声明：本博文翻译自：https://www.tutorialspoint.com/fortran/fortran_decisions.htm
fortran语言有下面几种if结构语句
if...then语句结构
if...then...else语句结构
if...else if...else语句结构
nested if语句结构
select case语句结构
nested select case语句结构
接下来详细讲解这六种语句
1. if...then
这是最简单的if语句结构。
if...then的语句结构如下：
if (logical expression) then
statement
end if
还可以对if块进行命名：
[name:] if (logical expression) then
! various statements
. . .
end if [name]
这里方括号以及里面的内容都是可选(即可写可不写)
示例代码如下：
Program testif
implicit none
integer :: a
a = 10
if ( a == 10 ) then
write(*,*) a
end if
End program testif
还有署名if语句。可将上面的代码改写成如下代码：
Program testif
implicit none
integer :: a
a = 10
if1: if ( a == 10 ) then
write(*,*) a
end if if1
End program testif
2. if...then...else
if...then...else语句的基本语法如下：
if (logical expression) then
statement(s)
else
other_statement(s)
end if
类似地，此语句也可以写成署名结构：
[name:] if (logical expression) then
! various statements
. . .
else
!other statement(s)
. . .
end if [name]
示例代码如下：
Program testif
implicit none
integer :: a
a = 10
if ( a == 10 ) then
write(*,'(1x,a)') "a = 10"
else
write(*,'(1x,a)') "a /== 10"
end if
End program testif
署名if的示例代码如下：
Program testif
implicit none
integer :: a
a = 10
if1: if ( a == 10 ) then
write(*,'(1x,a)') "a = 10"
else
write(*,'(1x,a)') "a /== 10"
end if if1
End program testif
3. if...else if...else
这一种语句比较重要。
其基本语法结构如下：
[name:]
if (logical expression 1) then
! block 1
else if (logical expression 2) then
! block 2
else if (logical expression 3) then
! block 3
else
! block 4
end if [name]
这里举一个例子。设变量grad为一个学生的成绩，现在要判断其优秀等级。
假设在90--100，其等级为I级；80--89为II级；60--79为III级；0--59为不及格。则示例代码为：
Program testif
implicit none
real :: grad = 76.5
if ( grad>=90 .and. grad <=100 ) then !.. 90--100
write(*,'(1x,a)') "I level"
else if ( grad >= 80 ) then !.. 注意含义80--89
write(*,'(1x,a)') "II level"
else if ( grad >= 60 ) then !.. 60--79
write(*,'(1x,a)') "III level"
else
write(*,'(1x,a)') "不及格"
end if
End program testif
署名if与上述类似，这里不再累赘。
4. nested if
这种叫嵌套if语句：即if语句中含有一个if语句。代码如下：
Program testif
implicit none
integer :: a
a = 68
if ( a > 50 ) then
if ( a < 100 ) then
write(*,'(1x,a)') " 50 < a < 100"
end if
end if
End program testif
5. select case
select case基本语法如下所示：
[name:] select case (expression)
case (selector1)
! some statements
... case (selector2)
! other statements
...
case default
! more statements
...
end select [name]
示例代码如下：
Program selectCaseProg
implicit none
character :: grade = 'B'
select case (grade)
case ('A')
write(*,'(1x,a)') "Excellent!"
case ('B')
write(*,'(1x,a)') "Well done"
case ('C')
write(*,'(1x,a)') "You passed"
case ('D')
write(*,'(1x,a)') "Better try again"
case default
write(*,'(1x,a)') "Invalid grade"
end select
write(*,'(1x,a,g0)') "Your grade is ", grade
End program selectCaseProg
其中case语句中的表达式也可以是一个范围
program selectCaseProg
implicit none
integer :: marks = 78
select case (marks)
case (91:100)
write(*,'(1x,a)') "Excellent!"
case (81:90)
write(*,'(1x,a)') "Very good!"
case (71:80)
write(*,'(1x,a)') "Well done!"
case (61:70)
write(*,'(1x,a)') "Not bad!"
case (41:60)
write(*,'(1x,a)') "You passed!"
case (:40)
write(*,'(1x,a)') "Better try again!"
case default
write(*,'(1x,a)') "Invalid marks"
end select
write(*,'(1x,a,g0)') "Your marks is ", marks
end program selectCaseProg
6. nested select case
基本语法结构如下：
select case(a)
case (100)
write(*,'(1x,a)') "This is part of outer switch", a
select case(b)
case (200)
write(*,'(1x,a)') "This is part of inner switch", a
end select
end select
示例代码如下：
Program nestedSelectCase
implicit none
integer :: a = 100
integer :: b = 200
select case(a)
case (100)
write(*,'(1x,a,g0)') "This is part of outer switch", a
select case(b)
case (200)
write(*,'(1x,a,g0)') "This is part of inner switch", a
end select
end select
write(*,'(1x,a,g0)') "Exact value of a is : ", a
write(*,'(1x,a,g0)') "Exact value of b is : ", b
End program nestedSelectCase
用ivf，执行结果如下：
This is part of outer switch 100
This is part of inner switch 100
Exact value of a is : 100
Exact value of b is : 200
