Logic

Last updated on 2025-04-29 | Edit this page

Estimated time: 30 minutes

Overview

Questions

  • How do we declare and assign values to variables?

Objectives

  • Understand the different intrinsic data types.
  • Declare and assign variables and parameters.

Fortran’s logical type has two values:

FORTRAN 

  logical :: is_below_20_metres = .false.
  logical :: use_stochastic_physics = .true.

Logical operators

Values can be tested logical operators .or., .and. and .not. are available, and these can be used to set the value of logical variables.

The precedence is illustrated by, e.g.,

FORTRAN 

  q = i .or. j .and. .not. k    ! evaluated as i .or. (j .and. (.not. k))

where q, i, j, and k are all logical variables.

Use brackets to avoid confusion over operator precedence.

Relational operators

To form logical expressions from numeric or other expressions, we require relational operators. The are two forms in Fortran, illustrated in the table below. It is recommended that you avoid the older form.

Relation Operator Older form For
Less than < .lt. integer real
Less than or equal to <= .le. integer real
Greater than > .gt. integer real
Greater than or equal to >= .ge. integer real
Equal to == .eq. integer real complex
Not equal to /= .neq. integer real complex

Logical equivalence

Equivalence between two logical expressions or variables is established via the logical operators .eqv. and .neqv..

While some some compilers may allow the use of ==, this should be avoided.

Using logical operators

These operators can be used to check and set the values of logical variables, dependent on other variables, e.g.

FORTRAN 

program example4
   implicit none

   real, parameter    :: pi      = 3.14159265
   logical, parameter :: switch1 = .true.

   real    :: a=3.0
   logical :: test1, test2, test3

   test1 = a >= pi               ! True if a is greater than or equal to pi

   test2 = (.not. test1)         ! True if test1 is False, False if test1 is True

   test3 = (test2 .eqv. switch1) ! True if test2 is True, False if test2 is False

   print *, test1

   print *, test2

   print *, test3

end program example4

Compiling and running this code will give the following output

$  ./a.out
 F
 T
 T

Key Points

  • Th