Fortran - If-then-else construct

An if… then statement can be followed by an optional else statement, which executes when the logical expression is false.

Syntax

The basic syntax of an if… then… else statement is −

if (logical expression) then      
   statement(s)  
else
   other_statement(s)
end if

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

[name:] if (logical expression) then      
   ! various statements           
   . . . 
   else
   !other statement(s)
   . . . 
end if [name]

If the logical expression evaluates to true, then the block of code inside the if…then statement will be executed, otherwise the block of code inside the else block will be executed.

Flow Diagram

Example

program ifElseProg
implicit none
   ! local variable declaration
   integer :: a = 100
 
   ! 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"
   else
   print*, "a is not less than 20"
   end if
       
   print*, "value of a is ", a
	
end program ifElseProg

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

a is not less than 20
value of a is 100