Fortran - if-then construct

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An if… then statement consists of a logical expression followed by one or more statements and terminated by an end if statement.

Syntax

The basic syntax of an if… then statement is −

if (logical expression) then      
   statement  
end if

However, you can give a name to the if block, then the syntax of the named if statement would be, like −

[name:] if (logical expression) then      
   ! various statements           
   . . .  
end if [name]

If the logical expression evaluates to true, then the block of code inside the if…then statement will be executed. If logical expression evaluates to false, then the first set of code after the end if statement will be executed.

Flow Diagram

Example 1

program ifProg
implicit none
   ! local variable declaration
   integer :: a = 10
 
   ! check the logical condition using if statement
   if (a < 20 ) then
   
   !if condition is true then print the following 
   print*, "a is less than 20"
   end if
       
   print*, "value of a is ", a
 end program ifProg

When the above code is compiled and executed, it produces the following result −

a is less than 20
value of a is 10

Example 2

This example demonstrates a named if block −

program markGradeA  
implicit none  
   real :: marks
   ! assign marks   
   marks = 90.4
   ! use an if statement to give grade
  
   gr: if (marks > 90.0) then  
   print *, " Grade A"
   end if gr
end program markGradeA   

When the above code is compiled and executed, it produces the following result −

Grade A