Language: C++

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

int main(){
    string c;
    cin >> c;
    string ans;
    if(c=="a") ans = b;
    else if(c=="b") ans = c;    
    else if(c=="c") ans = d;
    else if(c=="d") ans = e;    
    else if(c=="e") ans = f;
    else if(c=="f") ans = g;    
    else if(c=="g") ans = h;
    else if(c=="h") ans = i;    
    else if(c=="i") ans = j;
    else if(c=="j") ans = k;    
    else if(c=="k") ans = l;
    else if(c=="l") ans = m;    
    else if(c=="m") ans = n;
    else if(c=="n") ans = o;    
    else if(c=="o") ans = p;
    else if(c=="p") ans = q;    
    else if(c=="q") ans = r;
    else if(c=="r") ans = s;    
    else if(c=="s") ans = t;
    else if(c=="t") ans = u;    
    else if(c=="u") ans = v;
    else if(c=="v") ans = w;    
    else if(c=="w") ans = x;
    else if(c=="x") ans = y;    
    else if(c=="y") ans = z;
    cout << ans << "\n";
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:11:22: error: 'b' was not declared in this scope
   11 |     if(c=="a") ans = b;
      |                      ^
a.cc:13:27: error: 'd' was not declared in this scope
   13 |     else if(c=="c") ans = d;
      |                           ^
a.cc:14:27: error: 'e' was not declared in this scope
   14 |     else if(c=="d") ans = e;
      |                           ^
a.cc:15:27: error: 'f' was not declared in this scope
   15 |     else if(c=="e") ans = f;
      |                           ^
a.cc:16:27: error: 'g' was not declared in this scope
   16 |     else if(c=="f") ans = g;
      |                           ^
a.cc:17:27: error: 'h' was not declared in this scope
   17 |     else if(c=="g") ans = h;
      |                           ^
a.cc:18:27: error: 'i' was not declared in this scope
   18 |     else if(c=="h") ans = i;
      |                           ^
a.cc:19:27: error: 'j' was not declared in this scope
   19 |     else if(c=="i") ans = j;
      |                           ^
a.cc:20:27: error: 'k' was not declared in this scope
   20 |     else if(c=="j") ans = k;
      |                           ^
a.cc:21:27: error: 'l' was not declared in this scope
   21 |     else if(c=="k") ans = l;
      |                           ^
a.cc:22:27: error: 'm' was not declared in this scope
   22 |     else if(c=="l") ans = m;
      |                           ^
a.cc:23:27: error: 'n' was not declared in this scope
   23 |     else if(c=="m") ans = n;
      |                           ^
a.cc:24:27: error: 'o' was not declared in this scope
   24 |     else if(c=="n") ans = o;
      |                           ^
a.cc:25:27: error: 'p' was not declared in this scope
   25 |     else if(c=="o") ans = p;
      |                           ^
a.cc:26:27: error: 'q' was not declared in this scope
   26 |     else if(c=="p") ans = q;
      |                           ^
a.cc:27:27: error: 'r' was not declared in this scope
   27 |     else if(c=="q") ans = r;
      |                           ^
a.cc:28:27: error: 's' was not declared in this scope
   28 |     else if(c=="r") ans = s;
      |                           ^
a.cc:29:27: error: 't' was not declared in this scope
   29 |     else if(c=="s") ans = t;
      |                           ^
a.cc:30:27: error: 'u' was not declared in this scope
   30 |     else if(c=="t") ans = u;
      |                           ^
a.cc:31:27: error: 'v' was not declared in this scope
   31 |     else if(c=="u") ans = v;
      |                           ^
a.cc:32:27: error: 'w' was not declared in this scope
   32 |     else if(c=="v") ans = w;
      |                           ^
a.cc:33:27: error: 'x' was not declared in this scope
   33 |     else if(c=="w") ans = x;
      |                           ^
a.cc:34:27: error: 'y' was not declared in this scope
   34 |     else if(c=="x") ans = y;
      |                           ^
a.cc:35:27: error: 'z' was not declared in this scope
   35 |     else if(c=="y") ans = z;
      |                           ^