Variables store information that can be manipulated by the program. Fortran is a strongly typed language, which means that each variable must have a type.

There are 5 built-in data types in Fortran:

  • integer – for data that represent whole numbers, positive or negative

  • real – for floating-point data (not a whole number)

  • complex – pair consisting of a real part and an imaginary part

  • character – for text data

  • logical – for data that represent boolean (true or false) values

Before we can use a variable, we must declare it; this tells the compiler the variable type and any other variable attributes.

Fortran is a statically typed language, which means the type of each variable is fixed when the program is compiled—variable types cannot change while the program is running.

Declaring variables

The syntax for declaring variables is:

<variable_type> :: <variable_name>

where <variable_type> is one of the built-in variable types listed above and <variable_name> is the name that you would like to call your variable.

Variable names must start with a letter and can consist of letters, numbers and underscores. In the following example we declare a variable for each of the built-in types.

Example: variable declaration

program variables
  implicit none

  integer :: amount
  real :: pi
  complex :: frequency
  character :: initial
  logical :: isOkay

end program variables

Fortran code is case-insensitive; you don’t have to worry about the capitalisation of your variable names, but it’s good practice to keep it consistent.

Note the additional statement at the beginning of the program: implicit none. This statement tells the compiler that all variables will be explicitly declared; without this statement variables will be implicitly typed according to the letter they begin with.

Always use the implicit none statement at the beginning of each program and procedure. Implicit typing is considered bad practice in modern programming since it hides information leading to more program errors.

Once we have declared a variable, we can assign and reassign values to it using the assignment operator =.

Example: variable assignment

amount = 10
pi = 3.1415927
frequency = (1.0, -0.5)
initial = 'A'
isOkay = .false.

Characters are surrounded by either single (') or double quotes (").

Logical or boolean values can be either .true. or .false..


Watch out for assignment at declaration:

integer :: amount = 1

This is NOT a normal initialisation; it implies the save attribute, which means that the variable retains its value between procedure calls. Good practice is to initialise your variables separately to their declaration.

Standard input / output

In our Hello World example, we printed text to the command window. This is commonly referred to as writing to standard output or stdout.

We can use the print statement introduced earlier to print variable values to stdout:

print *, 'The value of amount (integer) is: ', amount
print *, 'The value of pi (real) is: ', pi
print *, 'The value of frequency (complex) is: ', frequency
print *, 'The value of initial (character) is: ', initial
print *, 'The value of isOkay (logical) is: ', isOkay

In a similar way, we can read values from the command window using the read statement:

program read_value
  implicit none
  integer :: age

  print *, 'Please enter your age: '
  read(*,*) age

  print *, 'Your age is: ', age

end program read_value

This input source is commonly referred to as standard input or stdin.


The usual set of arithmetic operators are available, listed in order of precedence:














program arithmetic
  implicit none

  real :: pi
  real :: radius
  real :: height
  real :: area
  real :: volume

  pi = 3.1415927

  print *, 'Enter cylinder base radius:'
  read(*,*) radius

  print *, 'Enter cylinder height:'
  read(*,*) height

  area = pi * radius**2
  volume = area * height

  print *, 'Cylinder radius is: ', radius
  print *, 'Cylinder height is: ', height
  print *, 'Cylinder base area is: ', area
  print *, 'Cylinder volume is: ', volume

end program arithmetic

Floating-point precision

The desired floating-point precision can be explicitly declared using a kind parameter. The iso_fortran_env intrinsic module provides kind parameters for the common 32-bit and 64-bit floating-point types.

Example: explicit real kind

program float
  use, intrinsic :: iso_fortran_env, only: sp=>real32, dp=>real64
  implicit none

  real(sp) :: float32
  real(dp) :: float64

  float32 = 1.0_sp  ! Explicit suffix for literal constants
  float64 = 1.0_dp

end program float

Always use a kind suffix for floating-point literal constants.

Example: C-interoperable kind

program float
  use, intrinsic :: iso_c_binding, only: sp=>c_float, dp=>c_double
  implicit none

  real(sp) :: float32
  real(dp) :: float64

end program float

In the next part we will learn how to use arrays for storing more than one value in a variable.

Local scope variables with block construct

The 2008 Fortran standard introduced the notion of block which enables using local scope variables within a program or procedure.


module your_module
    implicit none
    integer :: n = 2
end module

program main
    implicit none
    real :: x

        use your_module, only: n ! you can import modules within blocks
        real :: y ! local scope variable
        y = 2.0
        x = y ** n
        print *, y
    end block
    ! print *, y ! this is not allowed as y only exists during the block's scope
    print *, x  ! prints 4.00000000
end program