if-statement - GW-BASIC 中的嵌套 IF...THEN...ELSE

Question

我使用下面的代码块来测试 gw-basic 程序中的一些条件。

IF Average >= 80 AND Average <= 100 THEN Grade$ = "A"
IF Average >= 70 AND Average <= 79 THEN Grade$ = "B"
IF Average >= 60 AND Average <= 69 THEN Grade$ = "C"
IF Average >= 50 AND Average <= 59 THEN Grade$ = "D"
IF Average >= 40 AND Average <= 49 THEN Grade$ = "E" ELSE Grade$ = "F"  
PRINT "Average is: ", Average
PRINT "Grade is: ", Grade$

它总是打印 Grade is: F 无论 Àverage 的值是多少。

这个 gw-basic 程序有什么错误？

Answer 1

IF Average >= 40 AND Average <= 49 THEN Grade$ = "E" ELSE Grade$ = "F"  

因为这行 IF THEN ELSE 总是会为 Grade$ 赋值(不管 Ave​​rage 的内容如何)，所有的前面的 IF THEN 没有多大意义。

然而，解决方案非常非常简单。不要使用 ELSE 子句并为 Grade$ 提供默认值“F”作为开头。这样，只有当所有这些条件的计算结果都为假时，程序才会返回“成绩为:F”。

Grade$ = "F"
IF Average >= 80 AND Average <= 100 THEN Grade$ = "A"
IF Average >= 70 AND Average <= 79 THEN Grade$ = "B"
IF Average >= 60 AND Average <= 69 THEN Grade$ = "C"
IF Average >= 50 AND Average <= 59 THEN Grade$ = "D"
IF Average >= 40 AND Average <= 49 THEN Grade$ = "E"
PRINT "Average is: ", Average
PRINT "Grade is: ", Grade$