FORTRAN Tutorial 8 | Logical Operator | Decision Control

Welcome to the new lesson of the FORTRAN programming tutorial. Before going in, I suggest you go through previous lessons of FORTRAN.

We will focus on Logical Operators in this lesson.

Logical Operator

Logical Operators in FORTRAN helps to combine two or more expressions to a single one. The logical operators of FORTRAN programming language are: .or. .and. .not.

OR operator

.or. operator combines two or more expressions in such a way that, final result comes .true. value if any of the expressions gives .true. value. This operator gives .false. value only if all expressions give .false. value.

Let me clarify using an example.

First I will show you an individual expressions and combine them later.

program operatorLogical
  implicit none
   print*, 5 == 5
   print*, 5 /= 5
   print*, 5 > 3
   print*, 10 < 8 
end program

The output is:

T
F
T
F

The above four expressions give output either .true. or .false. (T or F). Let’s combine them using .or. operator.

program operatorLogical
  implicit none                      ! first exp:     second exp:
   print*, 5 == 5 .or. 5 > 3         !   T              T
   print*, 5 /= 5 .or. 5 > 3         !   F              T
   print*, 5 > 3  .or. 10 < 8        !   T              F
   print*, 10 < 8 .or. 5 /= 5        !   F              F
end program

As the .or. operator gives .false. value only if all expressions give .false. value. Only the fourth print statement will give .false. value as two expressions individually gives .false. value.

Output is:

T           
T
T
F

And operator

.and. operator combines two or more expressions in such a way that it gives .true. value only if all expressions give .true. value. And this operator gives .false. value if any one of the expressions gives .false. value.

Let’s see the use in code.

program operatorLogical
  implicit none                      ! first exp:     second exp:
   print*, 5 == 5 .and. 5 > 3         !   T              T
   print*, 5 /= 5 .and. 5 > 3         !   F              T
   print*, 5 > 3  .and. 10 < 8        !   T              F
   print*, 10 < 8 .and. 5 /= 5        !   F              F
end program

Only the first print statement will give .true. value because both expressions used there give .true. value individually.

The output is:

T
F
F
F

Not operator

.not. operator is a negation of any operator. This .not. operator is always used with the other two operators .or. and .and. and reverses the output given by those operators.

Look at the code for better understanding.

program not_operator
  implicit none
      print*, 10 < 8 .or. 5 /= 5
      print*, .not.(5 > 3  .or. 10 < 8)    ! .not. of previous
      print*, 5 == 5 .and. 5 > 3 
      print*, .not. (5 == 5 .and. 5 > 3)    ! .not. of previous
end program

The output is:

F          
T
T
F

Logical Operators in if statement

We can use those logical operators in if statement for decision making. Sometimes it helps to make our code shorter, using logical operators instead of nested if statement.

Here is a code with nested if statement.

program with_if
  implicit none
  integer :: marks = 89
     if (marks > 80) then
        if (marks <= 90) then
            print*, "you did excellent."
        end if
     end if
end program

Since the expression in both if statement gives .true. value for the marks value, the print statement will execute.

you did excellent.

But the same can be done using a logical operator.

program with_logical
  implicit none
  integer :: marks = 89
     if (marks > 80 .and. marks <= 90) then
            print*, "you did excellent."
     end if
end program

The output is still the same but we have made our code shorter than previous.

The output is:

you did excellent.

Keep Learning.

Watch the video tutorial of logical operator for better understanding.