Update ch04-01-select.md

Author: Euler-37

src/ch04-01-select.md

@@ -4,33 +4,56 @@
 
 举例
 ``` fortran
-select case(move)              ! move有wasd四种可能性
-case("w");write(*,*)"上"       !使用;一行可以写多个语句，可读性更好
-case("a");write(*,*)"左"
-case("s");write(*,*)"下"
-case("d");write(*,*)"右"
-case default;write(*,*)"错误的方向" !默认的case,可以省略
-end select
+program main
+   implicit none
+   character(len=1)::move
+   move="a"
+   select case(move)              ! move有wasd四种可能性
+   case("w");write(*,*)"上"       !使用;一行可以写多个语句，可读性更好
+   case("a");write(*,*)"左"
+   case("s");write(*,*)"下"
+   case("d");write(*,*)"右"
+   case default;write(*,*)"错误的方向" !默认的case,可以省略
+   end select
+end program main
 ```
+``` sh
+$ fpm run
+ 左
+```
+- 练习：尝试修改`move`的值，观察输出
 
 同时，`case` 语句支持指定多个条件进行匹配，也支持范围
 
 ``` fortran
-!数字范围
-select case(i)      
-case(1:9)  ;write(*,*)"一位数" 
-case(10:99);write(*,*)"两位数"
-end select
-!字符范围
-select case(c)      
-case('a':'z')  ;write(*,*)"小写字母" 
-case('A':'Z')  ;write(*,*)"大写字母"
-end select
+program main
+   implicit none
+   character(len=1)::c
+   integer::i,j
+   i=10
+   c="W"
+   j=7
+   !数字范围
+   select case(i)      
+   case(1:9)  ;write(*,*)"一位数" 
+   case(10:99);write(*,*)"两位数"
+   end select
+   !字符范围
+   select case(c)      
+   case('a':'z')  ;write(*,*)"小写字母" 
+   case('A':'Z')  ;write(*,*)"大写字母"
+   end select
 
-select case(i)
-case(2,3,5,7)  ;write(*,*)"质数"
-case default   ;write(*,*)"不是质数"
-end select
+   select case(j)
+   case(2,3,5,7)  ;write(*,*)"质数"
+   case default   ;write(*,*)"不是质数"
+   end select
+end program main
+```
+``` sh
+$ fpm run
+ 两位数
+ 大写字母
+ 质数
 ```
-
 - 不支持`1:4:2`这类范围