Language: C++

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

#define pi 3.1415926535
#define endl "\n"
#define yes cout << "YES\n"
#define no cout << "NO\n"
#define ll long long
#define llu unsigned long long
#define pb push_back
#define mp make_pair
#define sz(x) (int)x.size()
#define tc int test_case; cin >> test_case; while(test_case--)
#define f(i,a,n) for(int i = a ; i < n ; i++)
#define rf(i,n,a) for(int i = n-1 ; i >= a ; i--)
#define F first
#define S second
#define all(c) (c).begin(),(c).end()
#define get(a,n) f(i,0,n) cin >> a[i];
#define show(a,n) f(i,0,n) cout << a[i] << " "; cout << endl;
#define fast ios:: sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define trace(x) cout << '>' << #x << ':' << (x) << "\n"
#define trace2(x,y) cout<< '>' << #x << ':' << (x) << " | " << #y << ':' << (y) << "\n"
#define trace3(a,b,c) cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<"\n"
#define trace4(a,b,c,d) cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<", "<<#d<<"="<<(d)<<"\n"

typedef long double ld;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef pair<pii,int> ppi;
typedef vector<pii> vpi;
typedef map<int,int> mii;
typedef map<ll,ll> mll;

const int inf = 1e9;
const ll inf64 = 1e18;
const int mod = inf + 7;
const int N = 2e5 + 10;

int main()
{
    fast;
    string s, t;
    cin >> s >> t;
    int c;
    f(i,0,sz(s))
    {
        if(s[i] != s[t])
            c++;
    }
    cout << c;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:50:21: error: no match for 'operator[]' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
   50 |         if(s[i] != s[t])
      |                     ^
In file included from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:1:
/usr/include/c++/14/bits/basic_string.h:1247:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::const_reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](size_type) const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; const_reference = const char&; size_type = long unsigned int]'
 1247 |       operator[] (size_type __pos) const _GLIBCXX_NOEXCEPT
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1247:29: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1247 |       operator[] (size_type __pos) const _GLIBCXX_NOEXCEPT
      |                   ~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:1265:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; reference = char&; size_type = long unsigned int]'
 1265 |       operator[](size_type __pos)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1265:28: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1265 |       operator[](size_type __pos)
      |                  ~~~~~~~~~~^~~~~