Language: C++

#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main() {
  string S;
  cin >> S;
  if(S==A){
    cout << "A" <<endl;
  }
  else if(S==B){
    cout << "A" <<endl;
  }
  else if(S==C){
    cout << "A" <<endl;
  }
  else if(S==D){
    cout << "A" <<endl;
  }
  else if(S==E){
    cout << "A" <<endl;
  }
  else if(S==F){
    cout << "A" <<endl;
  }
  else if(S==G){
    cout << "A" <<endl;
  }
  else if(S==H){
    cout << "A" <<endl;
  }
  else if(S==I){
    cout << "A" <<endl;
  }
  else if(S==J){
    cout << "A" <<endl;
  }
  else if(S==K){
    cout << "A" <<endl;
  }
  else if(S==L){
    cout << "A" <<endl;
  }
  else if(S==M){
    cout << "A" <<endl;
  }
  else if(S==N){
    cout << "A" <<endl;
  }
  else if(S==O){
    cout << "A" <<endl;
  }
  else if(S==P){
    cout << "A" <<endl;
  }
  else if(S==Q){
    cout << "A" <<endl;
  }
  else if(S==R){
    cout << "A" <<endl;
  }
  else if(S==S){
    cout << "A" <<endl;
  }
  else if(S==T){
    cout << "A" <<endl;
  }
  else if(S==U){
    cout << "A" <<endl;
  }
  else if(S==V){
    cout << "A" <<endl;
  }
  else if(S==W){  
    cout << "A" <<endl;
  }
  else if(S==X){  
    cout << "A" <<endl;
  }  
  else if(S==Y){  
    cout << "A" <<endl;
  }
  else if(S==Z){  
    cout << "A" <<endl;
  }
  else{
    cout << "a" <<endl;
  }
}


Compiler output:
a.cc: In function 'int main()':
a.cc:8:9: error: 'A' was not declared in this scope
    8 |   if(S==A){
      |         ^
a.cc:11:14: error: 'B' was not declared in this scope
   11 |   else if(S==B){
      |              ^
a.cc:14:14: error: 'C' was not declared in this scope
   14 |   else if(S==C){
      |              ^
a.cc:17:14: error: 'D' was not declared in this scope
   17 |   else if(S==D){
      |              ^
a.cc:20:14: error: 'E' was not declared in this scope
   20 |   else if(S==E){
      |              ^
a.cc:23:14: error: 'F' was not declared in this scope
   23 |   else if(S==F){
      |              ^
a.cc:26:14: error: 'G' was not declared in this scope
   26 |   else if(S==G){
      |              ^
a.cc:29:14: error: 'H' was not declared in this scope
   29 |   else if(S==H){
      |              ^
a.cc:32:14: error: 'I' was not declared in this scope
   32 |   else if(S==I){
      |              ^
a.cc:35:14: error: 'J' was not declared in this scope
   35 |   else if(S==J){
      |              ^
a.cc:38:14: error: 'K' was not declared in this scope
   38 |   else if(S==K){
      |              ^
a.cc:41:14: error: 'L' was not declared in this scope
   41 |   else if(S==L){
      |              ^
a.cc:44:14: error: 'M' was not declared in this scope
   44 |   else if(S==M){
      |              ^
a.cc:47:14: error: 'N' was not declared in this scope
   47 |   else if(S==N){
      |              ^
a.cc:50:14: error: 'O' was not declared in this scope
   50 |   else if(S==O){
      |              ^
a.cc:53:14: error: 'P' was not declared in this scope
   53 |   else if(S==P){
      |              ^
a.cc:56:14: error: 'Q' was not declared in this scope
   56 |   else if(S==Q){
      |              ^
a.cc:59:14: error: 'R' was not declared in this scope
   59 |   else if(S==R){
      |              ^
a.cc:65:14: error: 'T' was not declared in this scope
   65 |   else if(S==T){
      |              ^
a.cc:68:14: error: 'U' was not declared in this scope
   68 |   else if(S==U){
      |              ^
a.cc:71:14: error: 'V' was not declared in this scope
   71 |   else if(S==V){
      |              ^
a.cc:74:14: error: 'W' was not declared in this scope
   74 |   else if(S==W){
      |              ^
a.cc:77:14: error: 'X' was not declared in this scope
   77 |   else if(S==X){
      |              ^
a.cc:80:14: error: 'Y' was not declared in this scope
   80 |   else if(S==Y){
      |              ^
a.cc:83:14: error: 'Z' was not declared in this scope
   83 |   else if(S==Z){
      |              ^