Language: C++

#include <bits/stdc++.h>
using namespace std;

int main(){
  
  vector<string> 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};
  vector<string> b={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};
  
  for(int i=0; i<3; i++){
    string x;
    cin >> x;
    for(int j=0; j<25; j++){
      if(x==a.at(j)){
        cout << b.at(j) ;
      }
    }
  }
  cout << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:6:23: error: 'b' was not declared in this scope
    6 |   vector<string> 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};
      |                       ^
a.cc:6:25: error: 'c' was not declared in this scope
    6 |   vector<string> 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};
      |                         ^
a.cc:6:27: error: 'd' was not declared in this scope
    6 |   vector<string> 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};
      |                           ^
a.cc:6:29: error: 'e' was not declared in this scope
    6 |   vector<string> 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};
      |                             ^
a.cc:6:31: error: 'f' was not declared in this scope
    6 |   vector<string> 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};
      |                               ^
a.cc:6:33: error: 'g' was not declared in this scope
    6 |   vector<string> 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};
      |                                 ^
a.cc:6:35: error: 'h' was not declared in this scope
    6 |   vector<string> 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};
      |                                   ^
a.cc:6:37: error: 'i' was not declared in this scope
    6 |   vector<string> 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};
      |                                     ^
a.cc:6:39: error: 'j' was not declared in this scope
    6 |   vector<string> 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};
      |                                       ^
a.cc:6:41: error: 'k' was not declared in this scope
    6 |   vector<string> 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};
      |                                         ^
a.cc:6:43: error: 'l' was not declared in this scope
    6 |   vector<string> 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};
      |                                           ^
a.cc:6:45: error: 'm' was not declared in this scope
    6 |   vector<string> 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};
      |                                             ^
a.cc:6:47: error: 'n' was not declared in this scope
    6 |   vector<string> 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};
      |                                               ^
a.cc:6:49: error: 'o' was not declared in this scope
    6 |   vector<string> 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};
      |                                                 ^
a.cc:6:51: error: 'p' was not declared in this scope
    6 |   vector<string> 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};
      |                                                   ^
a.cc:6:53: error: 'q' was not declared in this scope
    6 |   vector<string> 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};
      |                                                     ^
a.cc:6:55: error: 'r' was not declared in this scope
    6 |   vector<string> 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};
      |                                                       ^
a.cc:6:57: error: 's' was not declared in this scope
    6 |   vector<string> 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};
      |                                                         ^
a.cc:6:59: error: 't' was not declared in this scope
    6 |   vector<string> 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};
      |                                                           ^
a.cc:6:61: error: 'u' was not declared in this scope
    6 |   vector<string> 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};
      |                                                             ^
a.cc:6:63: error: 'v' was not declared in this scope
    6 |   vector<string> 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};
      |                                                               ^
a.cc:6:65: error: 'w' was not declared in this scope
    6 |   vector<string> 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};
      |                                                                 ^
a.cc:6:67: error: 'x' was not declared in this scope
    6 |   vector<string> 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};
      |                                                                   ^
a.cc:6:69: error: 'y' was not declared in this scope
    6 |   vector<string> 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};
      |                                                                     ^
a.cc:6:71: error: 'z' was not declared in this scope
    6 |   vector<string> 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};
      |                                                                       ^
a.cc:6:72: error: could not convert '{a, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>, <expression error>}' from '<brace-enclosed initializer list>' to 'std::vector<std::__cxx11::basic_string<char> >'
    6 |   vector<string> 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};
      |                                                                        ^
      |                                                                        |
      |                                                                        <brace-enclosed initializer list>
a.cc:7:21: error: 'A' was not declared in this scope
    7 |   vector<string> b={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};
      |                     ^
a.cc:7:23: error: 'B' was not declared in this scope
    7 |   vector<string> b={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};
      |                       ^
a.cc:7:25: error: 'C' was not declared in this scope
    7 |   vector<string> b={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};
      |                         ^
a.cc:7:27: error: 'D' was not declared in this scope
    7 |   vector<string> b={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};
      |                           ^
a.cc:7:29: error: 'E' was not declared in this scope
    7 |   vector<string> b={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};
      |                             ^
a.cc:7:31: error: 'F' was not declared in this scope
    7 |   vector<string> b={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};
      |                               ^
a.cc:7:33: error: 'G' was not declared in this scope
    7 |   vector<string> b={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};
      |                                 ^
a.cc:7:35: error: 'H' was not declared in this scope
    7 |   vector<string> b={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};
      |                                   ^
a.cc:7:37: error: 'I' was not declared in this scope
    7 |   vector<string> b={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};
      |                                     ^
a.cc:7:39: error: 'J' was not declared in this scope
    7 |   vector<string> b={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};
      |                                       ^
a.cc:7:41: error: 'K' was not declared in this scope
    7 |   vector<string> b={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};
      |                                         ^
a.cc:7:43: error: 'L' was not declared in this scope
    7 |   vector<string> b={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};
      |                                           ^
a.cc:7:45: error: 'M' was not declared in this scope
    7 |   vector<string> b={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};
      |                                             ^
a.cc:7:47: error: 'N' was not declared in this scope
    7 |   vector<string> b={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};
      |                                               ^
a.cc:7:49: error: 'O' was not declared in this scope
    7 |   vector<string> b={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};
      |                                                 ^
a.cc:7:51: error: 'P' was not declared in this scope
    7 |   vector<string> b={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};
      |                                                   ^
a.cc:7:53: error: 'Q' was not declared in this scope
    7 |   vector<string> b={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};
      |                                                     ^
a.cc:7:55: error: 'R' was not declared in this scope
    7 |   vector<string> b={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};
      |                                                       ^
a.cc:7:57: error: 'S' was not declared in this scope
    7 |   vector<string> b={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};
      |                                                         ^
a.cc:7:59: error: 'T' was not declared in this scope
    7 |   vector<string> b={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};
      |                                                           ^
a.cc:7:61: error: 'U' was not declared in this scope
    7 |   vector<string> b={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};
      |                                                             ^
a.cc:7:63: error: 'V' was not declared in this scope
    7 |   vector<string> b={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};
      |                                                               ^
a.cc:7:65: error: 'W' was not declared in this scope
    7 |   vector<string> b={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};
      |                                                                 ^
a.cc:7:67: error: 'X' was not declared in this scope
    7 |   vector<string> b={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};