Fortran - If-then-else 构造

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  简述

  一个 if… then语句后面可以跟一个可选的 else statement,当逻辑表达式为假时执行。

  句法

  >

  的基本语法if… then… else声明是 -

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

  但是，如果您给if块，然后是命名的语法if-else声明将是 -

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

  如果逻辑表达式的计算结果为true,然后是里面的代码块if…then语句会被执行，否则里面的代码块else块将被执行。
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  例子

  
  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
  

  编译并执行上述代码时，会产生以下结果 -

  
  a is not less than 20
  value of a is 100