Language: C

#include <stdio.h>
#include <string.h> //strcmp、strncmp関数を使うために必要
 
int main(void){
  	char moji;
    char q,w,e,r,t,y,u,i,o,p,a,s,d,f,g,h,j,k,l,z,x,c,v,b,n,m;
	scanf("%s",&moji);
  
  switch(moji){
    case a:printf("a");
          break;
      case b:printf("a");
          break;
      case c:printf("a");
          break;
      case d:printf("a");
          break;
      case e:printf("a");
          break;
      case f:printf("a");
          break;
      case g:printf("a");
          break;
      case h:printf("a");
          break;
      case i:printf("a");
          break;
      case j:printf("a");
          break;
      case k:printf("a");
          break;
      case l:printf("a");
          break;
      case m:printf("a");
          break;
      case n:printf("a");
          break;
      case o:printf("a");
          break;
      case p:printf("a");
          break;
      case q:printf("a");
          break;
      case r:printf("a");
          break;
      case s:printf("a");
          break;
      case t:printf("a");
          break;
      case u:printf("a");
          break;
      case v:printf("a");
          break;
      case w:printf("a");
          break;
      case x:printf("a");
          break;
      case y:printf("a");
          break;
      case z:printf("a");
          break;
     default :printf("A");
      			break;

  }

		return 0;
}

Compiler output:
main.c: In function 'main':
main.c:10:5: error: case label does not reduce to an integer constant
   10 |     case a:printf("a");
      |     ^~~~
main.c:12:7: error: case label does not reduce to an integer constant
   12 |       case b:printf("a");
      |       ^~~~
main.c:14:7: error: case label does not reduce to an integer constant
   14 |       case c:printf("a");
      |       ^~~~
main.c:16:7: error: case label does not reduce to an integer constant
   16 |       case d:printf("a");
      |       ^~~~
main.c:18:7: error: case label does not reduce to an integer constant
   18 |       case e:printf("a");
      |       ^~~~
main.c:20:7: error: case label does not reduce to an integer constant
   20 |       case f:printf("a");
      |       ^~~~
main.c:22:7: error: case label does not reduce to an integer constant
   22 |       case g:printf("a");
      |       ^~~~
main.c:24:7: error: case label does not reduce to an integer constant
   24 |       case h:printf("a");
      |       ^~~~
main.c:26:7: error: case label does not reduce to an integer constant
   26 |       case i:printf("a");
      |       ^~~~
main.c:28:7: error: case label does not reduce to an integer constant
   28 |       case j:printf("a");
      |       ^~~~
main.c:30:7: error: case label does not reduce to an integer constant
   30 |       case k:printf("a");
      |       ^~~~
main.c:32:7: error: case label does not reduce to an integer constant
   32 |       case l:printf("a");
      |       ^~~~
main.c:34:7: error: case label does not reduce to an integer constant
   34 |       case m:printf("a");
      |       ^~~~
main.c:36:7: error: case label does not reduce to an integer constant
   36 |       case n:printf("a");
      |       ^~~~
main.c:38:7: error: case label does not reduce to an integer constant
   38 |       case o:printf("a");
      |       ^~~~
main.c:40:7: error: case label does not reduce to an integer constant
   40 |       case p:printf("a");
      |       ^~~~
main.c:42:7: error: case label does not reduce to an integer constant
   42 |       case q:printf("a");
      |       ^~~~
main.c:44:7: error: case label does not reduce to an integer constant
   44 |       case r:printf("a");
      |       ^~~~
main.c:46:7: error: case label does not reduce to an integer constant
   46 |       case s:printf("a");
      |       ^~~~
main.c:48:7: error: case label does not reduce to an integer constant
   48 |       case t:printf("a");
      |       ^~~~
main.c:50:7: error: case label does not reduce to an integer constant
   50 |       case u:printf("a");
      |       ^~~~
main.c:52:7: error: case label does not reduce to an integer constant
   52 |       case v:printf("a");
      |       ^~~~
main.c:54:7: error: case label does not reduce to an integer constant
   54 |       case w:printf("a");
      |       ^~~~
main.c:56:7: error: case label does not reduce to an integer constant
   56 |       case x:printf("a");
      |       ^~~~
main.c:58:7: error: case label does not reduce to an integer constant
   58 |       case y:printf("a");
      |       ^~~~
main.c:60:7: error: case label does not reduce to an integer constant
   60 |       case z:printf("a");
      |       ^~~~