程序目的：输入百分制成绩，要求输入成绩登记'A'、'B'、'C'、'D'、'E'。90分以上为'A'，80~89分为'B'，70~79分为'C'，60~69分为'D'，60分一下为'E'。
#include<stdio.h>
void main()
{
    float mark;
    int mark1;
    scanf("%f",&mark);
    mark1=int(mark);
    switch(mark1)
    {
    case mark1>=90:printf('A');break;
    case mark1>=80 && mark1<=89:printf('B');break;
    case mark1>=70 && mark1<=79:printf('C');break;
    case mark1>=60 && mark1<=69:printf('D');break;
    default:printf('E');break;
    }
}
结果显示了错误：
--------------------Configuration: Cpp1 - Win32 Debug--------------------
Compiling...
Cpp1.cpp
E:\C++临时文件\Cpp1.cpp(10) : error C2051: case expression not constant
E:\C++临时文件\Cpp1.cpp(10) : error C2664: 'printf' : cannot convert parameter 1 from 'const char' to 'const char *'
        Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
E:\C++临时文件\Cpp1.cpp(11) : error C2051: case expression not constant
E:\C++临时文件\Cpp1.cpp(11) : error C2664: 'printf' : cannot convert parameter 1 from 'const char' to 'const char *'
        Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
E:\C++临时文件\Cpp1.cpp(12) : error C2051: case expression not constant
E:\C++临时文件\Cpp1.cpp(12) : error C2664: 'printf' : cannot convert parameter 1 from 'const char' to 'const char *'
        Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
E:\C++临时文件\Cpp1.cpp(13) : error C2051: case expression not constant
E:\C++临时文件\Cpp1.cpp(13) : error C2664: 'printf' : cannot convert parameter 1 from 'const char' to 'const char *'
        Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
E:\C++临时文件\Cpp1.cpp(14) : error C2664: 'printf' : cannot convert parameter 1 from 'const char' to 'const char *'
        Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
E:\C++临时文件\Cpp1.cpp(15) : warning C4065: switch statement contains 'default' but no 'case' labels
Error executing cl.exe.

Cpp1.exe - 9 error(s), 1 warning(s)

只写“case 100：”就可以了吧？90分以上的都是“A”啊。

严重同意你的最后一句话！谢谢你的鼓励！