Why can't we use variables inside a case in switch construct?

[radha gogia]

If I have an integer variable like int a=9 then in the switch case If i write
switch(a)
{
case 4+a: printf("hii");

}

then why is this statement a compile-time error that variables cannot be used inside a case statement why does the compiler not subtitutes the values in place of the variables.

[weaknessforcats]

The compiler needs to know which case you want before your program runs or the correct code cannot be generated. So case 4+a is an error because the program needs to be running when a is evaluated.

The switch cases need to be symbolic like 4 or 'A'.