Fortran If-then-else 语句

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

if…then…else 语句的基本语法是:

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

但是，如果为 if 块命名，则命名的 if-else 语句的语法如下:

[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，则 if…then 语句中的代码块将被执行，否则 else 块中的代码段将被执行。

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

结果为：

a is not less than 20
value of a is 100