In this example, we are using case b: and b is a variable. Thus, error case label does not reduce to an integer constant occurs.

Example:

#include <stdio.h>

int main()
{
    int a,b;
    
    a=2;
    b=2;
    
    switch(a){
        case 1:
            printf("Case 1\n");
            break;
        
        case b:
            printf("Case 2\n");
            break;
    }
    
	return 0;
}

Output

prog.c: In function ‘main’:
prog.c:15:9: error: case label does not reduce to an integer constant
         case b:
         ^~~~
prog.c:5:11: warning: variable ‘b’ set but not used [-Wunused-but-set-variable]
     int a,b;
           ^

How to fix?

To fix the error case label does not reduce to an integer constant, use value 2 instead of variable b. The correct statement will be case 2:

Correct code:

#include <stdio.h>

int main()
{
    int a,b;
    
    a=2;
    b=2;
    
    switch(a){
        case 1:
            printf("Case 1\n");
            break;
        
        case 2:
            printf("Case 2\n");
            break;
    }
    
	return 0;
}

Output

Case 2