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Fortran Programming Cheatsheet

A quick reference guide to Fortran programming, covering syntax, data types, control structures, and more. Useful for both beginners and experienced Fortran developers.

Fundamentals

Basic Syntax

Program Structure:

program program_name
  ! Declarations
  implicit none
  ! Statements
  ...
end program program_name

Comments:

! This is a comment.

Continuation:

result = very_long_expression + & 
         another_long_expression

Case Insensitivity:
Fortran is generally case-insensitive.

Data Types

integer

Whole numbers (e.g., 1, -5, 1000)

real

Floating-point numbers (e.g., 3.14, -2.0, 0.0) - single precision by default

double precision

Floating-point numbers with higher precision (e.g., 3.1415926535)

complex

Numbers with real and imaginary parts (e.g., (1.0, 2.0))

logical

Boolean values: .true. or .false.

character

Strings of characters (e.g., 'Hello', "Fortran")

Variable Declaration 

integer :: age
real :: temperature
double precision :: pi
character(len=20) :: name
logical :: is_valid

Implicit None:
Always use implicit none to force explicit declaration of all variables.

Operators & Control Flow

Operators

+

Addition

-

Subtraction

*

Multiplication

/

Division

**

Exponentiation

== or .EQ.

Equal to

/= or .NE.

Not equal to

.GT.

Greater than

.GE.

Greater than or equal to

.LT.

Less than

.LE.

Less than or equal to

Control Flow

IF Statement:

if (condition) then
  ! Statements
else if (condition) then
  ! Statements
else
  ! Statements
end if

DO Loop:

do i = start, end, increment
  ! Statements
end do

SELECT CASE Statement:

select case (expression)
  case (value1)
    ! Statements
  case (value2)
    ! Statements
  case default
    ! Statements
end select

Arrays and I/O

Arrays

Declaration:

integer :: matrix(3, 3)
real :: vector(10)

Array Slicing:

vector(1:5)  ! Elements 1 to 5
matrix(:, 1)  ! First column

Array Operations:
Fortran supports element-wise array operations.

Input/Output

Reading from standard input:

read *, variable1, variable2

Writing to standard output:

print *, "Result:", result

Formatted I/O:

print '(I5, F10.2)', integer_value, real_value

File I/O:

open(unit=10, file='data.txt', status='old')
read(10, *) data
close(10)

Subroutines & Functions

Subroutines

Definition:

subroutine subroutine_name(arg1, arg2)
  ! Declarations
  implicit none
  ! Statements
  ...
end subroutine subroutine_name

Calling a Subroutine:

call subroutine_name(value1, value2)

Functions

Definition:

function function_name(arg1, arg2) result(result_variable)
  ! Declarations
  implicit none
  ! Statements
  result_variable = ...
end function function_name

Calling a Function:

result = function_name(value1, value2)

Modules

Definition:

module module_name
  ! Declarations
  implicit none
  ! Public and private variables and subroutines
contains
  ! Subroutine and function definitions
end module module_name

Using a Module:

use module_name
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