statement/reference) variable names
Reading from a file (not shown in sample code):
read (unit number, format statement/reference) variable names
You may write binary files too = > Grads, Matlab, IDL...
Danger Warning:
Binary files created by PCs (Windows/Linux) do not have the same format as the ones make by UNIX machines.