Language: C

#include<stdio.h>
#include<string.h>
int main(void){
  char alf;
  char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
  scanf("%c",alf);
  int i;
  for(i=0;i<strlen(A);i++){
    if(alf==A[i]){
      printf("a\n");
      return 0;
    }
  }
  printf("A\n");
  return 0;
}
  
  

Compiler output:
main.c: In function 'main':
main.c:5:13: error: 'a' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |             ^
main.c:5:13: note: each undeclared identifier is reported only once for each function it appears in
main.c:5:15: error: 'b' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |               ^
main.c:5:17: error: 'c' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                 ^
main.c:5:19: error: 'd' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                   ^
main.c:5:21: error: 'e' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                     ^
main.c:5:23: error: 'f' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                       ^
main.c:5:25: error: 'g' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                         ^
main.c:5:27: error: 'h' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                           ^
main.c:5:29: error: 'i' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                             ^
main.c:5:31: error: 'j' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                               ^
main.c:5:33: error: 'k' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                 ^
main.c:5:35: error: 'l' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                   ^
main.c:5:37: error: 'm' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                     ^
main.c:5:39: error: 'n' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                       ^
main.c:5:41: error: 'o' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                         ^
main.c:5:43: error: 'p' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                           ^
main.c:5:45: error: 'q' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                             ^
main.c:5:47: error: 'r' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                               ^
main.c:5:49: error: 's' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                 ^
main.c:5:51: error: 't' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                   ^
main.c:5:53: error: 'u' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                     ^
main.c:5:55: error: 'v' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                       ^
main.c:5:57: error: 'w' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                         ^
main.c:5:59: error: 'x' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                           ^
main.c:5:61: error: 'y' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                             ^
main.c:5:63: error: 'z' undeclared (first use in this function)
    5 |   char A[]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                               ^